Electronic Navigation

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Associate Professor Associate Professor Associate Professor Associate Professor Radu Hanzu Radu Hanzu Radu Hanzu Radu Hanzu-Pazara, PhD Pazara, PhD Pazara, PhD Pazara, PhD Lecturer Varsami Anastasia Lecturer Varsami Anastasia Lecturer Varsami Anastasia Lecturer Varsami Anastasia, PhD , PhD , PhD , PhD Lecturer Tromiadis Ramona Lecturer Tromiadis Ramona Lecturer Tromiadis Ramona Lecturer Tromiadis Ramona, PhD , PhD , PhD , PhD ELECTRONIC NAVIGATION (Course Manual)

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Electronic Navigation

Transcript of Electronic Navigation

Page 1: Electronic Navigation

Associate Professor Associate Professor Associate Professor Associate Professor Radu HanzuRadu HanzuRadu HanzuRadu Hanzu----Pazara, PhDPazara, PhDPazara, PhDPazara, PhD Lecturer Varsami AnastasiaLecturer Varsami AnastasiaLecturer Varsami AnastasiaLecturer Varsami Anastasia, PhD, PhD, PhD, PhD Lecturer Tromiadis RamonaLecturer Tromiadis RamonaLecturer Tromiadis RamonaLecturer Tromiadis Ramona, PhD, PhD, PhD, PhD

ELECTRONIC NAVIGATION (Course Manual)

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Chapter I

Hyperbolic navigation

1.1. Introduction

Hyperbolic navigation refers to a class of radio navigation systems based on the difference in timing between the reception of two signals, without reference to a common clock. This timing reveals the difference in distance from the receiver to the two stations. Plotting all of the potential locations of the receiver for the measured delay produces a series of hyperbolic lines on a chart. Taking two such measurements and looking for the intersections of the hyperbolic lines reveals the receiver's location to be in one of two locations. Any form of other navigation information can be used to eliminate this ambiguity and determine a fix. The first such system to be used was the World War II-era Gee system introduced by the Royal Air Force for use by Bomber Command. This was followed by the more accurate Decca Navigator System in 1944 by the Royal Navy, along with LORAN by the US Navy for long-range navigation at sea. Post war examples including the well-known US Coast Guard LORAN-C, the international Omega system, and the Soviet Alpha and CHAYKA. All of these systems saw use until their wholesale replacement by satellite navigation systems like the Global Positioning System (GPS). Timing-based navigation. Consider two ground-based radio stations located at a set distance from each other, say 300 km so that they are exactly 1 ms apart at light speed. Both stations are equipped with identical transmitters set to broadcast a short pulse at a specific frequency. One of these stations, called the "secondary" is also equipped with a radio receiver. When this receiver hears the signal from the other station, referred to as the "master", it triggers its own broadcast. The master station can then broadcast any series of pulses, with the secondary hearing these and generating the same series after a 1 ms delay. Consider a portable receiver located on the midpoint of the line drawn between the two stations, known as the baseline. In this case, the signals will, necessarily, take 0.5 ms to reach the receiver. By measuring this time, they could determine that they are precisely 150 km from both stations, and thereby exactly determine their location. If the receiver moves to another location along the line, the timing of the signals would change. For instance, if they time the signals at 0.25 and 0.75 ms, they are 75 km from the closer station and 225 from the further. If the receiver moves to the side of the baseline, the delay from both stations will grow. At some point, for instance, they will measure a delay of 1 and 1.5 ms, which implies the receiver is 300 km from one station and 450 from the other. If one draws circles of 300 and 450 km radius around the two stations on a chart, the circles will intersect at two points. With any additional source of navigation information, one of these two intersections can be eliminated as a possibility, and thus reveal their exact location, or "fix".

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1.2. Hyperbolic navigation characteristics Consider the same examples as our original absolute-timed cases. If the receiver is located on the midpoint of the baseline the two signals will be received at exactly the same time, so the delay between them will be zero. However, the delay will be zero not only if they are located 150 km from both stations and thus in the middle of the baseline, but also if they are located 200 km from both stations, and 300 km, and so forth. So in this case the receiver cannot determine their exact location, only that their location lies somewhere along a line perpendicular to the baseline. In the second example the receivers determined the timing to be 0.25 and 0.75 ms, so this would produce a measured delay of 0.5 ms. There are many locations that can produce this difference - 0.25 and 0.75 ms, but also 0.3 and 0.8 ms, 0.5 and 1 ms, etc. If all of these possible locations are plotted, they form a hyperbolic curve centred on the baseline. Navigational charts can be drawn with the curves for selected delays, say every 0.1 ms. The operator can then determine which of these lines they lie on by measuring the delay and looking at the chart. A single measurement reveals a range of possible locations, not a single fix. The solution to this problem is to simply add another secondary station at some other location. In this case two delays will be measured, one the difference between the master and secondary "A", and the other between the master and secondary "B". By looking up both delay curves on the chart, two intersections will be found, and one of these can be selected as the likely location of the receiver. This is a similar determination as in the case with direct timing/distance measurements, but the hyperbolic system consists of nothing more than a conventional radio receiver hooked to an oscilloscope. Because a secondary could not instantaneously transmit its signal pulse on receipt of the master signal, a fixed delay was built into the signal. No matter what delay is selected, there will be some locations where the signal from two secondary would be received at the same time, and thus make them difficult to see on the display. Some method of identifying one secondary from another was needed. Common methods included transmitting from the secondary only at certain times, using different frequencies, adjusting the envelope of the burst of signal, or broadcasting several bursts in a particular pattern. A set of stations, master and secondaries, was known as a "chain". Similar methods are used to identify chains in the case where more than one chain may be received in a given location.

1.3. Operational systems Meint Harms was the first to have attempted the construction of a hyperbolic navigation systems, starting with musings on the topic in 1931 as part of his master's examination at Seefahrtschule Lübeck (Navigation College). After taking the position of Professor for Mathematics, Physics and Navigation at the Kaisertor in Lübeck, Harms tried to demonstrate hyperbolic navigation making use of simple transmitters and receivers. On 18 February 1932 he received Reichspatent-Nr. 546000 for his invention. LORAN. The US had also considered hyperbolic navigation as early as 1940, but only halting progress had been made by the time they were introduced to Gee. Gee was immediately selected for the 8th Air Force and their attention turned to other uses, eventually considering convoy navigation in particular. R. J. Dippy, inventor of Gee, moved to the US in mid-1942 to help with this project, which eventually emerged as LORAN, for LOng RAnge Navigation. LORAN became LORAN-A when the design of its replacement started, this was initially the LORAN-B concept, but eventually replaced by the very long-range LORAN-C starting in 1957.

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LORAN-A was essentially a version of Gee with a new selection of frequencies suitable for long-range transmission over water, eventually selecting 1.950 MHz. 7.5 MHz was selected for daytime use as an additional channel, but never used operationally. In comparison to Gee's 450 mile range through air, LORAN-A had a range of about 1500 miles over water, and 600 miles over land. Operation was generally similar to Gee, but only one of the secondary signals was displayed at a time. A fix required the operator to measure one delay, then the other, and then look up the resulting delays on the charts. The accuracy was quoted as 1% of range. LORAN-A used two methods to identify a chain. One was the operational frequency, with four "channels", as in Gee. The second was the rate at which the pulses were repeated, with "high", "low" and "slow" rates. This allowed for up to 12 chains in any given area. Additionally, the originally steady repetition of the pulses was later modified to create another eight unique patterns, allowing a total of 96 station pairs. Any given chain could use one or more pairs of stations, demanding a large number of unique signals for widespread coverage.

The Decca Navigation System was originally developed in the US, but eventually deployed by the Decca Radio company in the UK and commonly referred to as a British system. Initially developed for the Royal Navy as an accurate adjunct to naval versions of Gee, Decca was first used on 5 June 1944 to guide minesweepers in preparation for the D-Day invasions. The system was developed post-war and competed with GEE and other systems for civilian use. A variety of reasons, notably its ease-of-use, kept it in widespread use into the 1990s, with a total 42 chains around the world. A number of stations were updated in the 1990s, but the widespread use of GPS led to Decca being turned off at midnight on 31 March 2000. Decca was based on comparing the phases of continuous signals instead of the timing of their pulses. This was more accurate, as the phase of a pair of signals could be measured to within a few degrees, four degrees in the case of Decca. This greatly improved inherent accuracy allowed Decca to use much longer wavelengths than Gee or LORAN while still offering the same level of accuracy. The use of longer wavelengths gave better propagation than either Gee or LORAN, although ranges were generally limited to around 500 miles for the basic system. However, Decca also had the inherent disadvantage that the signal repeated over space, and could only identify the location within what they referred to as "lanes". These were relatively small, so additional information was needed to identify which lane the receiver was located in. Decca solved this problem though the use of an odometer-like display known as "decometers". Prior to leaving on a trip, the navigator would set the decometer's lane counter to their known position. As the craft moved the dial's hand would rotate, and increment or decrement the counter when it passed zero. The combination of this number and the current dial reading allowed the navigator to directly read the current delay and look it up on a chart, a far easier process than Gee or LORAN. It was so much easier to use that Decca later added an automatic charting feature that formed a moving map display. Later additions to the signal chain allowed the zone and lane to be calculated directly, eliminating the need for manual setting of the lane counters and making the system even easier to use. As each master and secondary signal was sent on a different frequency, any number of delays could be measured at the same time; in practice a single master and three secondaries were used to produce three outputs. As each signal was sent on a different frequency, all three, known as "green", "red" and "purple", were simultaneously decoded and displayed on three decometers. The secondaries were physically distributed at 120 degree angles from each other, allowing the operator to pick the pair of signals on the display that were sent from stations as close to right angles to the receiver as possible, further improving accuracy. Maximum accuracy was normally quoted as 200 yards, although that was subject to operational errors. In addition to greater accuracy and ease of use, Decca was also more suitable for use over land. Delays due to refraction can have a significant effect on pulse timing, but much less so for phase changes. Decca thus found itself in great demand for helicopter use, where runway approach aids like ILS and VOR were not suitable for the small airfields and essentially random locations the

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aircraft were used. One serious disadvantage to Decca was that it was susceptible to noise, especially from lightning. This was not a serious concern for ships, who could afford to wait out storms, but made it unsuitable for long-range air navigation where time was of the essence. Several versions of Decca were introduced for this role, notably DECTRA and DELRAC, but these did not see widespread use. LORAN-C. LORAN-A was designed to be quickly built on the basis of Gee, and selected its operating frequency based on the combination of the need for long over-water range as well as accuracy of the resulting fix. Using much lower frequencies, in the kHz instead of MHz, would greatly extend the range of the system. However, the accuracy of the fix is a function of the wavelength of the signal, which increases at lower frequencies - in other words, using a lower frequency would necessarily lower the accuracy of the system. Hoping for the best, early experiments with "LF Loran" instead proved that accuracy was far worse than predicted, and efforts along these lines were dropped. Several halting low-frequency efforts followed, including the Decca-like Cyclan and Navarho concepts. A key development was the introduction of the low-cost phase-locked loop in the 1950s, which allowed a receiver to "lock on" to a signal and maintain the phase and frequency very accurately. This allowed a carrier signal to be re-constructed in a local oscillator by observing received pulses, a process that was accurate enough for phase comparisons against the local oscillator. This allowed a single system to combine the features of pulse-based and phase-based systems. Re-using the Cyclan transmitters, the US Navy started experiments with such a system int the mid-1950s, and turned the system on permanently in 1957. Numerous chains followed, eventually providing around-the-world coverage near US allies and assets. Although less accurate that Decca, it offered the combination of reasonable accuracy and long ranges, keeping it in operation until GPS finally led to its shutdown on 8 February 2010. In basic operation, LORAN-C is more similar to Decca than Gee or LORAN-A, as its main way determining location was the comparison of phase differences between signals. However, at low frequencies and long ranges it would be difficult to know whether you are looking at the current phase of the signal, or the phase of the signal one cycle ago, or perhaps one reflected off the ionosphere. Some form of secondary information is needed to reduce this ambiguity. LORAN-C achieved this by sending unique details in the pulses so each station could be uniquely identified. The signal was started off when the Master broadcast a sequence of nine pulses, with the precise timing between each pulses being used to identify the station. Each of the Secondary stations then sent out their own signals, consisting of eight pulses in patterns that revealed which station they were. The receivers could use the signal timings to select chains, identify secondaries, and reject signals bounced off the ionosphere. LORAN-C chains were organized into the Master station, M, and up to five Secondary stations, V, W, X, Y, Z. All were broadcast at 100 kHz, a much lower frequency than earlier systems. The result was a signal that offered a daytime ground wave range of 2,250 miles, nighttime ground wave of 1,650 miles and skywaves out to 3,000 miles. Timing accuracy was estimated at 0.15 microseconds, offering accuracies on the order of 50 to 100 meters.

Omega. One of the last hyperbolic navigation systems to enter operational use was one of the earliest to be developed. Omega traces its history to work by John Alvin Pierce in the 1940s, working on the same basic idea as the Decca phase-comparison system. He imagined a system specifically for medium-accuracy global navigation, and thus selected the extremely low frequency of 10 kHz as the basis for the signal. However, the problem with phase ambiguity, as in the case of Decca, meant that the system was not practical at the time. Where the phase-locked loop made LORAN-C a possibility, for Omega it was the introduction of inertial navigation systems (INS) that offered a solution - the INS was accurate enough to resolve any ambiguity about which lane the receiver was in. Experiments continued throughout

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the 1950s and 60s, in parallel with Decca's development of their almost identical DELRAC system. It was not until the 1960s, when ice-breaking ballistic submarines became a main deterrent force, that there was a pressing need for such a system. The US Navy authorized full deployment in 1968, reaching a complete set of 8 stations in 1983. Omega would also prove to be one of the shortest-lived systems, shutting down on 20 September 1997. Omega stations broadcast a continuous-wave signal in a specific time-slot. In order to maintain precise timing of the slots for stations distributed around the world, stations were equipped with synchronized atomic clocks. These clocks also ensured that their signals were sent out with the right frequency and phase; unlike previous systems, Omega did not need to have a master/secondary arrangement as the clocks were accurate enough to trigger the signals without an external reference. To start the sequence, the station in Norway would initially broadcast on 10.2 kHz for 0.9 seconds, then turned off for 0.2 seconds, then broadcast on 13.6 kHz for 1.0 seconds, and so on. Each station broadcast a series of four such signals lasting about a second each, and then stood silent while other stations took their turn. At any given instant, three stations would be broadcasting at the same time on different frequencies. Receivers would select the set of stations that were most suitable for their given location, and then wait for the signals for those stations to appear during the 10 second chain. Calculation of the fix then proceeded in precisely the same fashion as Decca, although the much lower operating frequency led to much less accuracy. Omega's charts quote accuracies of 2 to 4 nautical miles.

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Chapter II

LORAN - C

2.1. Introduction and history

Introduction. Loran-C, the successor to Loran-A, was originally developed to provide radionavigation service for U.S. coastal waters and was later expanded to include complete coverage of the continental U.S. as well as most of Alaska. Twenty four U.S. Loran-C stations work in partnership with Canadian and Russian stations to provide coverage in Canadian waters and in the Bering Sea. Loran-C provides better than 0.25 nautical mile (460 meters) absolute accuracy for suitably equipped users within the published areas. Users can return to previously determined positions with an accuracy of 18 to 90 meters using Loran-C in the time difference repeatable mode. Advances in technology have allowed greater automation of Loran-C operations. New technology has allowed the United States Coast Guard to establish centralized control of the continental U.S. Loran-C system at two locations. The application of new receiver technology has improved the usability of the system. A majority of the 1.3 million Loran sets in use worldwide are for mariners. Loran-C is greatly appreciated by the US general aviation community with some 80,000 aircraft now equipped with the system. History. The Pacific war showed the need for a Loran-like system that could be operated over much greater distances in daylight than Loran-A could provide. There were few islands on which transmitters could be located and they were great distances apart. The only potential solution was to try Loran techniques at low frequencies so an experimental set of three Loran transmitters operating at 180 kHz was set up on the US East Coast during 1945 using balloon supported aerials. The main result of these tests was to show that pulse envelope matching, as used in Loran-A, was too inaccurate with the long pulses necessary at these low frequencies, and that phase comparison would he required. The experiments were not followed up and the system, called LF Loran at that time, was dismantled following the end of the war. The MIT Radiation Laboratory, which had sponsored the work, was also closed and responsibility for further work given to the new US Air Force. The transmitters that had been used for LF Loran were re-installed in Alaska for trials of LF propagation in Arctic areas and the experimentation yielded much useful data. According to the book "Sixty Years: the RCAF and CF Air Command 1924-84 edited by Larry Milberry, three station sites were established. Kittigazuit, N.W.T was the master. Slaves were close to Barrow, Alaska and Cambridge Bay, N.W.T. The chain was built in 1947, but was shut down in March of 1950. It operated at 180 kHz but "it became evident that attenuation of the ground wave over the permafrost and certain sea ice conditions was much more severe than predicted. ... interaction between the groundwave and the first hop skywave created severe pulse matching problems''. The station at Kittigazuit, NWT was called "Yellow Beetle". In 1946, the Sperry company proposed a navigation system called Cyclan which would use phase comparison and operate at two frequencies of 180 and 200 kHz, the difference between them being used to resolve ambiguities. It was tested by the USAF in 1948 using 160 and 180 kHz and later reduced to one frequency and renamed Cytac for possible use as a military tactical navaid.

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After further tests in 1951, the U.S. Air Force decided to concentrate on inertial and Doppler systems for tactical use and stopped development. Parallel development of a system known as Navarho had also been proceeding, a system that had been derived from the British POPI system. Navarho was a long range system providing both range and bearing, obtaining range by measuring the change of phase between the transmitted signal and a local very high stability reference oscillator - the first time this had been attempted. The state of the art for portable oscillators left too much to be desired so the project was abandoned. Also scrubbed, was a further development known as Navaglobe - a system intended to give wide area coverage. The US Navy began to take an interest a few years later, and recommissioned the three original Cytac transmitters at Forestport, NY (later used for experimental Omega transmissions), Carolina Beach, NC, and Carabelle, Florida, for a trial abroad the USCG cutter Androscoggin in April 1956. The transmissions were phase coherent 100 kHz pulses 100 microseconds long at peak powers of 60 kW except from Forestport, which had a 1,280 foot tower and radiated an estimated 200 kW. These pulses were less than half the duration later adopted for the Loran-C system (240 microseconds) and rose to a maximum amplitude in only 25 microseconds as compared with the later 60 microseconds. The results showed a daytime ground wave range of 2,250 miles, nighttime ground wave of 1,650 miles and skywaves out to 3,000 miles. Time difference accuracy was estimated at 0.15 microseconds. Thus encouraged, the US Navy established transmitters in the north-eastern Atlantic and the Mediterranean during 1957, followed by many others in the Pacific and elsewhere, naming the system Loran-C. Initial contracts for receivers were subcontracted to, amongst others, Decca Navigator, who produced the AN/SPN, probably the most successful of the early Loran-C receivers. It had 52 controls and weighed over 100 pounds. Although initiated as a marine system, Loran-C came into use for aerial navigation quite widely and during trials in 1963, was flown at over Mach 1 in a British Vulcan aircraft. A chain was installed in Vietnam in the 1960's specifically for the use of USAF aircraft. Loran-C was installed in many long range civil aircraft while inertial systems were being proven and is still used in long range military aircraft. The Decca company sued the US Navy in 1969, alleging that Loran-C infringed patents it held concerning a 100 kHz pulsed navigation system. As early as 1944, Decca had deliberately not used the 7F (98 kHz) frequency of the Navigator system because of its possible use for this pulse system. The claim was upheld at first but reversed on appeal, with the US Navy pleading military necessity. 2.2. LORAN-C signal characteristics Loran-C transmitters are organized into chains of 3, 4 or 5 stations. Within a chain, one station is designated "Master" (M) while the other "Secondary" stations identified by the letters W, X, Y and Z. Different secondary designations are used depending on the number of station in a chain.

