09a Sar Processing

7/31/2019 09a Sar Processing

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Radar systems:

SARSynthetic Aperture Radar


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What does SAR stand for?

S yntheticA pertureR adar


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What is the output?

Output: a microwave imagesof the earth with:- Amplitude information- Phase information

Coherent image

Azimuth

RangeMain use:

Earth observation


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Range imaging

Time measurement

Back to thefundamentals


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Range imaging


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Range imaging

Detection geometry


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Range imaging

Near range

Far range


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Range imaging

The radar looks in slant range

CRITICAL POWER BUDGET: only a limited amount of energy is backscattered


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Range imagingRange resolution

Ideal pulse:- high peak power- duration t = 0

Real pulse:- low peak power- duration t > 0

ENERGY?


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Range imaging


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Range Imaging

Non discriminating


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Range imaging

Range resolution uncertainty:

What we can do?


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Range imaging

Chirp pulse: linear frequency modulation


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Range imaging

At the receiver: Pulse compression

Cross-correlation between the transmitted and received signal

TxRx Rx

2 targets


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Range imaging


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Azimuth ImagingAzimuth resolution


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Not acceptableSolutions?


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IMPOSSIBLE


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Azimuth ImagingAzimuth resolution improvement

Sensor target range variation! Doppler effect


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Sensor target range variation! Doppler effect


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Sensor target range variation: focalisation process


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A i h I i


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DistanceIndependent!!!Azimuth resolution

A i h I i


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Azimuth ImagingAzimuth resolution improvement: arbitrary antenna size?

A i h I i


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Azimuth ImagingDoppler analysis

Same spectrum in range!!!

~Synthetic chirp in azimuth!

I f i


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Image formation

Coherent acquisition system: 1 pixel = I+Q (magnitude, phase)

Raw data

I f ti


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Image formation

Single Look Complex image(in radar geometry)

I f ti


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Image formation

SLC

-SingleLookC

ompleximage

U d t di SAR i


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Understanding SAR imagesgeometry

Near range

Far range

Recall

U d t di SAR i


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Understanding SAR imagegeometry

Image formation Geometric characteristics

Sar images are recorded in Slant-Range(SR).Slant-Range image shows unequal distances and widths betweenfeatures.

GR

SR

sin22 SR

HSRGR Hyperbolic correction

Ground range: image resampled in regular grid

U d t di SAR i


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Foreshortening Points A, B and C areequally on the ground.However, the distancebetween B' and C' isshortened compared to A-

B, because the top of themountain is relatively closeto the SAR sensor .

This effect may be compensated during the geocoding process if aterrain model is available (compensated, not recovered)

U d t di SAR i


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Layover

This effect is destructive superimposition of pixels

U d t di SAR i


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Shadowing

Dark pixels in the imagesno signal

U d t di SAR i


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Foreshortening vs layover vs shadowing

- layover >- foreshortening > 0, /2 -

Understanding SAR image


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Geocoding, Radiometric Calibration andNormalization

- Transformation from radar geometry to a cartographic projection

- Radiometric calibration and normalization of SAR data


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Geocoding

InputSlant rangegeometry

Resampling

OutputGround rangegeometry

Precise geocoding

Control pointsRefined orbital parametersSystem parameterDEM

Foreshortening effects can be corrected with the

availability of high resolution Digital Elevation

Model (DEM) data. Layover and Shadow areascan be calculated, but not corrected.


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Radiometric Calibration

1) Correction of the antenna gain pattern

2) Correction for the range spread loss

3) Radiometric normalization(sensors,time, look angles)

Look Angle influence on Sar SignatureX : Range Y = radar backscatter

white : not normalized

red : normalized

Input: ERS2-scene, Canada


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CalibrationScattering matrix

DLR Corner Reflectors

vv

09a Sar Processing

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