Long range scanning pulsed Coherent Lidar for real time wind monitoring in the Planetary Boundary Layer

Jean-Pierre Cariou, Laurent Sauvage, Ludovic Thobois, Guillaume Gorju,

Mehdi Machta, Guillaume Lea and Marie Duboué LEOSPHERE SAS - 16 Rue Jean Rostand - 91400 Orsay – France

jpcariou@leosphere.fr, lsauvage@leosphere.fr, lthobois@leosphere.fr, ggorju@leosphere.fr, mmachta@leosphere.fr, glea@leosphere.fr, mduboue@leosphere.fr

1. Introduction Among the last five years, LIDARs appear as powerful devices to measure accurately wind in direction and magnitude, especially in the domain of wind energy. Today, new challenges consist in the measurement and the detection of complex atmosphere dynamics 360° around and at longer range in the entire lower troposphere. These needs are driven by the development, among other needs, of wind energy in complex terrains and also in order to improve meteorological forecasting, and hence airports safety and traffic saturation. For this last application, the objective is to measure wind gradients and turbulence in nearby areas, such as atmospheric wind shears or aircraft wake vortices. These needs have motivated LEOSPHERE to develop and manufacture of a new long range hemispherical scanning pulsed lidar for wind measurement, so called Windcube200S, this goal being able to be achieved thanks to the know-how of LEOSPHERE in the development and manufacturing of accurate, compact and easy to install wind lidars [1]. In this paper, the main characteristics of the new Windcube200S lidar are summarized. Besides, the signal processing and the post-processing software developed are presented. Finally the measurements obtained during a preliminary validation campaign are detailed. 2. Presentation of the instrument The Windcube200S is based on a proven technology formerly developed at Onera (French Aerospace Lab) [2,3] and transferred to LEOSPHERE for industrialization and manufacturing. The technology is based on reliable dedicated high power fibered laser amplifiers and telecom components at 1550nm and coherent detection. A scanner head had been recently designed and added to the lidar to perform any scanning pattern in 3D space. 2.1 Doppler lidar

A Master Oscillator Power Amplifier (MOPA) configuration is used as a transmitter. The Erbium Doped Fiber Amplifier can handle high peak power while limiting non linear Brillouin scattering, having a linear polarization and a quasi single mode beam [4]. To ensure long range measurements and good velocity resolution at high measurement rate, pulse duration is set to 400ns at 10KHz pulse repetition frequency. A 100mm aperture refractive telescope collimates the beam with low divergence. Transmitted beam is frequency shifted to allow wind velocity sign to be retrieved. The beating signal is received on a fast InGaAs photodetector, amplified and digitized at 250MHz.

Figure 1 : Optical scheme of the pulsed lidar

Main lidar parameters are summarized in the table below.

Wavelength 1543 nm

Mean Power 1W

Pulse duration 400ns

Pulse repetition frequency 10 KHz

Range gate 512ns

Integration time 1 s

Telescope aperture 100mm

Maximum range 12km

Minimum range 100m

Range resolution 70m

Velocity range -30 to +30m/s

Velocity resolution <0.5m/s

Table 1: Windcube200S characteristics

2.2 Scanner head In 2010, a new endless scanning head has been designed to improve capacities of Windcube200S. It consists in two flat folding mirrors driven by brushless motors. Main characteristics are summarized below:

Azimuth angle 360°

Elevation angle 360°

Azimut speed 50°/s

Elevation speed 50°/s

Scanner aperture 120mm

Angle resolution 100µrad

Position accuracy 1mrad

Drivers Ethernet/USB/serial

Table 2: Scanner characteristics

The scanner allows either starring mode, PPI mode (Plan Position Indicator), RHI mode (Range Height Indicator), or VAD mode (Vertical Azimuth Display).

Figure 2 : Scanner head

Both lidar and scanner head are integrated into a twin compact casing for unattended operation, able to handle transport vibrations and various operating temperatures, as well as rain and dust. The lidar is eye-safe and supplied in 24VDC for safe operation.