CONFIGURATION DESIGNATORS EXAMPLE

Master with 5 secondaries M V, W, X, Y, Z South Central U.S. 9610

Master with 4 secondaries M W, X, Y, Z Southeast U.S. 7980

Master with 3 secondaries M X, Y, Z Canadian West Coast 5990

Master with 2 secondaries M W, X Calcutta 5943

Master with 2 secondaries M X, Y East China 8390 By 1989, there were 16 Loran-C chains comprising 67 stations and transmitting on 100 kHz. In the year 2000 this had grown to 28 chains. Power levels can range from as low as 11 Kw (Bombay 6042) to as high as 1.2 megawatts (China East Sea 8390). In Russia, a navigation

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system known as CHAYKA is compatible with Loran-C hence it forms part of the work wide chain. Transmitting station signal availability is greater than 99.9 percent while typically providing 99.7 percent triad availability. The Loran-C navigation signal is a carefully structured sequence of brief radio frequency pulses on a carrier wave centered at 100 kHz. All secondary stations radiate pulses in bursts of eight, whereas the Master signal, for identification purposes, has an additional ninth pulse burst. The sequence of signal transmissions consists of a pulse group from the Master (M) station followed at precise time intervals by pulse groups from the secondary stations. The time interval between the reoccurrence of the Master pulse is called the Group Repetition Interval (GRI).

Figure 2.1. Loran-C pulse format and sequencing

Since all Loran-C transmitters operate on the same frequency, the GRI is the key by which a receiver can identify and isolate signal groups from a specific chain. In naming the chains, the GRI is included. As an example the Great Lakes chain has a GRI of 8970. This means the time interval is 89700 microseconds. The rightmost zero is always implied and the GRI is always in multiples of 10 microseconds. In old Loran-C receivers, the operator had to actually set this number to receive the chain. In cases where the Loran signals were observed on an oscilloscope, pulses from the desired chain would be stationary while those from other chains would be drifting down the time base at varying speeds. It was in fact, the only way of positively identifying a chain, however in modern receivers, this is now done automatically. GRI's are chosen on the basis of: • Baseline lengths between master and slaves. If the distance between the master and first secondary is say 1000 km/s, the radio signal will take 33,000 microseconds to get to the slave so the GRI cannot possibly be less than that. • Number of slaves that have to be accomodate - they all have to have delays so that there is no possibility of them crossing over anywhere in coverage area. • Geography. • Other nearby chains with consideration given to interference. • Sky wave cross-rate interference.

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Duty cycle of the transmitters - a faster GRI means the average power of the transmitted signal is higher so the final stage in the transmitter requires more cooling. With average baseline lengths and three slaves, the minimum GRI cannot be much less than 50,000 microseconds.

Figure 2.2. Each Loran-C pulse has an approximate duration of 200 µs. The interval between

pulses within a pulse group is 1000 µs, except for the last two pulses at the Master which have a 2000 µs interval. The graphic below illustrates one pulse

Figure 2.3. This graphic illustrates the points on the Loran-C pulse envelope that define the start

time, the time of maximum envelope power and the stop time of the pulse

Two other important characteristics are associated with Loran-C signals, namely emission and coding delay. If the master station is taken as a reference, the emission delay refers to how long it takes before the secondary transmits after the Master has done so. The coding delay is a very small correction that removes the local (near-field) discrepancy between the envelope and carrier. Both parameters are measured in microseconds and are uniquely associated with each secondary station.

Baselines and coverage An imaginary line drawn between the Master and each secondary station is called the baseline. The continuation of the baseline in either direction is called a baseline extension. Typical baselines are from 1200 to 1900 km (say 600 to 1000 nautical miles). Chain coverage is determined by the power transmitted from each transmitter in the chain, the distance between

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them and how the different transmitters are oriented in relation to each other (the geometry of the chain).

Sky wave rejection A frequency of 100 kHz was chosen for the Loran-C carrier wave to take advantage of propagation of the stable ground wave to long distances. However, the presence of delayed sky waves, reflected from the ionosphere, cause distortions of the pulse shape and change the carrier phase within the pulses of the received signal. Not only that, the skywaves take longer to arrive at the receiver than the ground wave, so their presence complicates the computation. To avoid sky wave contamination, the Loran-C receiver selects a zero crossing of a specified carrier cycle at the front end of the pulses transmitted by master and secondary stations. Making the cycle selection early in the ground wave pulse - usually the third cycle is employed - ensures that the time interval measurement is made using the uncontaminated part of the pulse. But how is the third peak selected when the start time of the pulse is not known? To solve the problem, the receiver compares the envelope (the rough shape) of the received pulse with a stored envelope. This process is called the "rough measurement". When the third peak is finally located, the phase of the signal can be determined. The phase of the signal can be zero or pi radians. Precise control over the pulse shape at the transmitter also ensures that the selected zero crossing can be identified reliably by the receiver.

Figure 2.4. Zero Crossing: This diagram illustrates the third cycle in the Loran pulse

Phase coding To reduce the effects of interference and noise on time difference measurements, and to assist in distinguishing between master and secondary stations, the carrier phase of selected transmitted pulses is reversed in a predetermined pattern. The pattern is shown in Fig. 5, where a minus sign indicates an inverted pulse (180° phase shift), and a plus sign means no phase shift. This pattern is repeated every two GRI's. Simply stated, phase coding determines whether the first peak in the pulse is upwards or downwards.

Figure 2.5. Phase Coding

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Time difference measurements The basic measurements made by Loran-C receivers are to determine the difference in the time-of-arrival (TD) between the master signal and the signals from each of the secondary stations of a chain. Each TD value is measured to a precision of about 0.1 microseconds (100 nanoseconds) or better. As a rule of thumb, 100 nanoseconds correspond to about 30 metres. The principle of time difference measurements in hyperbolic mode is illustrated in Fig. 2.6.

Figure 2.6. Time Difference Measurements

Automatic operation Today’s state-of-the-art, solid state Loran-C transmitters are adapted for automatic operation; that is to say all vital transmitter functions are duplicated or designed for graceful degradation so that the result of a defect is minimized. These vital functions are further monitored at the Control Centre which has the capability of initiating corrective action using data communications. As a consequence, the transmitters may be operated as unmanned stations except for caretakers.

Precision clocks To achieve high positioning accuracy within the service area, Loran-C transmitter stations are equipped with a suite of atomic clocks which provide the timing for the transmitted Loran-C signal. On most stations these clocks are cesium frequency standards with a stability of typically 10-13, or an error of 1 second in 317,000 year. Precise navigation with Loran-C demands that the error in the timing system must not exceed a few tens of nanoseconds. For NELS it is specified that a station's clock shall not deviate by more than 30 nanoseconds from the clocks of the

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neighbouring stations. Achieving this precision in timing it is necessary to continuously measure the time deviation between the clocks in the system.

SAM control There are two basic methods in use for monitoring and adjusting the clocks in Loran-C systems. The most commonly used method up to now is to measure the time difference between Loran-C signals received from a master and a secondary at a fixed location in the coverage area. Timing control includes making adjustments to the clock of the secondary station so that the measured TD is kept at a predetermined value. The measurement equipment at the fixed location is called a System Area Monitor (SAM), hence this method of timing control is referred to as "SAM control".

TOE control In the other method for timing control, used by the Northwest European Loran-C System (NELS), there are no SAM's. Instead, arrival times of signals from adjacent transmitters are measured relative to the local clock at each transmitter station. The measurements from all stations in the system are sent by permanent data link to the control station where they are combined so that the time deviation of each transmitter's clock can be calculated. Computed adjustments are returned to the individual transmitter sites where they are used for clock synchronization. This results in a common time reference for the Time Of Emission (TOE) of the Loran-C pulses from all transmitters and is called "TOE control". The common NELS time reference is itself related to UTC using the UTC (Brest) time standard which is co-located with the NELS Control Centre at Brest, France. The NELS time reference synchronization to UTC (Brest) is maintained to within 100 ns.

TOE and SAM control compared Under TOE control, the time difference measurements over the coverage area will vary slightly with seasonal changes in the speed of ground wave propagation. With SAM control, time difference measurements made especially close to the area monitor will be very stable. TOE control thus provides better overall performance monitoring throughout the coverage area. Other advantages of TOE control over SAM control are: • Modeling and prediction of TD variations are made easier • Time derived from the signals is more accurate • Better accuracy for cross-chain and master independent use • Better accuracy for Rho Rho navigation (circular navigation method) • No monitor sites are needed.

Additional Secondary Factor (ASF) A Loran-C receiver computes distances from Loran-C transmitting stations using the time of arrival measurements and the propagation velocity of the radio ground wave to determine position. Small variations in the velocity of propagation between that over sea water and over different land masses are known as the Additional Secondary Factor, or ASF. Corrections may be applied to compensate for this variation. Such corrections may improve the absolute accuracy of the Loran-C service in positions where the received Loran-C signal passes over anything but sea water on its way from transmitter to receiver. The values of ASF depend mainly on the conductivity of the earth's surface along the signal paths. Sea water has high conductivity, and the ASFs of sea water are, by definition, zero. Dry soil, mountains or ice generally have low

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conductivity and radio signals travel over them more slowly, giving rise to substantial ASF delays and hence degradation of absolute accuracy. Fortunately, ASFs vary little with time, and it is possible to calibrate the Loran-C service by measuring ASF values throughout the coverage area. A program for mapping of ASF in northern Europe is the basis for the production of ASF corrections. These corrections will be distributed as electronic databases.

Service integrity Loran-C stations are constantly monitored to detect signal abnormalities which would render the system unusable for navigation. "Blink" is the prime means by which the user is notified that the transmitted Loran-C signal does not comply with the system specifications. Blink also indicates that the Control Centre cannot ensure that the signal complies with these specifications, for instance, as a result of discontinuation of data communications linking the Control Centre to the stations. Blink is a distinctive change in the group of eight Loran-C pulses that can be recognized automatically by a receiver so the user is notified instantly that the Loran-C chain blinking should not be used for navigation. Blink starts at a maximum of 60 seconds after detection of an abnormality. Automatic blink initiated within 10 seconds of a timing abnormality may be added where Loran-C is extensively used for aviation purposes. 2.3. Accuracy of the signal The Loran-C service will support an absolute accuracy varying from 185 meters to 463 meters (0.1 to 0.25 nautical miles), depending on where the observer is within the coverage area. Absolute accuracy defines a user's true geographic position (latitude and longitude). Repeatable accuracy is a measure of an observer's ability - by using a navigation system such as Loran-C - to return to a position visited previously using the same navigation system. Loran-C repeatable accuracy is sometimes as good as 18 meters and is usually better than 100 meters within the coverage area. The inherent accuracy of the Loran-C system makes it suitable for many land radio location applications. However, propagation anomalies may be encountered in urban areas caused by the proximity of large man-made structures. Compensation for these anomalies is usually possible either by prior measurement or by the application of the local ASF. Substantial improvement in the accuracy of Loran-C service is technically possible by the measurement and broadcast of local corrections in a technique known as differential Loran-C (DLoran-C). This is similar to what is achieved with GPS using GPS differential corrections known as DGPS. The Loran-C system is capable of being utilized as a time transfer standard. The HF stations WWV and WWVH (and others like CHU) suffer from variations in propagation time because of changes in the ionosphere. WWVB at 60 kHz is much better in that respect but suffered from low output power until the late 1990's. Loran-C was the best source of radio time signals for the last few decades. A number of companies built receivers that were specifically designed to be time reference receivers. It's only been in the last few years that GPS based time transfer had better performance than Loran-C. Apparently, there is a capability to send digital data on the Loran-C signal for military use. Can anyone comment on this? LORAN-C can be utilized in different modes of operation. Most common is the hyperbolic mode. Circular mode, often referred to as Range-Range or RHO-RHO, has limited application for special users. Unique and expensive user equipment is required for RHO-RHO operation however, it only takes a fix from two stations to establish position.

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2.4. Dual rated chains Some transmitters in a Loran chain have only one function. That means they either serve as dedicated master or dedicated secondary station in a particular chain. Many transmitters, however are dual rated, meaning that these can serve one function in one chain and yet another in a neighboring chain. For example, the Lorsta Searchlight (Nevada) facility has a transmitter which serves as the secondary (Y) station in the 9440 chain and it also serves as the secondary (W) station in the neighbouring chain 9610. Dual rating is desirable because, other things being equal, land acquisition costs and siting difficulties are reduced. In a dual rated environment, the operations/timing room has dual timing systems which are common to one transmitter. Both timing systems operate from the same cesium frequency standard, thus helping to reduce equipment cost. There is not one but two electronic timers for each rate. For each rate, one timer is always on-line while the other serves as a standby unit.

Figure 2.7. These maps of chain 9610 (left) and 9940 (right) serve to illustrate their coverage area. The dual rated station Lorsta Searchlight (Nevada) is used as the secondary (W) in chain

9610 and secondary (Y) in chain 9440

Figure 2.8. Coverage: This map illustrates the global coverage of Loran-C

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2.5. LORAN – C stations

LORAN-C stations working according to their status inside the system, grouped in Master and Secondary stations, as follows:

• The Master station transmits groups of 9 impulses, with a repetition frequency of 10-25 groups/second;

• After the Master station have transmitted the first group of impulses, follow a silence period, necessary to the direct line to travel the distance between the Master station and the secondary station;

• Next, the secondary station will transmits a group of 8 impulses, with a break between them of 1,000 microseconds.

Function of splitting of repetition period, each station of a LORAN-C chain is encoded as follows:

SL2 – M where: SL – PRR – pulse recurrence rate

2 – PRI - pulse recurrence interval

M – Master station

or: SL2 – Y

where: SL – PRR – pulse recurrence rate

2 – PRI – pulse recurrence interval)

Y – Secondary station

The hyperbole is encoded in a similar way, adding the time difference value, which defines the positioning hyperbole (in microseconds), like:

SL2-Y-19724

or: 7990 – Y - 19724

LORAN – C receivers

For a receiver to be able to display the ship position with system required accuracy must have the following characteristics:

o LORAN-C signals receiving is automatic; o An automatic identification of terrestrial pulses emitted by the Master and

secondary stations, and to realize a complete cycle of all 8 pulses for each pair of Master-Secondary stations that are used;

o An automatic tracking of the signal when receiving was carried out; o As a minimum requirement, displaying of two readings made in different

moments, with an accuracy of 0.1 microseconds, at least; o To have filters for interferences, calibrated by the producer to minimize the

effects due to radio frequencies interferences in the operation area of the system. At some of the alder receivers was necessary the operator to select the stations chain and stations pair during the measurement process. Never, the receivers will automatically process, if the operator has introduced the ship latitude and longitude, only will select the optimum stations chain and stations pair in the area. The automatic selection process can be replaced with the user manual selection of stations and chains.

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Having selected the Master and secondary stations, the system has to receive signals with enough accuracy to permit their positioning and tracking. The necessary time period for processing of received information from the Master and secondary stations and displaying of position, depends by receiver characteristics and the rate of received signals. The received signal should not be affected by interference caused by other signals that might appears as received signal and thus reducing the rate of received LORAN signal and affecting the ship position accuracy. The receiver signal filters can reduce the interferences influence. These filters can be preset by producer or to be adjusted by the user. The modern LORAN-C receivers give to the operator the possibility to monitoring ship movement and to execute the necessary course alterations when is needed. The receiver provides ship position (displayed as time difference or latitude and longitude) and using an accurate measurement of time can offer very useful information for navigation, like ship course and speed. The receiver permits introduction of the waypoints details and creation of the route which have to be followed, allowing to the operator to monitoring the ship movement on determined route, giving in the same time information about the deviation from course and time to next waypoint. It also can be store in the receiver memory data regarding the magnetic field variation in the navigation area and the possibility to select the ship course as true course or magnetic. When is used the magnetic course, the user will be warned about this value. The LORAN-C receivers can be used also as independently navigation equipment or can be integrated with other devices, like ECDIS or GPS. Modern receivers have the possibility to transmit information to other devices which use the NMEA protocol (National Marine Electronics Association). This type of receivers can be connected with the autopilot, ECDIS, Radar, Gyro and ship loch, so calculating the ship drift due to the current in the transiting area.

Figure 2.9. LORAN-C receiver display

At receiver, determination of positioning hyperbole is made by measuring the time difference

between the pulses receiving moments and comparing the phase difference of the oscillation

pulses, which allows verification and cancellation of the interference caused by simultaneous

reception of signals on direct wave and the reflected wave.

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2.6. Determinarea punctului navei cu ajutorul sistemului hiperbolic de

navigaţie „LORAN – C”

LORAN – C charts

The LORAN chart is the representation in Mercator projection of the hyperbole families generated by the station pair, the reception considering on direct wave.

Figure 2.10. LORAN-C chart

Each family of hyperbole is drawn to a particular color, and every hyperbole have a distinctive code, corresponding to the stations pair that generated it and the calculated time difference. Sometimes, on LORAN charts can be founded tables with correction values for the reflected wave for each of the stations. According to chart scale, the hyperbole can be drawn for time differences of 20, 100 or 200 microseconds. The ship position is calculated by interpolation of the read hyperbolas, the optimum intersection of two hyperbolas as close to 90 degrees, and for three hyperbolas of 60 – 120 degrees.

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Figure 2.11. LORAN-Cchart: four hyperbole families from LORAN-C 7980 chain

Determination of ship position on LORAN-C chart through graphical interpolation of two

hyperbole families

On LORAN-C chart are choosing the first two hyperbole families, characterized by the encoding according to the emitting station. It is drawn the first family of hyperbolas.

Figure 2.12. LORAN-C chart: chosen of hyperbole families

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It is determinate the first family of hyperbole.

Figure 2.13. LORAN-C chart: determination of the first family of hyperbolas, from secondary

station X

It is determine the second family of hyperbolas.

Figure 2.14. LORAN-C chart: determination of the second family of hyperbolas, from

secondary station Y

It is plotting the medial lines for both families of hyperbole.

