SAR Interferometry Fundamental and applications

Microwave Remote Sensing

1. 7/13/2017 1 SAR INTERFEROMETY

2. 7/13/2017 2 Concept of Interferomety Young’s Double slit experiment

3. 7/14/2017 3 SAR Interferomety

4. 7/14/2017 4 Methods for SAR Interferomety

5. 7/13/2017 5 Fig(3.5) SAR Geometry and various components of Interferometric pair

6. B A 1 A 2 Two images of the same area are required, measured from slightly different positions ( A 1 and A 2 ) Due to the range difference d  coming phases are out. These phase differences contain height information of the surface point and are used to create the interferograms q  ’ h z(x,y) y z a d  Insar Geometry

7. h r r-r ’ r-r ’ = range difference (red) Number of waves can be retrieved from the interferogram after phase unwrapping r ’ Phase difference Df = ( r -r ’ ) 2p/l mod (2p) Phase Difference

8.  [Number of waves ] r-r ’ [Meters] l Phase Difference

9. Data Format SLC = Single Look Complex Every resolution element contains - Magnitude - Phase ( between 0 and 2 p radians) of backscattered signal Phase 180 360 Magnitude

10. One color cycle (fringe) Equal to half of the wavelength of the radar wave, i.e. for ERS C band SAR, 56 mm / 2=28 mm) So by counting all the color cycles we know how many wavelengths fit inside r-r ’ Phase differences

11. Data set 1 Phase Unwrapping Image Co Registration Data set 2 Interferogram Generation Phase To Height Conversion Geocoding InSAR Processing Flowchart

12. InSAR Flowchart, Co Registration Register slave image to master image -The slave image is resampled to be perfectly co-registered to the master - Requires complex image resampling of the slave image - Co-registration must be within ¼ of a pixel width Recalculate the Amplitudes and Phases by Interpolation Align the pixels in both images SLC master image SLC slave image Sensor data Sensor data Coregistration

13. 7/13/2017 13 Visual interpretation of phase “ images”of one SAR image does not give any information Phase differences between corresponding elements of two images can contain a pattern (fringes) Phase plot of one SAR image contains no useful information

14. For each pixel: f corr = f raw - f flat earth A corr = average magnitude Coherence image Complex interferogram -Compute phase difference -Removing ‘flat earth phase’ by complex multiplication of two images - Output: Coherence Estimator InSAR Flowchart , Interferogram generation

15. 7/13/2017 15 (a) Raw Interferogram (b) Filtered Interferogram (c) Filtered and Flattened Differential Interferogram Interferometry with nearly zero baseline

16. COHERENCE MAP ERS-1 ERS-2 The values in a coherence map are an indication of the local differences between two radar images. Coherence map

17. Phase unwrapping Interferometric system does not measure the total pixel phase difference. Rather, it measures only the phase difference that remains after subtracting all full 2  intervals present (modulo 2  ). It is called a wrapped phase and has to be unwrapped or calculated back to its real values. Unwrapped phase difference is used to calculate height.

18. Phase unwrapping Phase difference as measured by interferometric system is as follows

19. Phase unwrapping

20. 7/13/2017 20 Phase Unwrapping

21. Error Sources Baseline accuracy (horizontal and vertical) - orbits geometry Low Coherence (Phase Correlation) between the two SAR images is the most important limitation and error source Decorrelation (loss of coherence) can be due to: - Registration errors {spatial and spectral(e.g. c & x)} - Temporal de-correlation - Baseline de-correlation - Orbit errors Inhomogeneous Atmosphere De-correlation has a direct effect on the interferometric phase and thus on the height

22. 7/13/2017 22 Non-Interferometric SAR Data Interferometric Pair

23. Raw SAR Data Coregistered Image Interferogram Phase Image Flattened Interferogram Differential Interferogram Geocoded Differential Interferogram STEPS of SAR INTERFEROMETRY 23

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26. Available 7/13/2017 Geological Survey of India 26 Sentinel- 1A Data ENVISAT Data ALOS PALSAR Data Satellite Sensor ALOS -1 PALSAR Mode Ascending Polarisation HH , HV Band L( =23cm ) Satellite Sensor ENVISAT ASAR Mode Descending Polarisation HH Band C( =5.6cm ) Satellite Sensor Sentinel-1 C-SAR Mode Descending Polarisation HH , VV , HV , VH Band C( =3.5-7.5cm ) Advanced Land Observation Satellite( ALOS ) Phased Array-type L-band SAR( PALSAR )