Temperature range -15 to 50° C

Size 1.5 x 0.55 x 0.65 m

Weight 150kg

Eye Safety IEC 60825-1

Humidity IP65

Table 3: operational parameters

Figure 3 : lidar LEOSPHERE Windcube200S

3. Signal processing The signal processing computes the FFT for all 240 possible range gates in real time, averages 10000 spectra during the measurement time and estimates the mean frequency, dispersion and Carrier to Noise Ratio (CNR) thanks to a powerful Maximum Likelihood Estimator (MLE). [1]. All results are displayed in real time and recorded in ASCII files for further data processing. A fast Ethernet connection is used for control and data transfer. 4. Data processing The Windsoft® software displays radial wind speeds, CNR and dispersion in real time in polar view (cf figures 7 and 8), with a local map in background. Some data processing are available, both on wind fields and CNR maps. - A range corrected signal map is displayed in order to detect scattering areas such as clouds, plumes or obstacles. - Using a local VAD algorithm, horizontal windbarbs are computed in homogeneous areas. - Strong wind gradients can be detected, to highlight windshear areas. 5. Performance 5.1 Range Figure 4 shows experimental CNR variations versus distance for horizontal beam in our Orsay premises. Focus is changed from 400m to infinite (~3200m). The CNR threshold corresponding to 0.1% false measurement rate is drawn in order to estimate maximum range. A simulated theoretical fit is superimposed on experimental points corresponding to a 40mm radius transmitted Gaussian beam. The good fit checks the optical quality of the Lidar architecture. For a collimated beam with 1 second integration, the maximum range exceeds 6500m on spring time and continental atmosphere. (Collimated beam is obtained by focusing it on infinite distance,~3200m in this case)

0 1000 2000 3000 4000 5000 6000 7000-30

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Measured CNR Foc=800m

Measured CNR Foc=1400m

Measured CNR Foc=3200m

CNR Threshold

Figure 4 : Measured and simulated CNR for

different Focusing Distances Figure 5 displays the simulated reliability zone for scanning mode on standard atmosphere. A collimated beam is considered for this simulation. For high elevation angle (zenithal Line Of Sight (LOS)), aerosol concentration decreases beyond the Planetary Boundary Layer (PBL). The weak backscattering coefficient limits the maximum range (2200m in this case). For low elevation angle (horizontal LOS) aerosols on PBL ensure good reliability on long distances. Maximum range can reach 8km for horizontal beam. Simulations were computed using reliability model developed by A.Dabas [6].

Figure 5 : Reliability zone in RHI mode

5.2. Velocity resolution Taking into account the actual lidar parameters and the MLE signal processing, a simulation of the velocity standard deviation versus range is computed according to Valla’s method [7]. Figure 6 shows the variation of velocity resolution for Windcube200S considering 1s integration and different focusing distances. For a collimated beam, velocity resolution is less than 0.5m/s for range 100m to 7000m. Focusing the beam at shorter distance (800m) leads to high measurement incertitude at long ranges (>0.5m/s after 3500m) but increases the precision for low ranges. In this case, velocity

resolution reaches a minimum of 0.01m/s near focusing point.

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Acceptance

Figure 6 : Velocity standard deviation

6. Lidar first validation At the end of the development process of each new LEOSPHERE’s Lidar, a preliminary campaign is achieved so as to validate the instrument. For the Windcube200S, this first campaign took place in autumn 2010 near Paris at the “Institut d’Optique Graduate School” – IOGS (lat. 48°42'50.96"N – long. 2°12'11.59"E). This 3 levels building is located at 2.2 kilometers from LEOSPHERE where short range vertical wind Doppler LIDARs are available as a reference for validating the measurements made by the new lidar. The figure 7 below shows a PPI velocity field measured from IOGS. The effective range of the device was about 4 kilometers that day. The velocity map corresponds to an homogeneous wind coming from South West. Wind structures appear in the velocity field, thanks to the high spatial available resolution.