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Figure 2.15. LORAN-C chart: median lines for hyperbolas determined

by X and Y stations

At the two medial lines intersection is obtaining the ship position, calculated with LORAN-C hyperbolic system.

Figure 2.16. LORAN-C chart: ship position determined by two families of hyperbole

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Use of LORAN tables for the ship positioning

The LORAN tables for determination of ship position are made it for each stations pair and contain the coordinates of the intersection points of position hyperbolas with parallels and meridians, depending on the orientation of the line position. Considered hyperbolas are determined for time differences (T) of 10 microseconds, reception carried out on direct wave. By using the graphical method of ship position determination, the hyperbolas spherical arches in Mercator projection are replaced with corresponding rhumb segments, called LORAN lines. For this approximation does not influence the position accuracy, the LORAN tables are calculated as follows:

• In the area of the hyperbolas family of the stations pair, where the curvature is not large, LORAN tables give the coordinates of hyperbolas intersection points (from 10 to 10 microseconds) with parallels and meridians at 1 degree intervals;

• Closer to transmitting stations, where the hyperbolas curvature is accentuated, intersection points data are giving for latitude and longitude difference intervals (∆φ, ∆λ) by 15 or 30 minutes.

The ∆ values listed in the time difference column (T), represents latitude or longitude variation for a time difference of 1 microsecond. The ∆ value is expressed in hundreds of minutes of arch and serves to interpolation of hyperbole intersection point coordinates with the considered meridian or parallel, the interpolation is carried out according to the difference of time value read at the receiver for a nominate hyperbole (TG).

The coordinates difference ∆φ or ∆λ can be obtained also, with the relation:

(∆φ, ∆λ) = (TG – T) x ∆ (3.1)

Figure 2.17. LORAN tables

SL4-X

T 13430 13440 13450

LAT L ∆∆∆∆ L ∆∆∆∆ L ∆∆∆∆ LONG

33 54.3 N +15 33 55.8 N +15 250 E

33 49.4 N +16 33 51.0 N +16 260 E

SL4-Y

T 31870 31880 31890

LAT L ∆∆∆∆ L ∆∆∆∆ L ∆∆∆∆ LONG

340 N 25 10.4 E -18 25 08.6 E -18

330 N 25 33.5 E +20 25 31.5 E +20

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Calculus model for ship position determination with LORAN tables

Date: yy/mm/dd Time: h/min LT

Estimate position: LAT 300 40’ N LONG 250 40’ E

At LORAN-C receiver, connected to SL4 chain, are read the following:

� pair M – SL4 – X (Matratin): TG = 13435 µsec � pair M – SL4 – Y (Targaburun): TG = 31872 µsec

LORAN table:

Determination of the intersection points:

- pair M – SL4 – X TG = 13435

- T = 13430 (TG – T) = + 5 for LONG 1 = 250 E; LAT = 330 54’.3 N ∆ = + 15 +(0.15’ x 5) = 0’.8 LAT 1 = 330 55’.1 N for LONG 2 = 260 E; LAT = 330 49’.4 N ∆ = + 16 +(0.16’ x 5) = 0’.8

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LAT 2 = 330 50’.2 N LORAN table:

Determination of intersection points:

- pair M – SL4 – Y TG = 31872

- T = 31870 (TG – T) = + 2 for LAT 3 = 340 N; LONG = 250 10’.4 E ∆ = + 15 -(0.18’ x 2) = -0’.4 LONG 3 = 250 10’ E for LAT 4 = 330 N; LONG = 250 33’.5 E ∆ = + 16 -(0.2’ x 5) = -0’.4 LONG 4 = 250 33’.1 E There are obtained two rhumb segments (LORAN lines) defined as follows:

� M position line (LAT1, LONG1; LAT2, LONG2) � P position line (LAT3, LONG3; LAT4, LONG4)

The problem can be solved directly on LORAN navigation chart in Mercator projection for considered area, or by the process of building scale charts. The obtained LORAN-C point has the coordinates: LAT = 330 54’ N

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LONG = 250 12’ E

Figure 2.18. Ship position calculated with LORAN tables

2.7. LORAN position accuracy

Suprafaţa rombului erorilor determina de o eroare de măsurare este minimă dacă liniile de poziţie hiperbolice se intersectează perpendicular. Amplasarea staţiilor LORAN este astfel calculată încât familiile de hiperbole pe care le generează să se intersecteze sub unghiuri mai mari de 30 grade. Precizia punctului LORAN – C, în cazul recepţionării undei directe, are o toleranţă de măsurare de ± 0,5 microsecunde, deci o eroare totală de ± 2,5 microsecunde. În cazul unui unghi de intersecţie favorabil pentru hiperbolele de poziţie, precizia punctului este de ± 0,1 mile marine pentru o distanţă de până la 350 mile marine faţă de staţia principală şi de ± 0,3 mile marine la o distanţă de 750 mile marine. 2.8. LORAN-C closure The closure of the Loran- C system in the United States occured on February 8, 2010. This is the official statement issued by the United States Coast Guard. In accordance with the DHS Appropriations Act, the U.S. Coast Guard has terminated the transmission of all U.S. LORAN-C signals effective on 08 Feb 2010. At this time, the U.S. LORAN-C signal will be unusable and permanently discontinued. This termination does not affect U.S. participation in the Russian American or Canadian LORAN-C chains. U.S. participation in these chains will continue in accordance with

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international agreements. The Canadian Coast Guard has also issued a statement, which is shown on their website. The Coast Guard strongly urges mariners currently using LORAN-C for navigation to shift to a GPS navigation system and become familiar with its operation as soon as possible. The decision to cease transmission of the LORAN-C signal reflects the president's pledge to eliminate unnecessary federal programs.

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Chapter III

e-LORAN

3.1. Introduction

The modern system eLoran (Enhanced Loran – eLoran) is a standardized international service for 2D (two dimensions) positioning, navigation and time (Positioning, Navigation and Timing –

PNT), working on frequency of 100 kHz, for different transport segments and other civilian applications for positioning. eLoran represents an improved version of the Hyperbolic Navigation System LORAN-C and meets the necessary requirements of performance, accuracy, availability and integrity of information for:

- Air navigation during approach to landing; - Coastal navigation in areas with heavy traffic, during approch procedures

and port entrance maneuvres, in restricted visibility conditions; - Terrestrial navigation; - Terrestrial positioning; - Telecommunication and other domains (Internet, etc.) through providing

of UTC time differential signals, with precission of 50 nanoseconds. eLoran is an independent hyperbolic system, complementary to GNSS (Global Navigation

Sattelite Navigation System – GNSS). The system is in the development phase until 2020. The first eLoran station was built at Anthorn in Great Britain. This station transmit eLoran messages from January 2008. The transmitted eLoran messages are EUROFIX messages and contain differential LORAN information, differential GPS, integrity of data information and UTC referentials.

3.2. Avantages of the eLoran system

The main advantages of the eLoran system are: - Civilian control; - The eLoran signal is not intentionally degraded; - UTC syncronized transmission by a method independent of satellite

systems; - If eLoran emission source is synchronized with the same UTC time

source like satellite systems, the eLoran signal can be used in combination with satellite navigation signal;

- The eLoran signal can be received in areas where there is not satellite coverage;

- Transmission in real time (under 10 seconds) of a message about possible malfunctions or loss of signal integrity;

- Repeatable positioning accuracy is good;

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- In addition to the LORAN-C syste, eLoran signal has constituted a data channel that provides corrections, warnings about system status and information integrity;

- Implementation and maintenance costs are lower than for satellite systems;

- Can be used for providing of differential corrections to satellite systems. The eLoran system ensures safe and low-cost services for governmental and private institutions and users from aviation, maritime, etc., by:

- Ensuring flights in all its phases (takeoff, flight, approach and landing); - Providing information for eNavigation, including the use of permanent

and temporary means for ensuring of maritime navigation (Aids to Navigation – AtoNs), for marking of dangerous water areas;

- Identification of terrestrial transport units; - Maintainance and synchroniziation of telecommunications wired and

wireless.

3.3. The eLoran components

The eLoran system is compound of: - Control Centres; - Transmitting stations; - Monitoring points; - eLoran receivers.

The eLoran services are provided by a main centre through specialized applications. The main distributor ensures original and accurate eLoran information in accordance with operational specifications for LORAN-C signal. The specialized applications for aviation, maritime, etc., provide information for specified domain by eLoran data channel.

eLoran transmitting stations

The eLoran signal is transmitted automatically. The eLoran transmitters using modern SSX (Solid State Transmitter) equipped with neinterruptibile energy sources, and with time control and emission frequency control systems. Signal phase corrections are made continuously. The eLoran time is provided by high performance watches with cesium or other technology with the same level of accuracy. At anomalies detection in operation of an eLoran transmission station, this is indicated in a very short time, like for LORAN-C system, cautioning the user to not use the eLoran information until the problem is fixed.

The eLoran Control Centres

The eLoran Control Centre provides quick solutions for malfunctions and maintaining availability and continuity of the public announced signal performance level. System maintenance is so planned and conducted to minimalize the impact on transmitting stations activity. Also, from control centres are transmitted information to users about signal interruption through official known channels.

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Monitoring points. eLoran reference stations

The monitoring points have the role to provide signal integrity for all users of the eLoran system. Receivers installed in these points monitoring continuous the signal quality in responsibility area. Some of these stations are used as reference stations to generate eLoran messages. Also, monitoring stations will provide real time differential corrections for ship receivers and warnings for aviation.

3.4. eLoran receivers

The eLoran receiver provides acquisition and tracking of received messages from several stations to offer a most accurate positioning and a more accurate time service. Also, the eLoran receivers can ensure the correctness of each eLoran signal. The eLoran receiver receives and decoding eLoran messages using specific applications. An eLoran receiver determines its position (latitude and longitude) and UTC time by measuring receiving times of pulses from last three eLoran stations within visible range. Propagation of eLoran signals over different forms of relief produces deviations of receiving times, called secondary additionl factors (Aditional Secondary Factors – ASF), versus theorethical receiving times and, as result, the reduction of position accuracy. Anyway, using eLoran differential information, the position accuracy increasing and become very good, about 10 meters. An ASF error of 1 µs means an error of 300 meters distance of eLoran position. An eLoran chart contains the nominal ASF values of ASF factors for a particular area and transmitter. The ASF value in µs, depending on relief forms is as follows:

0.0 – sea surface

1.65 – clay soil

2.36 – swamp and flotting ice

4.94 – trees land

5.61 – snow and ice

6.12 – land

6.62 – sand (desert)

3.5. The content of the eLoran signal

The eLoran signal is complex and containing the following information: - Identification details of transmitting eLoran station; - The eLoran transmitters and monitoring points almanah; - Refferenced UTC time scale; - Time difference between eLoran time and UTC time; - eLoran signal; - Warnings about abnormal radio propagation due to meteorological

conditions; - identification message by eLoran users; - differential-satellite corrections.

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The eLoran signal for marine users have a design, evaluation, checking and updating circuit, starting from Main eLoran System and finishing at ship eLoran receivers through eLoran Marine

System. At eLoran receiver arrived both eLoran signal with eLoran data integrity signal and eLoran differential corrections. 3.6. eLoran applications

The eLoran system applications are based on minimum operational performance standards. For aviation, eLoran provides information for horizontally guidance, less altitude information, for all flight stages. For ocean navigation, the eLoran system will provide high accuracy positioning and time information, according to IMO Resolution A.953/23/2003, on global radionavigation system. These performance standards are applying for approaching and port entry, coastal navigation in congested waters and high-risk areas.

Applying the eLoran differential corrections in real time, ensure achievement of the required performance standards. For time information the eLoran system respects ITU Standard G.811/1997.

In addition, if an eLoran receiver has a suitable antenna can be used as eLoran compass, for measuring eLoran bearings to transmitting stations and, also, is possible to determine ship course with less than 1 degree accuracy, irrespective of ship position and movement.

It also envisages to couples the eLoran receivers with ECDIS and AIS devices. An eLoran system will provide for maritime navigation the following: a. increased safety – can be used, with high accuracy level, comparing with other

methods and navigation procedires, like back-up system for a satellite positioning system, if needed;

b. security – provides functionally of collision warning systems, during operation interruption of satellite systems or VTS systems;

c. better use of resources: - potentially reduction of collision and grounding events, and, as result,

reduction of oil pollution cases; - assist in monitoring of marine oil pollution; - potentially reduction of costs with navigational systems; - potentially increasing of voyage monitoring efficiency and port entry

maneuvres.

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Chapter IV

Global Positioning System (GPS) 4.1. Introduction. System structure

The Global Positioning System (GPS) is a space-based global navigation satellite system that provides reliable location and time information in all weather and at all times and anywhere on or near the Earth when and where there is an unobstructed line of sight to four or more GPS satellites. It is maintained by the United States government and is freely accessible by anyone with a GPS receiver. In addition to GPS other systems are in use or under development. The Russian GLObal NAvigation Satellite System (GLONASS) is for use by the Russian military. There are also the planned Chinese Compass navigation system and Galileo positioning system of the European Union (EU). GPS was created and realized by the U.S. Department of Defense (DOD) and was originally run with 24 satellites. It was established in 1973 to overcome the limitations of previous navigation systems. GPS consists of three parts: the space segment, the control segment, and the user segment. The U.S. Air Force develops, maintains, and operates the space and control segments. GPS satellites broadcast signals from space, which each GPS receiver uses to calculate its three-dimensional location (latitude, longitude, and altitude) plus the current time.[2] The space segment is composed of 24 to 32 satellites in medium Earth orbit and also includes the boosters required to launch them into orbit. The control segment is composed of a master control station, an alternate master control station, and a host of dedicated and shared ground antennas and monitor stations. The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial, and scientific users of the Standard Positioning Service (see GPS navigation devices). 4.2. Applications While originally a military project, GPS is considered a dual-use technology, meaning it has significant military and civilian applications. GPS has become a widely used and a useful tool for commerce, scientific uses, tracking and surveillance. GPS's accurate timing facilitates everyday activities such as banking, mobile phone operations, and even the control of power grids. Farmers, surveyors, geologists and countless others perform their work more efficiently, safely, economically, and accurately Many civilian applications use one or more of GPS's three basic components: absolute location, relative movement, and time transfer. • Surveying: Surveyors use absolute locations to make maps and determine property boundaries • Map-making: Both civilian and military cartographers use GPS extensively.

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• Navigation: Navigators value digitally precise velocity and orientation measurements. • Cellular telephony: Clock synchronization enables time transfer, which is critical for synchronizing its spreading codes with other base stations to facilitate inter-cell handoff and support hybrid GPS/cellular position detection for mobile emergency calls and other applications. The first handsets with integrated GPS launched in the late 1990s. • Tectonics: GPS enables direct fault motion measurement in earthquakes. • Disaster relief/emergency services: Depend upon GPS for location and timing capabilities • GPS tours: Location determines which content to display; for instance, information about an approaching point of interest is displayed. • Geofencing: Vehicle tracking systems, person tracking systems, and pet tracking systems use GPS to locate a vehicle, person, or pet. These devices attach to the vehicle, person, or the pet collar. The application provides 24/7 tracking and mobile or Internet updates should the trackee leave a designated area. • Recreation: For example, geocaching, geodashing, GPS drawing and waymarking • GPS Aircraft Tracking • Geotagging: Applying location coordinates to digital objects such as photographs and other documents for purposes such as creating map overlays. The U.S. Government controls the export of some civilian receivers. All GPS receivers capable of functioning above 18 kilometers altitude and 515 metres per second (1,001 kn) are classified as munitions (weapons) for which U.S. State Department export licenses are required. These limits attempt to prevent use of a receiver in a ballistic missile. They would not prevent use in a cruise missile since their altitudes and speeds are similar to those of ordinary aircraft. This rule applies even to otherwise purely civilian units that only receive the L1 frequency and the C/A code and cannot correct for SA, etc. Disabling operation above these limits exempts the receiver from classification as a munition. Vendor interpretations differ. The rule targets operation given the combination of altitude and speed, while some receivers stop operating even when stationary. This has caused problems with some amateur radio balloon launches, which regularly reach 30 kilometers (19 mi). As of 2009, military applications of GPS include: • Navigation: GPS allows soldiers to find objectives, even in the dark or in unfamiliar territory, and to coordinate troop and supply movement. In the US armed forces, commanders use the Commanders Digital Assistant and lower ranks use the Soldier Digital Assistant. • Target tracking: Various military weapons systems use GPS to track potential ground and air targets before flagging them as hostile. • Missile and projectile guidance: GPS allows accurate targeting of various military weapons • Search and Rescue: Downed pilots can be located faster if their position is known. • Reconnaissance: Patrol movement can be managed more closely. 4.3. History The design of GPS is based partly on similar ground-based radio navigation systems, such as LORAN and the Decca Navigator developed in the early 1940s, and used during World War II. In 1956 Friedwardt Winterberg[12] proposed a test of general relativity using accurate atomic clocks placed in orbit in artificial satellites. To achieve accuracy requirements, GPS uses principles of general relativity to correct the satellites' atomic clocks. Additional inspiration for GPS came when the Soviet Union launched the first man-made satellite, Sputnik in 1957. A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. They discovered that, because of the Doppler effect, the frequency of the signal being

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transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that since they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion The first satellite navigation system, Transit, used by the United States Navy, was first successfully tested in 1960. It used a constellation of five satellites and could provide a navigational fix approximately once per hour. In 1967, the U.S. Navy developed the Timation satellite that proved the ability to place accurate clocks in space, a technology that GPS relies upon. In the 1970s, the ground-based Omega Navigation System, based on phase comparison of signal transmission from pairs of stations, became the first worldwide radio navigation system. However, limitations of these systems drove the need for a more universal navigation solution with greater accuracy. While there were wide needs for accurate navigation in military and civilian sectors, almost none of those were seen as justification for the billions of dollars it would cost in research, development, deployment, and operation for a complex constellation of navigation satellites. However during the Cold War arms race, the nuclear threat to the very existence of the United States was the one need that did justify this cost in the view of the US Congress. This deterrent effect is why GPS was funded. The nuclear triad consisted of the US Navy's submarine-launched ballistic missiles (SLBMs) along with the US Air Force's strategic bombers and intercontinental ballistic missiles (ICBMs). Considered vital to the nuclear deterrence posture, accurate determination of the SLBM launch position was a force multiplier. Precise navigation would enable US submarines to get an accurate fix of their positions prior to launching their SLBMs. The US Air Force with two-thirds of the nuclear triad also had requirements for a more accurate and reliable navigation system. The Navy and Air Force were developing their own technologies in parallel to solve what was essentially the same problem. To increase the survivability of ICBMs, there was a proposal to use mobile launch platforms so the need to fix the launch position had similarity to the SLBM situation. In 1960, the Air Force proposed a radio-navigation system called MOSAIC (Mobile System for Accurate ICBM Control) that was essentially a 3-D LORAN. A follow-on study called Project 57 was worked in 1963 and it was "in this study that the GPS concept was born." That same year the concept was pursued as Project 621B, which had "many of the attributes that you now see in GPS" and promised increased accuracy for Air Force bombers as well as ICBMs. Updates from the Navy Transit system were too slow for the high speeds that the Air Force operated at. The Navy Research Laboratory continued advancements with their Timation (Time Navigation) satellites, first launched in 1967, and with the third one in 1974 carrying the first atomic clock put into orbit. With these parallel developments out of the 1960s, it was realized that a superior system could be developed by synthesizing the best technologies from 621B, Transit, Timation and SECOR in a multi-service program. Over the Labor Day weekend in 1973, a meeting of about 12 military officers at the Pentagon discussed the creation of a Defense Navigation Satellite System (DNSS). It was at this meeting that "the real synthesis that became GPS was created." Later that year, the DNSS program was named Navstar. With the individual satellites being associated with the name Navstar (as with the predecessors Transit and Timation), a more fully encompassing name was used to identify the constellation of Navstar satellites. This more complete name was Navstar-GPS, which was later shortened simply to GPS.

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4.4. Timeline and modernization

Summary of satellites

Block Launch Period

Satellite launches Currently in orbit and healthy Success Failure

In preparation

Planned

I 1978–1985 10 1 0 0 0

II 1989–1990 9 0 0 0 0

IIA 1990–1997 19 0 0 0 10 of 19 launched

IIR 1997–2004 12 1 0 0 12 of 13 launched

IIR-M 2005–2009 8 0 0 0 7 of 8 launched

IIF 2010–2011 1 0 11 0 1 of 1 launched

IIIA 2014–? 0 0 0 12 0

IIIB 0 0 0 8 0

IIIC 0 0 0 16 0

Total 59 2 11 36 30

4.5. Basic concept of GPS A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the Earth. Each satellite continually transmits messages that include • the time the message was transmitted • precise orbital information (the ephemeris) • the general system health and rough orbits of all GPS satellites (the almanac). The receiver utilizes the messages it receives to determine the transit time of each message and computes the distances to each satellite. These distances along with the satellites' locations are used with the possible aid of trilateration, depending on which algorithm is used, to compute the position of the receiver. This position is then displayed, perhaps with a moving map display or latitude and longitude; elevation information may be included. Many GPS units show derived information such as direction and speed, calculated from position changes. Three satellites might seem enough to solve for position, since space has three dimensions and a position near the Earth's surface can be assumed. However, even a very small clock error multiplied by the very large speed of light — the speed at which satellite signals propagate — results in a large positional error. Therefore receivers use four or more satellites to solve for the receiver's location and time. The very accurately computed time is effectively hidden by most GPS applications, which use only the location. A few specialized GPS applications do however

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use the time; these include time transfer, traffic signal timing, and synchronization of cell phone base stations. Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known, a receiver can determine its position using only three satellites. (For example, a ship or plane may have known elevation.) Some GPS receivers may use additional clues or assumptions (such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give a less accurate (degraded) position when fewer than four satellites are visible. 4.6. Position calculation To provide an introductory description of how a GPS receiver works, errors will be ignored in this section. Using messages received from a minimum of four visible satellites, a GPS receiver is able to determine the times sent and then the satellite positions corresponding to these times sent. The x, y, and z components of position, and the time sent, are designated as where the subscript i is the satellite number and has the value 1, 2, 3, or 4. Knowing the indicated time the message was received , the GPS receiver can compute the transit time of the message as . Assuming the message traveled at the speed of light, c, the distance traveled or pseudorange, can be computed as . A satellite's position and pseudorange define a sphere, centered on the satellite with radius equal to the pseudorange. The position of the receiver is somewhere on the surface of this sphere. Thus with four satellites, the indicated position of the GPS receiver is at or near the intersection of the surfaces of four spheres. In the ideal case of no errors, the GPS receiver would be at a precise intersection of the four surfaces. The intersection of a third spherical surface with the first two will be its intersection with that circle; in most cases of practical interest, this means they intersect at two points. Another figure, Surface of Sphere Intersecting a Circle (not a solid disk) at Two Points, illustrates the intersection. The two intersections are marked with dots. Again the article trilateration clearly shows this mathematically. For automobiles and other near-earth-vehicles, the correct position of the GPS receiver is the intersection closest to the Earth's surface. For space vehicles, the intersection farthest from Earth may be the correct one. The correct position for the GPS receiver is also the intersection closest to the surface of the sphere corresponding to the fourth satellite. 4.7. Correcting a GPS receiver's clock The method of calculating position for the case of no errors has been explained. One of the most significant error sources is the GPS receiver's clock. Because of the very large value of the speed of light, c, the estimated distances from the GPS receiver to the satellites, the pseudoranges, are very sensitive to errors in the GPS receiver clock. This suggests that an extremely accurate and expensive clock is required for the GPS receiver to work. On the other hand, manufacturers prefer to build inexpensive GPS receivers for mass markets. The solution for this dilemma is based on the way sphere surfaces intersect in the GPS problem. It is likely that the surfaces of the three spheres intersect, since the circle of intersection of the first two spheres is normally quite large, and thus the third sphere surface is likely to intersect this large circle. It is very unlikely that the surface of the sphere corresponding to the fourth satellite will intersect either of the two points of intersection of the first three, since any clock

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error could cause it to miss intersecting a point. However, the distance from the valid estimate of GPS receiver position to the surface of the sphere corresponding to the fourth satellite can be used to compute a clock correction. Let denote the distance from the valid estimate of GPS receiver position to the fourth satellite and let denote the pseudorange of the fourth satellite. Let . is the distance from the computed GPS receiver position to the surface of the sphere corresponding to the fourth satellite. Thus the quotient, , provides an estimate of (correct time) − (time indicated by the receiver's on-board clock), and the GPS receiver clock can be advanced if is positive or delayed if is negative. However, it should be kept in mind that a less simple function of may be needed to estimate the time error in an iterative algorithm as discussed in the Navigation section. 4.8. System segmentation The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US). The Space segment

Figure 4.1. Space Segment

A visual example of the GPS constellation in motion with the Earth rotating. Notice how the number of satellites in view from a given point on the Earth's surface, in this example at 45°N, changes with time. The space segment (SS) is composed of the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design originally called for 24 SVs, eight each in three circular orbital planes, but this was modified to six planes with four satellites each. The orbital planes are centered on the Earth, not rotating with respect to the distant stars. The six planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection). The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface. Orbiting at an altitude of approximately 20,200 kilometers (about 12,550 miles or 10,900 nautical miles; orbital radius of approximately 26,600 km (about 16,500 mi or 14,400 NM)),

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each SV makes two complete orbits each sidereal day, repeating the same ground track each day. This was very helpful during development, since even with just four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones. As of March 2008, there are 31 actively broadcasting satellites in the GPS constellation, and two older, retired from active service satellites kept in the constellation as orbital spares. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail. About eight satellites are visible from any point on the ground at any one time (see animation at right). The Control segment The control segment is composed of 1. a master control station (MCS), 2. an alternate master control station, 3. four dedicated ground antennas and 4. six dedicated monitor stations. The MCS can also access U.S. Air Force Satellite Control Network (AFSCN) ground antennas (for additional command and control capability) and NGA (National Geospatial-Intelligence Agency) monitor stations. The flight paths of the satellites are tracked by dedicated U.S. Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs, Colorado and Cape Canaveral, along with shared NGA monitor stations operated in England, Argentina, Ecuador, Bahrain, Australia and Washington DC. The tracking information is sent to the Air Force Space Command's MCS at Schriever Air Force Base 25 km (16 miles) ESE of Colorado Springs, which is operated by the 2nd Space Operations Squadron (2 SOPS) of the United States Air Force (USAF). Then 2 SOPS contacts each GPS satellite regularly with a navigational update using dedicated or shared (AFSCN) ground antennas (GPS dedicated ground antennas are located at Kwajalein, Ascension Island, Diego Garcia, and Cape Canaveral). These updates synchronize the atomic clocks on board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of each satellite's internal orbital model. The updates are created by a Kalman filter, which uses inputs from the ground monitoring stations, space weather information, and various other inputs. Satellite maneuvers are not precise by GPS standards. So to change the orbit of a satellite, the satellite must be marked unhealthy, so receivers will not use it in their calculation. Then the maneuver can be carried out, and the resulting orbit tracked from the ground. Then the new ephemeris is uploaded and the satellite marked healthy again. The User segment The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial and scientific users of the Standard Positioning Service. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly stable clock (often a crystal oscillator). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2007, receivers typically have between 12 and 20 channels. GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of an RS-232 port at 4,800 bit/s speed. Data is actually sent at a

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much lower rate, which limits the accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. As of 2006, even low-cost units commonly include Wide Area Augmentation System (WAAS) receivers. Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol, or the newer and less widely used NMEA 2000. Although these protocols are officially defined by the National Marine Electronics Association (NMEA), references to these protocols have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols exist as well, such as the SiRF and MTK protocols. Receivers can interface with other devices using methods including a serial connection, USB, or Bluetooth. 4.9. Communication The navigational signals transmitted by GPS satellites encode a variety of information including satellite positions, the state of the internal clocks, and the health of the network. These signals are transmitted on two separate carrier frequencies that are common to all satellites in the network. Each GPS satellite continuously broadcasts a navigation message at a rate of 50 bits per second (see bitrate). Each complete message is composed of 30-second frames, distinct groupings of 1,500 bits of information. Each frame is further subdivided into 5 subframes of length 6 seconds and with 300 bits each. Each subframe contains 10 words of 30 bits with length 0.6 seconds each. Each 30 second frame begins precisely on the minute or half minute as indicated by the atomic clock on each satellite. The first part of the message encodes the week number and the time within the week, as well as the data about the health of the satellite. The second part of the message, the ephemeris, provides the precise orbit for the satellite. The last part of the message, the almanac, contains coarse orbit and status information for all satellites in the network as well as data related to error correction. All satellites broadcast at the same frequencies. Signals are encoded using code division multiple access (CDMA) allowing messages from individual satellites to be distinguished from each other based on unique encodings for each satellite (which the receiver must be aware of). Two distinct types of CDMA encodings are used: the coarse/acquisition (C/A) code, which is accessible by the general public, and the precise (P) code, that is encrypted so that only the U.S. military can access it. The ephemeris is updated every 2 hours and is generally valid for 4 hours, with provisions for updates every 6 hours or longer in non-nominal conditions. The almanac is updated typically every 24 hours. Additionally data for a few weeks following is uploaded in case of transmission updates that delay data upload. All satellites broadcast at the same two frequencies, 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal). The satellite network uses a CDMA spread-spectrum technique where the low-bitrate message data is encoded with a high-rate pseudo-random (PRN) sequence that is different for each satellite. The receiver must be aware of the PRN codes for each satellite to reconstruct the actual message data. The C/A code, for civilian use, transmits data at 1.023 million chips per second, whereas the P code, for U.S. military use, transmits at 10.23 million chips per second. The L1 carrier is modulated by both the C/A and P codes, while the L2 carrier is only modulated by the P code.

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4.10. Error sources and analysis Geometric Error Diagram Showing Typical Relation of Indicated Receiver Position, Intersection of Sphere Surfaces, and True Receiver Position in Terms of Pseudorange Errors, PDOP, and Numerical Errors The term user equivalent range error (UERE) refers to the error of a component in the distance from receiver to a satellite. User equivalent range errors (UERE) are shown in the table. There is also a numerical error with an estimated value of about 1 meter. The standard deviations for the coarse/acquisition and precise codes are also shown in the table. These standard deviations are computed by taking the square root of the sum of the squares of the individual components (i.e., RSS for root sum squares). To get the standard deviation of receiver position estimate, these range errors must be multiplied by the appropriate dilution of precision terms and then RSS'ed with the numerical error. Electronics errors are one of several accuracy-degrading effects outlined in the table above. When taken together, autonomous civilian GPS horizontal position fixes are typically accurate to about 15 meters (49 ft). These effects also reduce the more precise P(Y) code's accuracy. However, the advancement of technology means that today, civilian GPS fixes under a clear view of the sky are on average accurate to about 5 meters (16 ft) horizontally. These UERE errors are given as ± errors thereby implying that they are unbiased or zero mean errors. These UERE errors are therefore used in computing standard deviations. The standard deviation of the error in receiver position is computed by multiplying PDOP (Position Dilution Of Precision) by the standard deviation of the user equivalent range errors is computed by taking the square root of the sum of the squares of the individual component standard deviations. PDOP is computed as a function of receiver and satellite positions. A detailed description of how to calculate PDOP is given in the section, geometric dilution of precision computation (GDOP).

Atmospheric effects Atmospheric inconsistencies affect the speed of the GPS signals as they pass through the Earth's atmosphere, especially the ionosphere. Correcting these errors is a significant challenge to improving position accuracy. These effects are smallest for overhead satellites and greatest for satellites at the horizon since the path through the atmosphere is longer (see airmass). Once the receiver's approximate location is known, a mathematical model can estimate and compensate for these errors. Ionospheric microwave signal delay depends on its frequency. This phenomenon is known as dispersion and can be calculated from measurements of delays for two or more frequency bands, allowing delays at other frequencies to be estimated.[68] Some military and survey-grade civilian receivers calculate atmospheric dispersion from the different delays in the L1 and L2 frequencies, and apply a more precise correction. This can be done in civilian receivers without decrypting the P(Y) signal carried on L2, by tracking the carrier wave instead of the modulated code. To facilitate this on lower cost receivers, a new civilian code signal on L2, called L2C, was added to the Block IIR-M satellites, first launched in 2005. It allows a direct comparison of the L1 and L2 signals using the coded signal instead of the carrier wave. Ionospheric effects generally change slowly, and can be averaged over time. Those for any particular geographical area can be easily calculated by comparing the measured position to a known surveyed location. This correction is also valid for other receivers in the same general location. Several systems send this information over radio or other links to allow L1-only receivers to make ionospheric corrections. The ionospheric data are transmitted via satellite in Satellite Based Augmentation Systems (SBAS) such as WAAS (available in North America and Hawaii), EGNOS (Europe and Asia) or MSAS (Japan), which transmits it on the GPS frequency

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using a special pseudo-random noise sequence (PRN), so only one receiver and antenna are required. Humidity also causes a variable delay, resulting in errors similar to ionospheric delay, but occurring in the troposphere. This effect is both more localized and changes more quickly than ionospheric effects, and is not frequency dependent. These traits make precise measurement and compensation of humidity errors more difficult than ionospheric effects. Changes in receiver altitude also change the delay, due to the signal passing through less of the atmosphere at higher elevations. Since the receiver computes its approximate altitude, this error is relatively simple to correct, either by applying a function regression or correlating margin of atmospheric error to ambient pressure using a barometric altimeter.

Multipath effects GPS signals can be affected by multipath issues, where the radio signals reflect off surrounding terrain; buildings, canyon walls, hard ground, etc. These delayed signals can cause inaccuracy. A variety of techniques, most notably narrow correlator spacing, mitigate multipath errors. For long delay multipath, the receiver itself can recognize the wayward signal and discard it. To address shorter delay multipath from the signal reflecting off the ground, specialized antennae (e.g., a choke ring antenna) may be used to reduce the signal power as received by the antenna. Short delay reflections are harder to filter out because they interfere with the true signal, causing effects almost indistinguishable from routine fluctuations in atmospheric delay. Multipath effects are much less severe in moving vehicles. When the antenna is moving, false solutions using reflected signals quickly fail to converge and only the direct signals result in stable solutions. Ephemeris and clock errors While ephemeris data is transmitted every 30 seconds, it may be up to two hours old. If a fast time to first fix (TTFF) is needed, it is possible to upload a valid ephemeris to a receiver, and in addition to setting the time, obtain a position fix in under ten seconds. It is feasible to put such ephemeris data on the web for use in mobile GPS devices. The satellite's atomic clocks experience noise and clock drift errors. The navigation message contains corrections for these errors and estimates of the accuracy of the atomic clock. However, they are based on observations and may not indicate the clock's current state. These problems tend to be very small, but may add up to a few meters (tens of feet) of inaccuracy. For very precise positioning (e.g., in geodesy), these effects can be eliminated by differential GPS: the simultaneous use of two or more receivers at several survey points. In the 1990s when receivers were quite expensive, some methods of quasi-differential GPS were developed, using only one receiver with reoccupation of measuring points. At the TU Vienna the method was named qGPS and adequate post processing software was developed. Geometric dilution of precision computation (GDOP) When visible GPS satellites are adjacent in the sky (i.e., small angular separation), the DOP values are high; when far apart, the DOP values are low. Low DOP values represent a better GPS positional accuracy due to the wider angular separation. HDOP, VDOP, PDOP and TDOP are respectively Horizontal, Vertical, Position (3-D) and Time Dilution of Precision. The horizontal dilution of precision, and the vertical dilution of precision, are both dependent on the coordinate system used. To correspond to the local horizon plane and the local vertical, x, y, and z should denote positions in either a North, East, Down coordinate system or a South, East, Up coordinate system.

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Selective availability GPS includes a (currently disabled) feature called Selective Availability (SA) that adds intentional, time varying errors of up to 100 meters (328 ft) to the publicly available navigation signals. This was intended to deny an enemy the use of civilian GPS receivers for precision weapon guidance. SA errors are actually pseudorandom, generated by a cryptographic algorithm from a classified seed key available only to authorized users (the U.S. military, its allies and a few other users, mostly government) with a special military GPS receiver. Mere possession of the receiver is insufficient; it still needs the tightly controlled daily key. Because SA affects every GPS receiver in a given area almost equally, a fixed station with an accurately known position can measure the SA error values and transmit them to the local GPS receivers so they may correct their position fixes. This is called Differential GPS or DGPS. DGPS also corrects for several other important sources of GPS errors, particularly ionospheric delay, so it continues to be widely used even though SA has been turned off. Typical SA errors were about 50 meters (164 ft) horizontally and about 100 m vertically. Widespread availability of DGPS nullified SA, leading to its demise on May 1, 2000. DGPS services are widely available from both commercial and government sources. The latter include WAAS and the U.S. Coast Guard's network of LF marine navigation beacons. The accuracy of the corrections depends on the distance between the user and the DGPS receiver. As distance increases, the errors at the two sites will not correlate as well, resulting in less precise differential corrections. Per the directive, the induced error of SA was changed to add no error to the public signals (C/A code). Clinton's executive order required SA to be set to zero by 2006; it happened in 2000 once the U.S. military developed a new way to deny GPS (and other navigation services) to hostile forces in a specific area without affecting the rest of the world or its own military systems. One interesting side effect of the Selective Availability hardware is the capability to add corrections to the outgoing signal of the GPS cesium and rubidium atomic clocks to an accuracy of approximately 2 × 10−13. This represented a significant improvement over the clocks' raw accuracy.