Figure 7: PPI wind field

The CNR map also validates the maximum range obtained in this campaign of about 5 kilometers at which the level of CNR gain reaches the minimum threshold level of -29 dB.

Figure 8 : CNR map

To validate the wind speed measured by the Windcube200S, the wind speed and direction above LEOSPHERE premises are compared with the averaged velocity derived from one of the Windcube70 in test at the same place. The scanning beam intercepts the lidar axis at 150m high.

Figure 9 : Geometry of velocity validation

Figure 10 shows the early comparison of the radial velocity measured by the Windcube200S and the Windcube70.

Figure 10 : Comparison of radial velocities between

the Windcube200S and theWindcube70

For the Windcube70, the radial velocity displayed corresponds to the 10-min averaged velocity projected on the scanning beam. For the Windcube200S, the instantaneous radial velocities of 4 consecutive laps are represented. This preliminary validation is quite good since wind fluctuations measured by the Windcube70

reaches 1m/s. These wind fluctuations can explain the difference observed between the instantaneous velocity measured by the Windcube200S on first lap and the averaged velocity of the Windcube70. 7. Conclusion and perspectives A new long range lidar has been designed and tested at LEOSPHERE, based on 1550nm fibered pulsed EDFA technology. The Windcube200S measures in 3D wind profiles up to 7km with 70m spatial resolution, 0.5m/s velocity resolution ad 1s time resolution. The lidar is integrated in a robust casing for long term atmosphere monitoring. It has been compared with success to a reference lidar without scanner during a preliminary campaign. Applications are airport wind hazard monitoring as well as wind mapping for wind energy resources or scientific atmospheric dynamics studies. Several campaigns are scheduled in 2011 on airports. 8. References 1. J.P. Cariou, R. Parmentier, M. Valla, L. Sauvage, I. Antoniou, and M. Courtney, “An innovative and autonomous 1.5µm Coherent lidar for PBL wind profiling”, 14

th CLRC, 2006.

2. A. Dolfi-Bouteyre, B. Augere, M. Valla, D. Goular, D. Fleury, G. Canat, C. Planchat, T. Gaudo, C. Besson, A. Gilliot, J.-P. Cariou, O. Petilon, J. Lawson-Daku, S. Brousmiche, S. Lugan, L. Bricteux, B. Macq, “Aircraft wake vortex study and characterization with 1.5 µm fiber Doppler lidar“, Journal of Aerospace Lab, December 2009. 3. A. Dolfi-Bouteyre, G. Canat , M. Valla, B. Augère, C. Besson, D. Goular, L. Lombard , J.-P. Cariou, A. Durecu, D. Fleury, L. Bricteux, S. Brousmiche, S. Lugan and B. Macq, “Pulsed 1.5 µm LIDAR for Axial Aircraft Wake Vortex Detection Based on High-Brightness Large-Core Fiber Amplifier”, IEEE JSTQE, vol. 15, no 2, march/april 2009. 4. J.P. Cariou, B. Augere, M. Valla, “Laser source requirements for coherent lidars based on fiber technology”, C. R Physique 7 (2006) Elsevier Ed. 5. M. Valla, B. Augere, J.-P. Cariou, P. H. Flamant, “Fourier transform maximum likelihood estimator for distance resolved velocity measurement with a pulsed 1.55 µm erbium fiber laser based lidar”, 13th CLRC 2005, Kamakura, Japan. 6. A. Dabas, “Semi-empirical model for the reliability of a matched filter frequency estimator for Doppler lidar”, Journal of Atmospheric and Oceanic Technology, 16:19–28, 1999. 7. M. Valla, “Etude d’un lidar doppler impulsionnel à laser Erbium fibré pour des mesures de champ de vent dans la couche limite de l’atmosphère”, PhD thesis, Paris, 2005.