Antispoofing Another restriction on GPS, antispoofing, remains on. This encrypts the P-code so that it cannot be mimicked by a transmitter sending false information. Few civilian receivers have ever used the P-code, and the accuracy attainable with the public C/A code is so much better than originally expected (especially with DGPS) that the antispoof policy has relatively little effect on most civilian users. Turning off antispoof would primarily benefit surveyors and some scientists who need extremely precise positions for experiments such as tracking tectonic plate motion. Natural sources of interference Since terrestrial GPS signals tend to be relatively weak, natural radio signals or scattering can desensitize the receiver, making acquiring and tracking satellite signals difficult or impossible. Space weather degrades GPS operation in two ways, direct interference by solar radio burst noise in the same frequency band or by scattering of the GPS radio signal in ionospheric irregularities referred to as scintillation. Both forms of degradation follow the 11 year solar cycle and peak at sunspot maximum although they can occur anytime. Solar radio bursts are associated with solar flares and coronal mass ejections (CMEs) and their impact can affect reception over the half of the Earth facing the sun. Scintillation occurs most frequently at tropical latitudes at night. It occurs less frequently at high latitudes or mid-latitudes where magnetic storms can lead to

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scintillation. In addition to scintillation, magnetic storms can produce strong ionospheric gradients that degrade SBAS accuracy. Artificial sources of interference In automotive GPS receivers, metallic features in windshields, such as defrosters or car window tinting films, can act as a Faraday cage, degrading reception inside the car. Man-made electromagnetic interference (EMI) can also disrupt or jam GPS signals. In one well-documented case it was impossible to receive GPS signals in the entire harbor of Moss Landing, California due to unintentional jamming caused by malfunctioning TV antenna preamplifiers. Intentional jamming is also possible. Generally, stronger signals can interfere with GPS receivers when they are within radio range or line of sight. Some countries allow GPS repeaters, to facilitate the reception of GPS signals indoors and in obscured locations; however, under European Union and U.K. laws, these are prohibited because the signals can interfere with other GPS receivers that receive data from both satellites and the repeater. Various techniques can address interference. One is to not rely on GPS as a sole source. According to John Ruley, "IFR pilots should have a fallback plan in case of a GPS malfunction". Receiver Autonomous Integrity Monitoring (RAIM) is included in some receivers, to warn if jamming or another problem is detected. The U.S. military has also deployed since 2004 their Selective Availability / Anti-Spoofing Module (SAASM) in the Defense Advanced GPS Receiver (DAGR). DAGR detects jamming and maintains its lock on encrypted GPS signals during interference. 4.11. Timekeeping While most clocks are synchronized to Coordinated Universal Time (UTC), the atomic clocks on the satellites are set to GPS time. The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections that are periodically added to UTC. GPS time was set to match Coordinated Universal Time (UTC) in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset with International Atomic Time (TAI) (TAI - GPS = 19 seconds). Periodic corrections are performed on the on-board clocks to correct relativistic effects and keep them synchronized with ground clocks. The GPS navigation message includes the difference between GPS time and UTC, which as of 2009 is 15 seconds due to the leap second added to UTC December 31, 2008. Receivers subtract this offset from GPS time to calculate UTC and specific timezone values. New GPS units may not show the correct UTC time until after receiving the UTC offset message. The GPS-UTC offset field can accommodate 255 leap seconds (eight bits) which, given the current rate of change of the Earth's rotation (with one leap second introduced approximately every 18 months), should be sufficient to last until approximately the year 2300. As opposed to the year, month, and day format of the Gregorian calendar, the GPS date is expressed as a week number and a seconds-into-week number. The week number is transmitted as a ten-bit field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on January 6, 1980, and the week number became zero again for the first time at 23:59:47 UTC on August 21, 1999 (00:00:19 TAI on August 22, 1999). To determine the current Gregorian date, a GPS receiver must be provided with the approximate date (to within 3,584 days) to correctly translate the GPS date signal. To address this concern the modernized GPS navigation message uses a 13-

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bit field, which only repeats every 8,192 weeks (157 years), thus lasting until the year 2137 (157 years after GPS week zero). 4.12. GPS receivers Basically, from the beginning of this century, the GPS receivers can be met onboard of all types of ships and crafts, becoming a common navigation device. Obviously, every model and type equipment has its own characteristics, data displaying mode and user algorithm. In principle, however, all GPS receivers designed for maritime navigation provides a minimum of common facilities, primarly to provide accurate positioning information to the seafarer in order to be able to conduct the ship on desired route. Next will be presented main types of information provide by every GPS receiver, the graphical and alphanumerical formats for information displaying. Essential is that used terminology for labelling of presented data is the same for all equipments, and basic idea for presenting information is the use of “windows”. Therefore, using a minimum keyboard (generally 10 numeric keys and other 4-8 function keys, including “arrows” keys to older receivers and 5-8 keys to newer receivers) can be selected and enabled all functions available to the user.

Taking into account that all GPS receivers perform the same tasks, the differences between

models consists in information display and keys configuration, every deck officer must read the

user manual of the equipment to learn how to operate the GPS receiver that working at a

moment..

Figure 4.2. Different types of GPS receivers

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4.13. G.P.S. terminology The terminology used (in English) and the means of abbreviations used for data identification are partly presented below.

• BRG – Bearing

• CMG – Course Made Good

• COG – Course Over Ground

• CTE – Course Track Error

• CTS – Course to steer

• DTK – Desired Track

• ETA – Estimated Time of Arrival

• ETE – Estimated Time Enroute

• HDG – Heading

• OCE – Off Course Error

• SOG – Ground Speed

• ROUTE – Route

• SPD – Speed

• TRK – Track (COG/CMG)

• VMG – Velocity Made Good

• WP – Way Point

• XTE – Cross Track Error

The data regarding directions can be grouped in two sections, as follows:

• The real course of the ship (CMG, TRK), respectively the course over ground; • Directions derived from the fact that the GPS receiver has stored a particular route

(ROUTE, DTK), defined by the waypoints (WP). In this way, the actual ship position is compared with the route and function of this is established the lateral deviation (CTE, XTE), and, related to the next waypoint is indicated the ship course to be taken (BRG, HDG) to reach that waypoint. 4.14. Data windows The GPS receiver provides the following interest windows for seafarer:

• Satellites; • Ship position; • Navigation (ship conduct); • Ship route; • Route control; • Man Over Board; • Electronic navigation chart.

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The satellites window

Figure 4.3. The satellite information section

The vertical bars represent the received satellites, the height of which is equivalent with the signal strength. The satellites arrangement on sky is schematized by two concentrical circles (the horizon and 45 degree height circle). Missed sattelites are underlined.

Figure 4.4. The satellite information section

Depending on the number of receive satellites and consider in determination of ship position, there are displayed data regarding the position accuracy. The standard situations are:

• 2 D precision – ship position is calculated by three satelittes; • 2 D –D precision – ship position is calculated by three satellites in DGPS system • 3 D precision – ship position is calculated by five satellites minimum • 3 D-D precision – ship position is calculated by five satellites minimum inDGPS system

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Ship position window The data provided in this window represent the essential information for any navigation hyperbolic equipment and practically, the primarly element of interest for the operator The purpose of this type of display is to concentrate all information about ship position (geographical coordinates), and ship real movement (course over ground and speed over ground), even if the same data appears together or independently in other sections (windows). Note that the graphic representation of the ship course (graphic compass) is not a gyro repeater, but the displayed value corresponding to the actual hip heading, shall indicate the value of the course over ground. The format used for time display can be chosen by user, as 12/24 hours format, or as local time (LT), GMT respectively.

Figure 4.5. Ship position section

In the User Area can be displayed different data, on operator request, data considered as digital loch, like:

• TRIP represent the distance travelled by ship from the last reset of distance counter; • ELPSD represent total time elapsed from the last reset of the timer; • TTIME represent the time when the speed over ground has not changed; • AVSPD represent the average speed from the last reset of the loch; • MXSPD represent the maximum speed reached since the last reset of the loch.

Figure 4.6. Displaying of ship position on GPS screen

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Navigation window

Navigation window is the section where are displayed information on ship piloting, namely data comparing the current ship position with the ship’s route, information that are often more important than data from ship position window. From a practical point of view, depending on the characteristic of navigation area, the GPS will be set to display one of the mentioned windows, in the situation when the GPS receiver have not the facility to display electronic navigation charts in Vectorial format. During open sea navigation is preferred the displaying of position window permanently, so that the navigator to have all the time access to the necessary data for ship plotting. During near coast navigation, or in the areas with heavy traffic or navigation dangers, to follow exactly the ship planned route is essential. As a result, the seafarer will want to have permanently displayed GPS data regarding the ship deviation from the planned route. For the navigation page to work is necessary the GPS receiver to have stored the route to be followed by ship, or, at least, a single waypoint defined as immediately destination. In the navigation window we have the possibility to see which is the own ship position in relation with the route wants to be follow and what we have to do for our ship to keep as closer to the planned route. At less sophisticated GPS equipments, the navigation window presentation form is limited to the segment between the ship current position and the first waypoint on route. The graphical presentation of this route segment, that follows to be travelled by our ship, seems like a road, has made this displaying option to be named „Highway”.

Usually, on the middle of the “Highway” is drawn a line connecting the ship position with the next waypoint and at the same time representing the bearing (BRG) at that waypoint (WP). From the seafarer point of view, the interpretation of the image is very eloquent.

Figure 4.7. Information display in navigation section of the GPS receiver If the bearing line to waypoint WP (BRG) appears vertical, then the ship is on planned route, or, we can say, that the ship have not deviation from active segment of the route (LEG) stored in the GPS memory. If this line is oblique, the ship is diverted laterally from the desired track (DTK). The graphical representation symbolizes the direction where the ship must to alter the course to return on the desired track. The same indication is given by the arrow direction (pointer) placed under the „Highway”. The value of this course alteration to bring the ship back on the planned route can be determined in several ways, as follows:

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• The value of the course over ground to the next waypoint, from the current position, is

given by the bearing value to waypoint (BRG); • The difference between the bearing (BRG) and the course over ground (COG), track

(TRK), indicates the number of degrees for changing of the actual course (to stabord

side if BRG > COG or, to port side if BRG < COG) to reach the next waypoint; this

value can be displayed in the User Area, if the seafarer request the TRN (turn) value

display; • If we want to get back faster to the planned route, will made an ample course changing

than the value of the difference between the bearing and the course over ground (BRG –

COG).

Figure 4.8. Displaying of the “Highway” and data regarding ship movement

GPS specifically data window Have to note that all GPS indications regarding to the ship course value are expressed as real courses (course over ground), which will be corrected for drift to determine the value of the course to be followed. This calculus is not necessary, if, after the course changing, will be followed the GPS indications. In this way, will be ordered to the helmsman a course changing to the desired direction and the course followed will be corrected than the value indicated by the GPS for course over ground (COG) to be the same with the bearing value (BRG).

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Figure 4.9. Ship piloting section If we want to return faster to the planned route, we will execute an ample change in direction of pointer and will keep the new course until the bearing line will become vertical (XTE = 0). Then we will take a course direction equal with the bearing line (BRG), or, in case of drifting, a course to lead to a course over ground (COG) with the same value as indicated for the bearing (BRG). At seafarer request, in the User Area, can be displayed additional data, such as:

• Estimated time enroute (ETE); • Time to next waypoint or estimated time of arrival at the waypoint (ETA), this

calculation is performed based on the actual ship speed (SPD, VMG); • Course alteration value and direction to return on route in the designated waypoint

The graphical scale, shown by some GPS equipments in the bottom of the “Highway”, is espressed in nautical miles and serves to indicate to the seafarer the approximate track error value from desired route. Some of the GPS receivers express this track error (CTE, XTE) under alphanumeric format also, as distance, or distance and direction. Performing GPS receivers have the possibility to present a large part of the route and will display a “Highway” configuration containing several waypoints to be achieved. In addition to the graphic image of the “Highway”, the next waypoint can be displayed with a number or with the nomination give it by the User. Also, at some devices, each waypoint can be assigned with a specific symbol (chosen from a library for graphic symbols), so that waypoint can be identified more easily. Ship route window

At presentation of the piloting window have stated that information displayed in this window is not available if there is not defined any waypoint, as next destination. Generally, the classical method for waypoints nomination is in direct connection with the paper chart. On this, the navigation officer drew the routes to be performed from the departure to the arrival ports. Current practice is the waypoints to be the same like that ones from the paper chart. In most of the cases, the route defining only by these waypoints is enough. The navigation officer will fill a table where will note the data determined from the paper chart, a model table is presented below.

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WPs list for route: Madison - Bahia

No. WP name Call sign Chart no. Latitude Longitude Course

1 York Head YKHD 1599 50 20.4 N 070 22.8 E 050

2 BD racon RACON 1728 50 23.5 N 077 17.3 E 080

3 Kimley Lt. H. KIMLEY 1906 50 25.3 N 085 36.1 E 120

4 Bella Bank BELBK 1906 50 02.7 N 086 44.7 E 100

Table 4.1. Model of table for a route WPs registering (data as example only)

To records the waypoints in the GPS memory are enough only data regarding the geographical

coordinates of the waypoint and a number. GPS receivers can stored between 100 and 3000 waypoints, so the counting is impossible to start every time from 1, only when deleting from the GPS memory the waypoints already registered on that positions. Waypoints nomination is not compulsory, but when the GPS receiver have this facility is better

to be used, for a more easily identification during reading of the waypoints list. Generally, all GPS receivers provide this facility to nominate the waypoints. But, in many cases, the number of the characters possible to be used is limited (5 to 10 characters), when is necessary the officer to find a solution. At the end of the operation of filling the table with waypoints details can go further to the introduction of these values in the GPS memory, typing the coordinates.

Figure 4.10. Creation of a new WP based on geographical coordinates and its nominatios as

„DAY 2” Some of the GPS receivers have a library of symbols, that can be used for attaching an icon to a waypoint. After all waypoints were introduced, the operator can proceed to the next step, to define the ship route. Like for the waypoints, the number of the routes that can be stored in the GPS receiver memory is limited by the hardware capacity of the device. Generally can be stored simultaneously between 2 to 10 routes, including the route will become active for that voyage. A route construction is a simple action, to select from the waypoints list those that are necessary for the ship voyage.

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Figure 4.11. List of stored routes in the GPS memory

Be aware! The used waypoints for a route definition have to be selected according to their reach by the ship. Punctele de schimbare de drum utilizate trebuie selectate în ordinea parcurgerii lor de către navă. The created route has named and should be saved in the GPS receiver memory. To modify an already created route is possible to be made through one of the following procedures:

• Deleting of one or more waypoints from the route list; • Introduction of new waypoints, already stored in the GPS memory, but inside of other

routes; • Defining of new waypoints,storing of them in the GPS receiver memory, and their

selection for introduction in the waypoint list of the new edited route.

Figure 4.12. Editing of WPs from route list

The GPS receivers provide also the facility of reversibility of the route. This means that the ship have to travel from Constanta to Istanbul and back to Constanta, will be not necessary to create a new route in the GPS memory, but will be called a reverse route of the initial one, the GPS receiver processor will reverse the order of the waypoints, that are already defined in the list of the initial route. When a route is reversed, the original route is not modified, the reverse route is saved and stored as a new route.

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Figure 4.13. The process of reverting of the original route

To simplifying the work of the navigation officer regarding the ship routes creation and also, to be able to use this work later, it is recommended that for longer routes (longer than 48 to 72 hours) to be defined route sections. For example, taking account the particularities of a route between Constanta and a Mediterranean Sea port, the route sections can be defined as follows: � Constanta – Bosforus North bound � Bosforus South bound – East Dardanelles � West Dardanelless – Rhodos (routes to East Mediterranean) � West Dardanelless – Kavo Maleas (roputes to West Mediterranean) The GPS receiver Course Recorder Since the GPS receiver displays continuously the ship position in geographical coordinates, will be no problem for the receiver to memorize these positions. Basically, it all depends on the memory capacity that is allocated for this function. As result, the

GPS receivers can store the last 100 to 1000 waypoints planned for the ship voyage. The interval between two points that are stored can be set by the user, at order of minutes. During sailing in coastal waters, probably, an optimum interval for storage could be set at 2

minutes. During a strait transit, or navigation in heavy waters, areas with a limited length, is better to reduce the interval to 0.5 minutes. During an ocean passage, the interval can be increased from 5 to 10 minutes. In the GPS receiver memery allocated to this kind of activity, the storage process is continuosly, when the storage capacity is reached, automatically will be deleted the first memorized positions to increase the necessary space for the next positions storage. Based on this points stored in the receiver memory, at seafarer request, can be displayed the ship travelled route in a graphical form, the real route performed by the ship in time. The usefulness of this function will often evident only in unfortunate situations when the ship is involved in an accident. In such cases, the accident avoidance maneuvers are possible to be tested based on data provided by the receiver recorder.

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Chapter V

DGPS

5.1. Introduction

The GPS position accuracy can be greatly improved using differential techniques. Some of the experimental differential systems have been used as part of terrestrial navigation hyperbolic system. The Dfifferential Global Possitioning System (DGPS) is, relatively, an improved version of system used in terrestrial hyperbolic navigation. Generally, ship positioning data from satellite are recived by a mobile unit and a fix unit, position known. A system installed on the fix receiver calculates the distance to the satellite and compare it to a standard known distance for determining the geographic position accuracy. If, an error resulting from the calculation, this information is transmitted to the mobile receiver is used to correct the information received from satellite directly. The use of differential system does not eliminate the errors due to poor reception or interferences.

Figure 5.1. Working principle of the DGPS system

For maritime use, were established a number of monitoring stations of the DGPS system alongside the coasts of 28 states. For example, the United States Coast Guard supervising stations on both coasts of the United States, Pacific Ocean and Atlantic Ocean. Corrected data are transmitted using beacons with frequencies from low frequency band and therefore the reception is limited to a range between 100 and 250 kilometers. But, this system is very useful in areas where ship maneuvering is restricted, areas usually near the coast.

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Figure 5.2. DGPS coverage on USA coasts

With ITU support was supplemented the data transmitted using radio beacons working on frequencies of 283.5 – 310 kHz, in some areas of the World on frequencies between 285 – 325 kHz, leading to an increased transmission speed with a rate of 100 to 200 bits per second.

5.2. The DGPS system

The DGPS system was developed in order to achieve a highly accuracy of ship position in special areas (harbours, coastal areas), accuracy difficult to be achieved by using the GPS system, especially in standard transmission code. The components of the DGPS system are:

- GPS satellites; - Command and control centre; - Differential reference stations and DGPS data transmission; - System monitoring stations.

The DGPS system operation principles

The operating principle of the DGPS system is to compare the GPS position of a fix point to a reference terrestrial station. The observed differences (in two or three dimensions) are considered as differential position or like a serial of pseudodistances corrections measured to the satellite (differential pseudodistances), so the amounts of pseudodistances to satellites have a higher degree of accuracy, hence the increased accuracy in ship position. According to ITU-RM 823 Recommendation, the DGPS reference stations transmitting differential signals with a frequency of about 300 kHz. The transmission of differential corrections for each satellite take a period of 6 to 9 seconds, the processing and display of data

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take less time. The DGPS correction message is transmitted in a special format, called RTCM SC 104 format (RTCM – Radio Technical Commision for Maritime Services). A DGPS receiver can receive GPS differential data from 10 DGPS stations at least, which allows choosing of the best transmitted signal. Signal quality depends by the distance from the reference station, the atmospheric conditions, the antenna installation conditions and ship structure elements which can cause abnormal reflection of the received signal. Currently there are geostationary satellite systems emiting DGPS signals, in this case, the corrections of measured pseudodistances to satellites are carried out through a reference coastal stations network.

Automatic reception of the DGPS signal

For receiving of GPS corrections is necessary to have a receiver capable to incorporate DGPS correction in RTCM SC 104 format. When these corrections are encrypted is necessary to have a special receiver or a component capable for decoding them. At reception of DGPS signal in automatic mode, the receiver automatically chooses the best differential signal. For this, the DGPS receiver provides alerts on:

- Exceeding 10 seconds to receive a signal; - Reference station malfunction or, if, the signal is not valid.

Manual reception of the DGPS signal

At DGPS signal receiving in manual mode, the operator select a particular station or a frequency, check the signal integrity and take account of the received parameters value in selection of other DGPS stations. For this, the DGPS receiver has an alert for selection of incorrect station or the signal is not valid. For selected station the following are displayed:

- Reference station name and identification number; - Operating frequency; - The distance calculated to reference station; - Technical condition of the reference station; - Signal quality.

Depending on the distance calculated to the reference station, the DGPS accuracy can be appreciated as very good (less than five meters), at tens of miles distances and good (over five meters) for distances of 100-200 nautical miles. The DGPS signal quality is assessed by percentage value called WER (Word Error Rate). A value of 0% WER indicates a perfect reception of the DGPS signal. Stations/transmitters/radiobeacons that transmit DGPS corrections are presented in international nautical publications by the following:

- Station name; - Geographical position; - DGPS corrections (frequency, speed in bits/sec); - Identification number of the reference station; - Identification number of the transmitting station; - Transmission distance; - Monitoring of data integrity; - Messages type.

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In the 2nd Volume of ALRS 282 are presented the types of DGPS messages, numbered from 1 to 63. For example, the message type 3 represents the GPS reference station parameters, message type 7 is the Almanah of DGPS radiobeacons, message type 9 is GPS partial corrections, message type 16 is the GPS special message, type 17 is the GPS Ephemeris.

5.3. The DGPS receivers

Generally, the DGPS receivers are performance dual GPS/DGPS devices, with monocrom or color screen of 4.5 inches, providing an accuracy of 10 meters for GPS information and three meters for DGPS data. The DGPS receiver consists of:

- Antenna; - Command, reception and signal processing unit; - Command unit interface; - Data displaying interface.

The operational requirements for a DGPS receiver, according to IMO MSC 114(73)/2000, are: - Operating frequencies 283,5 – 315 kHz for Region 1 and 285 – 325 kHz

for Regions 2 and 3, according to ITU-RM 823 Standard; - Automatic and manual selection of station transmitting differential GPS

signal; - Displaying of information at maximum 100 miliseconds from their

reception; - Acquisition of a signal in less than 45 seconds in the case of atmospheric

electric disturbance; - Omnidirectional horizontally antenna; - Optimum operation in common conditions of interferences; - Protection to malfunctions up to five minutes.

The main technical features are: - Data displaying in several languages; - Storing of all DGPS stations; - Images: plot with 11 plotting scales, from 0.2 to 320 nautical miles; ship

positioning information (including LORAN C); ship conduct information; open sea navigation on scales from 0.2 to 16 nautical miles; loch information in analog or digital format;

- Multiple alarms: arrival at destination, waypoint, anchor watch, low/high speed, time (clock);

- Memorizing of the last 1,000 ship positions, 999 waypoints, 50 routes with maximum 30 waypoints;

- Possible connection to a PC or shiphandling simulator. Differential satellite systems are advanced systems that provide improved satellite signals for errors reduction in a particular area. Comparing of values between the signals transmitted by the DGNSS reference stations and those from satellites provide the necessary information for a high accuracy satellite diferrential positions than those determined with GPS observations, especially during coastal navigation, approach, harbor areas and in those areas with dangers for navigation or heavy traffic.

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Chapter VI

Glonass. Galileo

6.1. Glonass

The Russian satellite system GLONASS is similar to the US NAVSTAR-GPS system, in operation from January 1996. The GLONASS system consist of 24 satellites and provide position with 20 meters accuracy for civilian users and 10 meters accuracy for military users, with a probability of 95%. The first version used triaxial stabilized satellites Block II, upgraded to versions Block IIa, Block

IIb and Block IIv. Six of the Block IIa satellites were launched between 1985 and 1986, and Block IIv type from 1988 to 2005 (25 satellites). The second version of the system, named GLONASS-M develops from 1990. Until 2007 were launched 14 satelittes of second generation, upgraded, operating for a period of seven years. The third version, namec GLONASS-K, is a modern version using satellites with a life period of 10 years, lightweight (compared with the older versions). GLONASS-K transmits a signal for civilian users providing differential signals for search and rescue – SAR. Using CDMA (Code Division Multiple Access) signals, GLONASS-K system becomes interoperable with the US NAVSTAR-GPS system. First GLONASS-K satellite was launched in 2011. With constellation of 18 satellites have covering the entire Russian Federation territory and with 24 satellites the entire World. The GLONASS system (Globalnaya Navigationnaya Sputnikova Sistema) provides a global coverage, continuously and in all weather conditions, ensuring accurate position information, speed and time. The IMO Resolution MSC.113(73)/2000 establish the performance standards for GLONASS receivers.

GLONASS structure

GLONASS system consists of: a. Space segment; b. Control segment; c. User segment.

Space segment. Today, the space system is represented by the GLONASS satellites constellation, arranged in three orbital planes of 31 satellites, 24 GLONASS-M types, 6 GLONASS-M under maintenance and one GLONASS-K satellite. Whereas the orbits inclination is higher than the orbits of NAVSTAR-GPS system, greatly increase the possibilities of system using at high latitudes. Control segment. The control system, similar to the US system ensures the functionality of the satellite system and is managed by the Information and Scientific Coordination Centre of the Russian Air Force (CICS). The headquarter is in Moscow and secondary stations are in Sankt Petersburg, Ternopol, Eniseisk and Konsomolsk na Amur. User segment. The User segment consists of all GLONASS receivers for satellite signal processing, and the accuracy of position for civilian users is higher than achieved using the NAVSTAR-GPS system.There is several producers of satellite receivers using GLONASS

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information for positioning. Actually, are used receivers able to operate with information from GPS and GLONASS and to benefit from joint work of the two satellite systems. During 1998 - 2005 the system was improved to reduce the frequency band and to eliminate the interferences from system. In 2008 the system consisted of 16 satellites, 12 operational in the same time. Since 2011 the entire system is complete and consists of 24 satellites. Since 2013 is intended to start an updating program and the launch of new satellites GLONASS-K2 and GLONASS-KM type.

GLONASS system functions

The particularities of the GLONASS system are: - Height of the orbit: 19100 km; - Period : 11h15min; - Orbital inclination: 640.8; - Program: continuous; - Satellites: 24 - Spacing of at least four satellites in orbit and dilution of precision PDOP

of maximum six; - Working frequency in L band; - Reference ellipsoid: PZ 90.

The used reference ellipsoid PZ90 (Parameters Zemlia 90) is different by the international reference ellipsoid WGS84, so the difference between two ellipsoids datuums was on 17.09.2007 up to 40 cm, onmidirectional. Each GLONASS satellite works on two L-band frequencies, different for each satellite. L1-band length from 1602.5625 MHz to 1615.5 MHz, with 0.5625 MHz periods, and L2-band from 1246.4375 MHz to 1256.5 MHz, with periods of 4.4375 MHz, being generates 24 channels necessary for work. Each satellite transmit continuously own accurate position and system information, using ECEF (Earth Centred Earth Fixed) coordinates. The information are modulated in accurate P code and ordinary acquisition code C/A. Position informatioided by the GLONASS system are more accurate than those provided by the NAVSTAR-GPS system, but the system is less stable in operation than the US one.

GLONASS receivers

A standard GLONASS receiver consists of: - antenna; - Reception and calculation unit; - Comand and interface unit; - Displaying screen.

The performance standards according to IMO Resolution MSC.113(73)/2000 are:

- Ability to receive and process GLONASS positioning signals in geographic coordinates, latitude and longitude, in degrees, minutes and thousandths of minutes referential PZ-90 and UTC; the PZ-90 calculated position must be converted into WGS-84 values;

- Operation in Standard Positioning Service – SPS code (L1-band frequency);

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- The ability to send to other equipments reference positioning information PZ-90 or WGS-84;

- Static accuracy of 45 meters (95%) with a horizontally dilution of precision HDOP=4, PDOP=6;

- Dynamic accuracy of 45 meters (95%) with a horizontally dilution of precision HDOP=4, PDOP=6;

- Automatic selection of the best signal for ship positioning with desired accuracy;

- GLONASS signal acquisition with carrier frequency of 130 dBm to -120 dBm; once the signal acquired, the receiver have to work satisfactory for a carrier frequency of less than – 133dBm;

- Acquisition of positioning information at required precision in 30 minutes for invalid data of the satellite Almanac;

- Acquisition of positioning information at a required level of precision in five minutes for valid data of the satellite Almanac;

- Reloading of positioning information at a required level of precision, in five minutes, for a system malfunction less than 24 hours, without lost of power supply;

- Reloading of positioning information at a required precision level, in two minutes, for an interruption of power supply for 60 seconds;

- Displaying of a new position at every second; - A resolution of 0.001 minutes for geographical coordinates displaying; - Displaying of course over ground value; speed over groud value and

UTC time, at performance standards; - Reception of GLONASS differential information; for this, static and

dynamic positioning precision is 10 meters (95%); - The ability to operate satisfactory in typically interference conditions; - Warnings for status and failure.

6.2. Galileo

GALILEO is the joint project of the European Commission and the European Space Agency to deploy a new infrastructure based on a 30-satellite constellation, to provide positioning and timing services. On 26 March 2002, the European Union released €450 million to fund the development of the GALILEO satellite radio navigation system, which will enable people to pinpoint their exact position or the location of any moving or stationary object to within 1m. The European Space Agency had already committed a similar amount. The estimated cost of GALILEO is €3.4 billion and it is expected to be operational in 2008. Although the impact of satellite global positioning on society and industrial development is not yet clear, Europe cannot afford to rely on technologies and systems developed outside it, as GPS is certain to be one of the largest industries of the 21st Century. In the field of telecommunications, allied with other new technologies such as GSM or UMTS, GALILEO will increase the potential to provide positioning information as well as to provide combined services of a very high level.

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Existing radio navigation satellites Currently there are two radio navigation satellites in the world: American (GPS) and Russian (Glonass). Both were designed as military systems, but the GALILEO system appears to offer the only real alternative to the American system. Advantages of GAILILEO over GPS include: • GALILEO has been designed and developed as a non-military application, while nonetheless incorporating all the necessary protective security features. Unlike GPS, which was essentially designed for military use, GALILEO provides, for some of the services offered, a very high level of continuity required by modern business, in particular with regard to contractual responsibility • It is based on the same technology as GPS and provides a similar - and possibly higher - degree of precision, thanks to the structure of the constellation of satellites and the ground-based control and management systems planned • GALILEO is more reliable as it includes a signal "integrity message" informing the user immediately of any errors. In addition, unlike GPS, it will be possible to receive GALILEO in towns and regions located in extreme latitudes • It represents a real public service and, as such, guarantees continuity of service provision for specific applications. GPS signals, on the other hand, in recent years have on several occasions become unavailable on a planned or unplanned basis, sometimes without prior warning GALILIEO also complements GPS: • Using both infrastructures in a coordinated fashion (double sourcing) offers real advantages in terms of precision and in terms of security, should one of the two systems become unavailable • The existence of two independent systems is of benefit to all users since they will be able to use the same receiver to receive both GPS and GALILEO signals

Galileo services The various service requirements and the associated commercial and security aspects can be rationalised into five distinct service groups: • Open Services (OS) • Commercial Services (CS) • Safety-Of-Life Services (SAS) • Public Regulated Services (PRS) • Search-And-Rescue Services (SAR) In addition, Navigation Related Communication Service (NRS) will be implemented under the Commercial Service frame. Furthermore, Combined Services will be implemented, such as: • GALILEO combined with GPS and augmented by EGNOS, so called GALILEO-GPS-EGNOS Services • GALILEO services combined with services provided by Local Components • Combined GALILEO and GMS, UMTS services

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System architecture The GALILEO system architecture will be designed in such a way as to permit: • Adaptation of the response to the needs of users and to market trends • Minimisation of development and operating costs • Minimisation of the risks, other than financial risks, inherent in a project so unusual by virtue of its scope, complexity and the challenges it poses • Interoperability with existing systems, notably GPS, while at the same time maintaining autonomy and competitiveness Architecture is made up of four components: Global Component - the central component will be the global constellation of 30 satellites, distributed over three planes in Medium Earth Orbit (MEO). Within each plane, one satellite is an active spare, able to be moved to any of the other satellite positions within its plane, for replacement of a failed satellite. This will be complemented by regional and local components. Regional Components - the service provided by GALILEO is global and this includes the delivery of integrity worldwide. However, the design of the system is such as to permit the introduction of rdata from regional service providers using authorised integrity up-link channels provided by GALILEO, thereby making it possible to "personalise" integrity under partnership agreements with the relevant countries. The cost of this component will be borne by the region in question. Local Components - the GALILEO system will provide a high level of performance to users worldwide, even in places where there is no ground infrastructure. However, in the case of specific applications in given areas, even more demanding levels of positioning performance will be necessary or, alternatively, integration with other functions, e.g. local communications, will confer added value on the basic service. User Receivers and Terminals - receivers will be the crucial link in the GALILEO chain and will need to satisfy market requirements: • Competitive performance and costs compared with the existing systems • Adequate tailoring to the needs of users (general public and the professional market) • Potential for change and integration of the services (e.g. communications) • Possibility of multi-modal use

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Chapter VII

Electronic Charts

7.1. Introduction

There are two classes of electronic chart display systems. The first is an ECDIS (Electronic Chart Display and Information System), which can meet IMO/SOLAS chart carriage re-quirements. The second is an ECS (Electronic Chart System), which can be used to assist navigation, but does not meet IMO/SOLAS chart carriage requirements. ECDIS equipment is specified in the IMO ECDIS Performance Standards as follows: Electronic Chart Display and Information System (ECDIS) means a navigation information

system which, with adequate back up arrangements, can be accepted as complying with the up-

to-date chart required by regulation V/19 & V/27 of the 1974 SOLAS Convention. Where the term ECDIS is used in this document, this is to be understood as those navigational electronic chart systems, which have been tested, approved and certified as compliant with the IMO ECDIS Performance Standards and other relevant IMO Performance Standards and thus is compliant with SOLAS ECDIS requirements. ECS is specified in ISO 19379 as follows: ECS is a navigation information system that electronically displays vessel position and relevant

nautical chart data and information from an ECS Database on a display screen, but does not

meet all the IMO requirements for ECDIS and is not intended to satisfy the SOLAS Chapter V

requirements to carry a navigational chart. ECS equipment ranges from simple hand held GPS enabled devices to sophisticated stand-alone computer equipment interfaced to ship systems. The 1974 International Convention for the Safety of Life at Sea (SOLAS 1974), subsequently amended in 2000 and 2002, specifies the requirements for the navigational equipment to be used onboard ships entitled to fly the flag of a party to the convention. This Convention was adopted by the International Maritime Organisation (IMO), the United Nations Organisation that is concerned with maritime transportation. IMO member states are obliged to adopt IMO rules and regulations into their national legislation. However, only when the convention text has been incorporated into national legislation does it take effect for the individual ships registered in that country. This process of incorporation into national legislation may vary from a few months to several years. The country in which a ship is registered and hence which flag it is flying is known as the Flag State. It is the national maritime administration representing the Flag State, which controls the adherence to the SOLAS carriage requirements (Flag State control). The national maritime administration is also responsible for Port State control. Ships arriving at a port may be subject to Port State control by local officials (Port State Control Officers - PSCOs) based on Flag State regulations and international agreements.

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7.2. Geospatial Information and Services (GI&S) Concept

The geospatial information obtained using satellites consists in a two or three dimensional measurements of any point of Earth’s surface. Cartographers will convert these data in a formal mode, equate them with directions, distances, sizes and relative values. The tridimensional objects, with time and position variation, can be represented as points, lines or surfaces. Based on this encryption can be drawn after the Raster and Vectorial charts. This kind of information can be printed or displayed on screen as text, plane imagine or 3D models, resulting a representation of an Earth section, which, for a currently use of information, will be stored in electronic format, allowing to be viewed on a computer. This new approaching type of editing and displaying mode of an Earth surface was named GI&S (Geospatial Information & Services). The primary tridimensional information, can be stored in different data bases, such as:

o Raster format data bases; o Vectorial format data bases; o 3D data bases for land elevation; o 3D data bases for sea depths (bathymetry).

All these data types will required a special program to be displayed or printed. The activity of obtaining and storing of tridimensional measurements made with satellites is relative new (last 5 to 10 years). The advantage of using this model for creation of the electronic charts is indisputably superior to the classical carthographic methods, based on surveying, because:

o Measurements are more accurate; o Charts can be generated faster if we have necessary hardware and software

resources; o Measurements can be uodated and changing of data in the final chart is very

easy

7.3. Nautical charts

Nautical charts are special purpose maps specifically designed to meet the requirements of marine navigation, showing amongst other things depths, nature of bottom, elevations, configuration and characteristics of coast, dangers and aids to navigation. Nautical charts offer a graphical representation of relevant information to mariners for executing safe navigation. Nautical charts can be distributed in analogue form, as paper charts or digitally, and are available from a variety of sources, both governmental and private. For information on paper charts see the separate document “Facts about paper charts” The requirements for carriage of nautical charts are laid down in SOLAS Chapter V. The relevant regulations are: - Regulation 2, defines the nautical chart - Regulation 19, specifies the equipment to be carried on different types of ships and - Regulation 27, specifies the requirement to keep charts and publications up to date.

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IMO SOLAS V/2

2.2 Nautical chart or nautical publication is a special-purpose map or book, or a specially com-

piled database from which such a map or book is derived, that is issued officially by or on the

authority of a Government, authorized Hydrographic Office or other relevant government

institution and is designed to meet the requirements of marine navigation. The nautical charts and nautical publications referred to in regulation V/2 are commonly referred to as “official charts and publications”

IMO SOLAS V/19

2.1 All ships irrespective of size shall have: 2.1.4 nautical charts and nautical publications to plan and display the ship’s route for the

intended voyage and to plot and monitor positions throughout the voyage; an Electronic Chart

Display and Information System (ECDIS) may be accepted as meeting the chart carriage

requirements of this subparagraph; 2.1.5 back-up arrangements to meet the functional requirements of subparagraph 2.1.4, if this

function is partly or fully fulfilled by electronic means;

IMO SOLAS V/27

Nautical charts and nautical publications, such as sailing directions, lists of lights, notices to

mariners, tide tables and all other nautical publications necessary for the intended voyage, shall

be adequate and up to date

7.4. Types of electronic charts

There are two types of electronic chart – raster and vector. A raster chart is a scanned and passive image of a paper chart, whereas a vector chart corresponds to a digital analysis by object (points, lines, areas etc.). Charts issued by or on the authority of a Government, authorized Hydrographic Office or other relevant government institutions are official and may be used to fulfil carriage requirements (provided they are kept up to date). All other nautical charts are by definition not official and are often referred to as private charts. These charts are not accepted as the basis for navigation under the SOLAS convention. There are two kinds of official digital charts commonly available; Electronic Navigational Charts (ENC) and Raster Navigational Charts (RNC). ENC stands for “Electronic Navigational Chart”. The term was originally introduced for digital chart data complying with the IHO chart data transfer standard S-57. By IMO definition ENCs can only be produced by or on the authority of a government, authorised Hydrographic Office or other relevant government institution. Any other vector data is unofficial and does not meet carriage requirements. ENCs have the following attributes: - ENC content is based on source data or official charts of the responsible Hydrographic Office; - ENCs are compiled and coded according to international standards; - ENCs are referred to World Geodetic System 1984 Datum (WGS84); - ENC content is the responsibility and liability of the issuing Hydrographic Office;

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- ENCs are issued only by the responsible Hydrographic Office; and - ENCs are regularly updated with official update information distributed digitally. Only authorized distributors sell ENCs in ENC services which include the delivery of update information. The distributors are authorized either directly by the originating Hydrographic Office or by a cooperation of Hydrographic Offices. ECDIS distinguishes an ENC from unofficial data. When unofficial data is used, ECDIS informs mariners that they must navigate by means of an official, up to date, paper chart by a continuous warning on the screen. If unofficial data is shown on the ECDIS display, its boundary is to be identified by a special line style. This boundary is visualized as a “one-sided” RED line with the diagonal stroke on the unofficial side of the line. Further the mariner can use an ECDIS function to interrogate the chart display to obtain the chart details like information on originator, edition number and status of updating. The International Hydrographic Organisation (IHO) provides an interactive web catalogue displaying the status of worldwide ENC production. This system has pointers for guiding users to ENC suppliers and distributors. A three-colour scheme is used to distinguish between degrees of accessibility. This catalogue shows that many common shipping routes are already covered by ENCs. The illustration below shows the front page of the catalogue, which can be found at the IHO web-site at www.iho.int, look under “ENC”. Some HOs (eg Canada and Australia) make their RNCs and ENCs available to users via their own distributor networks; these distributors often offer additional folio services to shipping companies. A majority of all ENCs are only made available to the end-users in a protected form compliant with the IHO S-63 Data Protection scheme. The standard maintains the integrity in all transactions between the service provider and the end-user. The protection scheme enables the end user systems to check the authenticity of the supplied information. The S-63 protection scheme defines a mechanism for encrypting ENC information and applying a digital signature to enable authentication of the chart data by the end-user. The end-user will require a decryption key to access and view the ENC data protected by the scheme. Each ENC chart is encrypted with a different key, and the decryption keys are issued to specific end-user systems and can consequently not be exchanged or shared by different systems. The required decryption keys are distributed to the end-users as ‘Cell Permits’ by the service provider. The operation of a protection scheme should not add any operational overhead for the end-users since all aspects of ENC decryption and authentication are handled automatically by the chart system. The end-user will occasionally receive new Cell Permits from their service provider when their ENC subscription is renewed or there are changes to the ENC chart outfit. The updated Cell Permits must be imported into the chart system to enable it to automatically process new ENC deliveries and updates. A majority of all ECDIS and ECS suppliers have developed support for IHO S-63 and can read protected ENCs. A few nations distribute their ENCs without using encryption; all ECDIS systems are able to access and display these ENCs. The International Hydrographic Organisation has approved the distribution of ENCs in the internal format used by the individual ECDIS manufacturer. The generic name of this format is SENC – System-ENC. Depending on the make of ECDIS this can increase the speed of loading of ENC-data. The IHO requires service providers using this method of SENC-distribution to gain the agreement of the Hydrographic Offices supplying the ENCs and to use type approved software to ensure that the integrity of the SENC data is maintained.

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7.5. Raster Charts

RNC means “Raster Navigational Chart”. RNCs are digital raster copies of official paper charts conforming to IHO Product Specifications RNC (S-61). By definition RNCs can only be issued by, or on the authority of, a national Hydrographic Office. RNCs have the following attributes: - RNCs are a facsimile of official paper charts; - RNCs are produced according to international standards; - RNC content is the responsibility of the issuing Hydrographic Office; and - RNCs are regularly updated with official update information distributed digitally. The IMO performance standards for ECDIS states that where ENCs are not available, RNCs may be used in ECDIS to meet carriage requirements. However, when the ECDIS is using RNCs it should be used together with an appropriate folio of up to date paper charts. Information in the Raster format are obtained by scanning a paper chart. This process produces an image which is the exact copy of the paper chart and which contain a number of lines consist of many colored points or pixels. This technique not recognized the objects individually, limiting the ability to comply with the international requirements. Anyway, allows use of vectors overlaying, which allows user to input specific data like waypoints, radar overlaying and other operations that reduce this deficiency. The Raster charts advantages are:

o Use familiar because it uses the same symbols and colores like for the paper

charts; o Are exact copies of the paper charts with the same relevance and integrity; o The user cannot neglect navigation information on display; o Production costs lower than those for the vectorial charts; o Possibility to use official charts catalogues for the charts with global coverage; o Using the vectorial overlaying technique, together with a specialized computer

program, the Raster charts type can be used for all navigation activities, but

doubled by the paper chart. The Raster charts disadvantages:

o User cannot particularize the displaying mode; o When vectorial overlaying is used shadows may appear on the image; o Can’t be used without an additional data base with a common reference system; o Can’t provide warnings and indication directly to user; o Even if the stored information quantity is the same like for any vectorial chart

type, required a greater storage capacity.

7.6. Vector Charts Comparing with the ENC – Electronic Navigation Chart, which are made using data in raw and direct format, vector information can be obtained by scanning a paper chart also. The obtained image after scanning is vectorized using digital encryption on every cartographied object and its attributes (structure encryption) and stores this information, together with object geographical position into the data base. Resulted charts can be grouped and stored in thematic folders for each group. For example, the coastal area can be a folder and deep sea area another one. The user can optimize the display mode, receiving only interest data and avoiding the displaying of unwanted data that can make the activity difficult. Vectorial charts can provide information that allows the detection of a danger.

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The production process for Vector charts is longer and more expansive than for production of a Raster chart. Advantages of the Vector charts are:

o Laying of information in folders allows selection of displayed data; o The display mode can be customize by the user; o Possibility to change the scale without image distorsion; o Can provide warnings and indications in dangerous situations, like exceeding of

the safety contour; o The objects can be presented using different symbols like those used for Raster

or paper charts; o The chart information can be correlated with other equipments information such

as ARPA radar; o Compared with Raster charts requires a less storage capacity for the same

information volume. Disadvantages of the Vector charts:

o Technically are more complex; o Increased costs and time of production; o The coverage is not global yet; o Difficult to provide quality and integrity of displayed Vector information; o Training for their use is more complex than Raster charts.

The delivery mode of the electronic charts is the ECDIS (Electronic Chart Display and

Information System) system, a navigation information system using Vector charts provided by a recognized authority. Such equipment shall be in accordance with the international standards adopted by the International Maritime Organization on requirements for a safe passage specified in SOLAS Convention. The ECDIS hardware system can be represented by a common computer with a good graphical resolution or by a display incorporated in the integrated bridge system. The equipment can receive information from other electronic sources, like ship position information from GPS or LORAN, heading information from gyro, ship speed information from loch or other ships from ARPA Radar. The information is transmitted to the ECDIS system using NMEA (National Marine Electronics

Association) protocol. Radar information can be used by overlaying of data resulted from a radar screen scanning or electronically, using information provided by an ARPA Radar device (Automatic Radar Plotting Aid). ECDIS system software must contain required elements for electronic charts display and to allow displaying of their data and those received from other electronic equipments. The charts contained by ECDIS are Electronic Navigation Charts type, which have to comply with S-57 standard of the International Hydrographic Organization on the transfer of data and information. Non-official Vector charts

Generally are made scanning paper charts provided by national hydrographic authorities. The resulted image is than digitalized by copying the main lines and putting in electronic format. This vectorization process stored the future charts in layers that can be automatically translated to an appropriate chart scale. Information categories, like depths or navigation aids can be added or deleted on request. In some systems these elements can be used to obtain more information. The Vector view mode is designed such the information are electronically displayed in another way than were copied from the paper chart. The most of the automatic systems decides what information should be displayed, depending on resolution scale, so to avoid image blurs.

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This new operational regime has been designed to take account of the implications arising from: o Adding/deleting of information; o Changing of exposure scale and displaying only of data for the working scale; o Displaying of chart at a larger scale than that used for the paper chart.

Official Raster charts

There are two official Raster chart formats: � BSB Raster charts, containing all data from the paper charts published by

NOAA (National Oceanic and Atmospheric Administration), including weekly

corrections. These corrections are accessible via internet and are made

according to the Notices provided by US Coast Guard, Canadian Coast Guard

and National Imagery and Mapping Agency (NIMA). The NOAA data dase

contains a number of 1000 charts available from 1995 in Raster format.

Increasing the use of electronic navigation systems with GPS positioning or

other positioning systems, led to increased sales of Raster chart format. The

Raster charts are provided on electronic support (CD-ROM), each of this

containing a number of 55 charts with all facilities for navigation. � UKHO – ARCS (United Kingdom Hydrographic Office – Admirality Raster

Chart Service) and AHO (Australian Hydrographic Office) publishing Raster

charts according to British Admirality standards for paper charts. The British

Raster charts are weekly corrected using electronic support (CD-ROM),

containing the same information as the weekly editions of Notice to Mariners

used for charts correction onboard. The Australian charts (called “Seafarers”)

are montly corrected on the same principle. The British Admirality provide a

number of about 2,700 Raster charts, available on electronic format.

The British and Australian Raster charts are produced by the same process like the paper charts, consisting in printing and scanning of a chart or direct designing of a Raster chart. These charts reproduce exactly the original charts, with each pixel in relation to latitude and longitude. In use, the horizontal datums changes are included in each chart and all information must comply with the WGS-84 geodetic system. Not all charts have the information in accordance with WGS-84 geodetic system and should be used with caution when using position data provided by GPS. The production system of the British Raster charts involve the use of a main base and an online copy system, called ABRAHAM, used for correction and update of the Raster charts, and for the control and drawing of the basic charts.

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Figure 7.1. The UK Admirality ABRAHAM system for Raster charts development

The ABRAHAM represents all the processes necessary for designing and maintaining of a high resolution monochrome Raster base (25µ/1016 dpi) for each chart. The electronic format for charts correction is issued in accordance with the Notice to Mariners and can have weekly, periodically or monthly frequency, depending on corrections complexity.

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Figure 7.2. The coverage of British Admirality Raster charts 7.7. Electronic Navigation Charts (ENCs)

These are charts designed for use by ECDIS devices and are produced in a single universal format. Use Vector information based on IHO standards, as S-57 regarding to digital hydrographic information standards. Some of the major elements of identifying the unique properties of these charts are:

o Are edited only by governmental hydrographic authorities or under their

supervision; o Chart elements must be encrypted and able to provide information; o Information is provided in the form of cells which can offer only necessary data

and can be modified on user requirements; o All chart information is reported to the geodetic system WGS-84, used by GPS.

The information is accessible at any chart scale and displaying only the information required for user in given area. If required addition or deletion of data, these can be grouped in folders and used on request. The changing of chart scale allows the image to be zoomed out and easily used. Zooming out of a Raster chart can led to changing of navigation aids dimensions and to create an uncertainly chart for navigation. Zooming out of an Electronic Navigation Chart fix this

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problem, the navigation aids keep their characteristics related to the chart scale and in case of any change the user is adviced immediately. The individual countour lines can be defined as safety lines or anti-grounding, warning being given when ship is in proximity. Alarms will be generated automatically if the ECDIS equipment detects a conflict between the ship planned route and hydrographical characteristics contained by the electronic chart, with a potential risk to the ship. ECDIS can provide information from chart presenting the entire content of the ENC, by presenting a standard display or a minimum display of the content, called basic display. The first two displaying modes allow addition or deletion of information, while the basic display does not allow deletion of the information, which are considered the minimum needs for the safety of navigation. Validity of Electronic Navigation Chart depends by a number of factors related to editing hydrographic authority and include the following:

o The experience in production of electronic charts; the validity grows with

accumulated experience; o Information quality; software used to ensure quality of a digital data base must

comply with S-57 standard requirements; o Uniform data; is necessary that all hydrographic authorities to ensure standard

electronic charts editing process, the use of regional coordination centres is an

optimum solution; o Geographical coverage; focusing on geographical areas intensive used by the

shipping companies can ensure providing of required electronic charts.

Private publisher Vector

charts

Official Raster charts Electronic Navigation Charts

(ENC)

Are produced by private companies

Are produced by or under license of a national hydrographic authority

Are produced by or under license of a national hydrographic authority

Unofficial Official Official Not accepted for general purpose

Accepted for use with condition that data to be complete and the same with those in paper chart

Accepted for use with condition that data to be complete and recognized

Less probable to become the official replacer of the paper charts

Less probable to become the official replacer of the paper charts

Are the legal equivalent of the paper charts

Possible changes of chart original data

Chart information is safe Chart information is safe

Zoom out possibility Scale zooming out is limited at a levet that does not change the original image and information is not blurred

Charts can be zoom out without restrictions, the displayed details are according to used scale

Quality control depends by producer

Quality control according to governmental standards

Quality control according to governmental standards

Table 7.1. Integrity of different types of electronic charts

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Private publisher Vector

charts

Official Raster charts Electronic Navigation Charts

(ENC)

Generally, copies of the paper charts

Exact copy of the original paper charts

All data are separated in cells

O diferenţă de imagine faţă de original este prezenţă la toate nivelele de mărire utilizate

All the time is presented the same image as the original. Charts more equivalent with the paper charts than Vector type, including ENC

No similarity with the paper chart

Symbols and colors depending on the producer

Symbols and colors identically with the original chart

IHO S-52 standard defining the new colors and symbols used by the ENC

Accuracy, conformity and complexity depends on the producer

Accuracy, conformity and complexity like for the original chart

Can be more accurate than paper charts

A new operation regime is required

Same working mode as for the paper charts, possible changes may occur due to displaying screen sizes

A new operation regime is required

Table 7.2. Equivalence of various types of electronic charts with paper charts

Private publisher Vector

charts

Official Raster charts Electronic Navigation Charts

(ENC)

Updates depends by the producer

Have updated corrections when sold

Have updated corrections when sold

Is difficult to define the producer policy for corrections publishing

Information are corrected according to standards

Chart information are maintained according to strict standards

Depends by producer Can be provided new editions at user request

Not applying

Depends by producer On-line corrections for commercial users

On-line corrections available

Depends by producer Automatic integration of corrections

Automatic integration of corrections

Table 7.3. Correction of different types of electronic charts

Private publisher Vector

charts

Official Raster charts Electronic Navigation Charts

(ENC)

Chart Datum may differ by WGS-84

Comply with WGS-84 All data comply with WGS-84

Chart information can be removed from display, information important for navigation can be changed

Chart information can not be deleted, the user cannot deleted important information by mistake

Chart information can be removed from display, possible to modify important information for navigation

Table 7.4. The safety use of different types of electronic charts

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7.8. Electronic Navigation Charts (ENC) characteristics

Electronic Navigation Chart standards

Since 1980 a number of factors caused radical changes in the navigation safety ensurance: o Considerable increasing of maritime traffic, especially in narrow areas of

navigation; o Generally use and accuracy of electronic positioning systems (positioning

systems using satellites); o Dangerous increasing of marine pollution; o New large ships with speeds exceed 20 knots; o Reduction of bridge team members and limited time they have available for

studying, analyzing and filling of the nautical documentation (printed form). These were the main reasons regarding strictly to the maritime domain that led to the idea to introduce a displaying system for ship position in real time. The way this was done in practice was strictly related to the development of microprocessors and communication systems. In 1985 the International Maritime Organization has officially recognized the possibility of using ENC and the ECDIS systems. With the most advanced technology in the collection and presentation of the geographical data in electronic format, ECDIS equipments have been considered an efficient device for:

o Increasing of safety of navigation; o Updating of navigation information and chart correction; o Reduction of watch officer activity regarding chart working.

Uniformity in the presentation of geographical information in electronic format, and the devices for visualization of this kind of information was elaborated by four international organisations, namely:

� International Hydrographic Organization (I.H.O), provide to producers the

main sets of data used for generation of ENC’s; � International Maritime Organization (I.M.O), establish the norms that must be

observed by the ENC’s, that the product to be useful to the navigator, both in

terms how information is presented and the information type; � International Electronics Commission (I.E.C), control the products in terms of

electronic and information reliability standards; � International Radio Maritime Commission (I.R.M.C), verifies standards for

data transmission via radio. In developing these standards were considered two basic options:

o Offsetting decrease in resolution of the ENC image, related to a paper chart,

ensuring of an interface with which the OOW to access the alphanumeric data

base, so have been opted for Vector charts instead of the Raster charts; o The imposed standards for ECDIS equipments are limited only to those

regarding the user working interface (to make it as accessible possible to any

operator using the system), allowing instead a scientific evolution of creation

and display of the ENC’s. The ECDIS equipments, to be approved as navigation device, have to comply with the following four types of rules:

o ECDIS equipments must comply with the rules for electronic navigation devices

according to SOLAS Convention, specified ECDIS standard being developed by

International Maritime Organization by Resolution A/817(19) from 15th

of

December 1995, including the 1999 amendments;

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o The electronic chart which shown the geographic characteristics must comply

with IHO S-52 and S-57 standards; o ECDIS hardware and software must comply with IEC 61174 (1999) rules. o Considering that this equipment is installed onboard ship, the equipment type

have to be approved by the national ship registry. 7.9. Electronic Chart Display and Information System

ECDIS equipment is specified in the IMO ECDIS Performance Standards (IMO Resolution A.817 (19)) as follows:

Electronic Chart Display and Information System (ECDIS) means a navigation information

system which, with adequate back up arrangements, can be accepted as complying with the up-

to-date chart required by regulation V/19 & V/27 of the 1974 SOLAS Convention, by displaying

selected information from a system electronic navigational chart (SENC) with positional

information from navigation sensors to assist the mariner in route planning and route

monitoring, and by displaying additional navigation-related information if required. ECDIS is a ship borne navigational device and as such it is the responsibility of IMO. It must support the whole range of navigational functions that make use of the characteristics of the chart data and their specific presentation. Moreover, to be an ECDIS the equipment must be shown to meet all the requirements of the IMO Performance Standards (IMO Resolution A.817(19)) and offer, besides the graphic presentation of chart data, additional information about the characteristics of the displayed features. Within the ECDIS, the ENC database stores the chart information in the form of geographic objects represented by point, line and area shapes, carrying individual attributes, which make any of these objects unique. Appropriate mechanisms are built into the system to query the data, and then to use the obtained information to perform certain navigational functions (e.g. the anti-grounding surveillance). The presentation of the current position, range/bearing functions and route planning capabilities are other examples of the minimum ECDIS requirements laid down in the IMO Performance Standards for ECDIS. The presentation of ENCs on the screen is specified in another IHO standard, the “Colours and Symbols Specifications for ECDIS IHO S-52”, i.e. in its Appendix 2, called”ECDIS Presentation Library”. This style of presentation is mandatory. The use of ENCs in a tested, approved and certified ECDIS and with appropriate back up arrangements, is the only paperless chart option for vessel navigation. 7.10. ECDIS approvement

To ensure that ECDIS equipment intended for onboard use is seaworthy, it must pass type approval and test procedures developed by the International Electrotechnical Commission (IEC) based on the ECDIS Performance Standards of IMO and applying the IHO requirements, S-52 and S-57 in particular. Type approval is a method to show conformance with IMO requirements on a legal basis – it is initiated and required by all Flag States which are Member States of the European Union and by many others outside including United States, Japan and Australia. ECDIS type approval is the

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certifications process that ECDIS equipment must undergo before it will be considered to comply with the IMO Performance Standards for ECDIS by the international shipping community. Type approval is normally conducted by recognized organisations or by marine classification societies nominated by Flag States. Some maritime nations also have type approval programs within their maritime safety administration or Department of Marine Transportation. European Governments have agreed about mutual recognition of their ECDIS type approval certificates – indicated by the so-called “Wheel Mark” sign showing conformity with the Maritime Equipment Directive of the European Union. 7.11. Meeting Carriage Requirements with ECDIS Only a type approved ECDIS operating with up to date ENCs and with appropriate back up may be used to replace all paper charts on a vessel. Where ENCs are not yet available, IMO regulations allow Flag States to authorise the use of RNCs (together with an appropriate folio of paper charts) - see below. Note that in all other cases the vessel must carry all paper charts necessary for its intended voyage. From the regulatory perspective, the most important statement about the legal status of ECDIS is contained in the amended Chapter V of the SOLAS Convention set into force on 1 July 2002. As stated earlier in this section, ECDIS is specifically referred to in Regulation 19 ”Carriage requirements for ship borne navigational systems and equipment”. However, in order to replace paper charts, such systems must fulfil considerable technical requirements laid down in ECDIS Performance Standards: - The chart data in use must be official - ENCs where these are available; - The graphic display on the screen must meet the equipment-independent specification; and - The equipment must support the full range of navigational functions that can be performed on the traditional paper charts. 7.12. Back up requirements No electronic system is completely failsafe. IMO Performance Standards therefore require that the “overall system” include both a primary ECDIS and an adequate independent back up arrangement that provides: - Independent facilities enabling a safe take over of the ECDIS functions in order to ensure that a system failure does not result in a critical situation; and - A means to provide for safe navigation for the remaining part of the voyage in case of ECDIS failure. However, these rather basic statements allow considerable leeway and there are various interpretations as to what are the minimum functional requirements, or what constitute ”adequate” back up arrangements. There are two commonly accepted options: - A second ECDIS, connected to an independent power supply and a separate GPS position input; - An appropriate up to date folio of official paper charts for the intended voyage Some Flag States may permit other options (e.g. radar-based systems such as ”Chart-Radar”). Ship owners should consult their national maritime administration for specific advice. At the request of IMO the IHO is currently seeking information from their member states on which paper charts covering their territorial waters would be appropriate to serve as a back up to

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ECDIS. IHO will compile this information and present it on its web site in the form of a catalogue. In 1998 the IMO recognised that it would take some years to complete the world’s coverage of ENCs. As a consequence IMO ECDIS Performance Standards were amended adding a new optional mode of operation of ECDIS, the Raster Chart Display System (RCDS) mode. In this mode RNCs can be used in ECDIS to meet SOLAS carriage requirements for nautical charts. However, this is only allowed if approved by the Flag State. The intention of those changes was to allow the ECDIS to operate as far as possible on official chart data; ENCs where they were available and RNCs to fill in the gaps. IMO took note of the limitations of RNCs as compared to ENCs (see IMO SN Circular 207 at Annex), and the revised ECDIS Performance Standards require that the ECDIS must be used together with “an appropriate folio of up to date paper charts” for the areas where RCDS mode is employed. The intention was to allow the number of paper charts carried by a vessel to be reduced where RCDS mode was employed, but only to a level compatible with safe navigation. No definition of an “appropriate folio” was provided by IMO and consequently different Flag States developed individual interpretations. As there is no common interpretation of the term “appropriate” ship owners should consult their Flag State as to whether RCDS mode is allowed and under what conditions. A web-based catalogue showing world coverage of all ENCs, RNCs and paper charts available is currently under preparation by the IHO. In areas where ENCs or RNCs are not available vessels must carry all paper charts necessary for the intended voyage.

7.13. Use of ECDIS

There has been much confusion with regard to the names used to describe electronic chart distribution formats; the diagram below is designed to clarify the situation. From the diagram it can be seen that the same distribution format can be used for the delivery of both private and official chart data. For instance ’BSB’ is the term used for the distribution format of US and Canadian RNCs; the same term is used for the distribution of private raster chart data in other areas (for example in European waters). There can also be confusion with ENCs; private vector chart data delivered in S-57 format does not meet IMO requirements and should never be named as ENC. Similarly private vector data delivered in SENC format can be mistaken for ENCs delivered in the same SENC format. Therefore the most important factor to consider is the source of the electronic chart data; this determines its status and the purpose for which it may be used. However, only the combination between the status of the chart data and the functionality of the particular device finally decides if its practical operation can be stated as ECDIS-mode or as ECS-mode. Unlike the paper chart, ECDIS is a highly sophisticated system which, besides the navigational functions, includes components of a complex, computer-based information system. In total, the system includes hardware, operating system, ECDIS software (kernel and user interface), sensor input interfacing, electronic chart data, rules for presentation and display, status and parameters of alarms and indications, etc. All these items are accessed through an appropriate human-machine interface. As such, care must be taken when navigating with ECDIS to avoid - False operation - Misinterpretation - Malfunction or, even worse, - Over-reliance on this highly-automated navigation system

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As with any type of shipboard navigation equipment, it can only be as good as those who use it and what it is being used for. In the case of ECDIS and electronic charts, if the mariner is well trained then the system provides the information flow that the mariner needs to make good decisions and therefore contributes significantly to safe and efficient navigation. Stated another way, an electronic chart system is another tool to enable mariners to perform their job better. However, just having some “knowledge” about “functions” and “operational controls” is insufficient to maximise the benefits of ECDIS; proper training is absolutely necessary. ECDIS and other electronic charting systems have become increasingly important to ship navigation and are already widely used either as a primary navigation tool or as an aid to navigation. The systems are increasingly complex, and require adequate and appropriate training in order to be operated correctly and safely. Without proper training, these systems will not be used to their full potential and could under some circumstances increase the hazard to navigation. The STCW (Standards of Training, Certification and Watch-keeping) and ISM (International Safety Management) codes put the responsibility firmly on the shipowner to ensure that mariners on their vessels are competent to carry out the duties they are expected to perform. If a ship is fitted with ECDIS, the shipowner has a duty to ensure that users of such a system are properly trained in the operation and use of electronic charts and are familiar with the shipboard equipment before using it operationally at sea. There is no specific regulation or reference to ECDIS systems in the STCW Convention. However, since ECDIS systems are related to electronic charts, references about them are considered to be included in the material covered by the word “chart”: - To encourage effective ECDIS education, the IMO approved a standardised model course for ECDIS training on the operational use of ECDIS in 1999 (IMO course 1.27). This course is offered by approved training institutions and maritime academies. Maritime administrations can provide information on approved institutions. Some Flag States have developed their own training courses in ECDIS in order to be able to recognise the training certificates. - Type specific ECDIS training is provided by equipment manufacturers. Navigating with ECDIS is fundamentally different from navigating with paper charts. Important bridge work-processes are significantly affected, in particular, voyage planning and voyage execution task. These require careful analysis and consideration:

Voyage Planning

ECDIS provides a number of additional planning functions and features such as safety contours, alarms, click-and-drop facilities for waypoints and markers, etc. Whilst in many ways ECDIS makes voyage planning easier it is still possible to make errors, however these are likely to be of a different type to those encountered when using paper charts. Even though ENC coverage is increasing rapidly, many vessels will, to some degree, have to operate a dual – or even triple – system with ENCs paper and raster charts. Planning and validation of the route has therefore to consider issues such as which chart types are available for the various segments of the voyage. The format of the voyage plan is likely to differ from the traditional alphanumeric lists of waypoints used with paper charts and should include information on the usability of connected electronic navigational devices such as GPS and AIS and their actual alarm settings. It is essential to make use of the in-built automatic check functions provided by ECDIS when validating and approving the voyage plan. Thought also needs to be given to ensuring that a backup to the voyage plan on the ECDIS is available in case of equipment failure of the ECDIS itself or the connected sensors. It is important that there is good communication of the voyage plan to all bridge officers so that they are prepared for the intended voyage. This should include information on equipment status and backup procedures.

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Voyage execution At the beginning of the voyage, as well as at any change of watch, the officers should review the voyage plan and agree the selected pre-settings of functions, alarms and indicators to be used on the ECDIS. Where vessels carry paper charts as well as an ECDIS the role of the ECDIS and the charts should be considered. If the ECDIS is used for real time navigation, the statutory requirements regarding monitoring of the progress of the voyage and marking of positions will need to be considered: • are positions marked in paper charts solely for record keeping purposes? • what steps are taken to ensure that intended tracks marked on the paper charts correspond with the ECDIS information? • have the bridge procedures set in place by the shipping company been adapted for the use of ECDIS and are all persons concerned with the navigation familiar with these adaptations

Over reliance on ECDIS There is a tendency to put too much trust in computer based systems and believe whatever is on the display. It is essential that officers remember to cross check the information displayed by all other means available; especially by looking out the window and watching the radar! Bridge-procedures must be adapted appropriately and ENC training must be carried out to alleviate these concerns. 7.14. ECDIS operational functions

The many models of ECDIS devices used onboard ships forced the international organizations with duties in the field of electronic charts to establish a code (called GREEN-YELLOW-RED

Code) for approval of the existing equipments. Thus, each code color which labelling an equipment represents the classification according to standards set by IMO and IHO. Green Code label – a product 100% compatible with IMO and IHO standards, that means the

use of ENC’s according to S-57 standard, possibility to update the charts, allows the use of

DGPS devices in coastal areas and has all functions corresponding to Performance Standard.

Yellow Code label – a product partially compatible with the IMO and IHO standards.

Red Code label – is applying to equipments incompatible with the IMO and IHO standards.

An ECDIS program used on a PC or a specialized console have to respect some technical specifications and to fulfil different functions, such as:

� Electronic navigation chart format supported: o S-57 I.H.O standard o CHS NTX vector data o CMAP vector data o NOAA (USA) BSB raster data o British Admiralty ARCS raster data

� Chart functions: o Display of Vector or Raster charts o Display of Mercator latitude/longitude grid o Three information levels for Vector charts o Ship course prediction and anti-collision alarm (on Vector charts) o Charts update o Real motion

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o Manual repositioning o Ship movement simulation

� Displays overlaying on Electronic Navigation Charts

o Watch zones

� Distance circles � Latitude/longitude grid � Ship contour � Heading � Course over ground � North direction arrow � Ship course storage for the last 24 hours

o Ship route functions � Unlimited number of stored routes � Unlimited number pf displayed routes � Unlimited library for waypoints � Waypoints editing function � Reversing route � Safety depth and track error configuration � import/export of routes

� The Course Recorder o According to IMO standard o Ability torun recording o Interface with other equipments o Input for two positioning devices (GPS, LORAN) o loch, ultrasound, anemometer, Radar, ARPA Radar

� Navigation elements o NAVLINE o Bearing o Distances o Alarm zones defining o Introduction of the User notes

� The Radar module o Radar overlaying on ENC o Independent Radar image o Radar image adjustment

The User functions

The voyage planning, route selection. This route is defined by the created waypoints (WP). The waypoints definition can be done directly on the navigation chart or using a table, both methods being interconnected, so a waypoint defined on the ENC will be found also in the table with waypoints characteristic data, or, a waypoint defined in the table by geographical coordinates (latitude and longitude), will be found on the electronic chart. On the electronic chart is marked also the areas known as dangerous for navigation. Therefore, when planning the route, if one of these areas is crossing, a warning alarm will be activated. Before to start plotting of the waypoints will be introduced the own ship data, which will activate the anti-grounding system, the ship draft and safety depth under the keel and eventually the average speed.

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For seafarer is easier to plan the route directly on the electronic chart, having display the tabular data for route waypoints. Thus have the opportunity to nominate some of the waypoints, names that will facilitate their recognision later. When a waypoint is defined, in the table will be stated the values for the course between two waypoints, distance and time to travel (if there was introduced an average speed). Rout monitoring. Is continuously and in real time, the ship position can be displayed simultaneously on the electronic chart based on information received from two independent positioning devices (GPS/DGPS – LORAN or GPS/DGPS – AIS). On the electronic chart can be overlayed the Radar or ARPA Radar image, both for fixed and mobile targets, including their movement vectors when were ARPA plotted. Own ship movement vector will be displayed and updated continuously. Meantime, in digital form, in the side windows will be displayed data about ship course, ship speed, course over ground, speed over ground, ship position in geographical coordinates, drift details, direction, distance and ETA to the next waypoint, XTE value. Given that ECDIS equipment should provide all facilities for chart working, the software allows direct plotting of electronic bearings and distances to different references, also to determine the ship position by independents methods, without using of the electronic positioning devices. When is considered that these measurements are more accurate than the electroning positioning information, very possible when is not working with differential positioning systems, then the shop position can be manually corrected, using the coastal positioning as reference. For the safety of navigation, alarms included in the program are very important. These alarms will indicates:

o Near the bathimetric line under the safety depth value; o Proximity of a navigation danger; o Proximity of a special navigation area; o Aprroaching of a mobile target with CPA/TCPA values under the selected ones; o Track error over the planned values; o Approaching to a waypoint; o Time to start turning for a course change; o Malfunction of one of the electronic positioning devices.

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Chapter VIII

Automatic Identification System (A.I.S)

The Automatic Identification System is based on a transponder on board ship that transmit continuously information about the own ship using the maritime frequency of the VHF band. The transmitted information contain:

� Ship identification data: ship name, call sign, LOA, beam, draft, etc.; � Cargo onboard and if is dangerous or not; � Information about the ship course ans maneuvering capacities; � Ship position according to GPS.

These information must be received by other AIS equipments from other ships, also by the Vessel Traffic Service stations in its range. The information received by a ship or VTS station must allow plotting of the ship position on electronic chart, according to the position provided by the GPS or DGPS, together with a vector to indicate ship speed and course made good. Accessing the target must be possible to display other information, like target identification data.

Figure 8.1. Automatic Identification System components

The Automatic Identification System (AIS) require several adiacent components in order to achieve tasks, like: GPS or DGPS device, a VHP transponder, two VHF TDMA (time division multiple access) receivers, a VHF DSC receiver and an electronic communication system according to maritime standards connected to the ECDIS system. Information about ship position is provided by the global positioning system. Information about ship course and speed are

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provided by the AIS device using data received from other electronic navigation equipments, but data about ship destination and ETA can be provided only by the previous setting.

8.1. Recommendation on AIS performance according to IMO Resolution

MSC 74(69), Annex 3

At the third meeting of the Navigation Sub-Committee of IMO, in July 1997, waere elaborated the initial form of the performance standards for onboard AIS devices. These standards describe the operational requirements for device, but does not define the communication protocol to be used. The MSC Report include the following elements regarding to AIS equipments: 1. All ships with a gross register tonnage more than 300 tonnes (engaged in international

voyages), cargo vessels with a gross register tonnage of 500 tonnes or more (not engaged in

internmational voyages), and all passenger ships, must be equipped with AIS, as follows: 1.1. ships built on or after 1st of July 2002;

1.2. ships engaged in international voyages and built before of 1st of July 2002;

1.2.1. for the passenger ships, regardless of size, and for all tanker ships, no later than

1st of July 2003;

1.2.2. for ships other than passengers and tankers, with gross register tonnage of

50,000 tonnes or more, no later than 1st of July 2004;

1.2.3. for ships other than passengers and tankers, with a gross register tonnage of

10,000 tonnes or more, but less than 50,000 tonnes, no later than 1st of July

2005;

1.2.4. for ships other than passengers and tankers with a gross register tonnage of

3,000 tonnes or more, but less than 10,000 tonnes, no later than 1st of July 2006;

1.2.5. for ships other than passengers and tankers, with a gross register tonnage of 300

tonnes or more, but less than 3,000 tonnes, no later than 1st of July 2007; and

1.3. ships not engaged in international voyages built before 1st of July 2002, but no

later than 1st of July 2008.

2. Administration may except from the requirements of the previous paragraph those ships

that will be permanently out of service within two years from the implementation data

indicated in paragraph 1.

3. AIS equipments should: 3.1. to provide automatically information to a coast station, other ship or special

equipped aircraft, including ship identity, ship type, position, course, speed, ship

state and other safety information;

3.2. automatically receiving of such information from ships adecquate equipped;

3.3. ships tracking and monitoring; and

3.4. data exchange with coast facilities, the present paragraph requirements shall not

be applied where there are international regulations, rules or standards issued

for protection of the navigation data. The AIS system should be used taking into

account the recommendations adopted by the Organization.

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8.2. Operational functions of AIS

Target monitoring: o Displaying of unlimited number of targets on screen;

o Target selectionby TCPA and RCPA values;

o Targets possible to be individually centred on screen;

o Possibility to activate only one target;

o The messages can be send in binary form or ASCII form on specific channel;

o Automatically (scheduled) or manually transmission of data;

o Binary transmissions contain: Man Over Board, ARPA, elements and points of

interest (WP’s, routes or areas);

o Displaying of AIS working channels;

o Displaying of CPA value on screen;

o Alarms and warnings based on configured CPA.

AIS Long Range monitoring: o Electronic mail configuration using Microsoft MAPI (Mail Application

Programming Interface);

o Monitoring using Inmarsat;

o Sender information filtering;

o Possibility to transmit to several e-mail addrresses;

o Formats configuration.

AIS module configuration: o Posibiliti to delete target properties;

o Transmitting of data regarding name, call sign, ship type, IMO number, MMSI,

ship draft, voyage, destination and ETA at destination;

o Information about the own ship transponder;

o Distinctive features of the transponder;

o The GPS equipment used for positioning on electronic chart;

o Global positioning system transponder state;

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