Signal Processing : Services

SAR Post-Processing

CSL developed a recognized expertise in the frame of SAR interferometry (InSAR), differential interferometry (DInSAR), coherence tracking, SAR polarimetry and polarimetric interferometry (PolSAR & PolInSAR), split band interferometry (SBInSAR) and spectral coherence. 
For each of these techniques, CSL developed and validated its own tools.
Collaborations through applicative projects allows enhancing our expertise, improving existing tools and developing new techniques.

 

SAR interferometry

SAR interferometry (InSAR) consist in using the phase information in two consecutive or simultaneous SAR acquisitions of a given area to generate an interferogram and a coherence image of the observed scene. Main InSAR application consists in retrieving a Digital Elevation Model (DEM) from the interferogram. Coherence is a quality indicator of the scene stability between acquisitions. It can be used as an indicator of backscattering processes changes. Figure 1 shows a DEM of the Nyiamuragira volcano overlaid with a false color image in which both the original SAR image and the coherence image were used.

 

INSAR

Figure 1: InSAR DEM of the Nyiamuragira volcano

 

Coherence tracking

Coherence tracking consist in optimizing coherence at a local level to refine co-registration of images used in the interferometric process. Such tracking allows to detect azimuth fast moving features between acquisitions. Animation hereafter shows coherence tracking results as observed for the Shirase fast flowing ice stream in Antarctica.

 

 

 

Differential Interferometry

If the topography of the scene is made available, the topographic phase may be retrieved from the interferogram to show only the phase component linked to local displacements. This process is know as differential interferometry (DInSAR). Figure 2 shows DInSAR result derived for the Landers earthquake (1992).

 

DINSAR

Figure 2: DInSAR fringe pattern of the Landers earthquake

 

Two SAR images, one before and one after the earthquake, where used to generate an interferogram containing both the topography and the displacement. A second interferometric pair of images was used to generate only the topography onto which the differential fringe pattern was overlaid.

 

Polarimetric SAR 

Polarimetric SAR (PolInSAR) make use of polarimatric channels if available in the SAR acquisition to perform target decomposition and segment the signal along three main backscattering processes. Full polarimetric SAR data allows also performing polarization synthesis. The animation hereafter shows an example of polarization synthesis showing what would be observed if sending a left circular polarization while analyzing along variable horizontal polarizations.

 

 

 

Polarimatric SAR interferometry

Polarimatric SAR interferometry (PolInSAR) uses target decomposition principle to perform coherence optimization at local level. This optimization results in three interferograms, each corresponding to a dominant backscattering process. Figure 3 shows an example of optimized interferograms in line with their associated optimized coherence.

 

polarimetric SAR

 

Figure 3: Polarimetric SAR interferometry

 

Spectral coherence

New sensors are using wide bandwidth signals to improve resolution. This wide band may be used as a new degree of freedom. Splitting this wide band in sub bands allows performing a spectral analysis of a single image, leading to sub-images of lower resolution but centered on slightly different carrier frequencies. Performing a coherence analysis between sub-bands allows computing spectral coherence. Manmade structures are shown to be spectrally stable while random distribution of scatterers are loosing spectral coherence. This characteristic allows for example detecting vessels on open sea surface. Figure 4 shows an example of spectral coherence measured on the Venice Laguna.

 

Venice Laguna

 

Figure 4: Spectral coherence measured on the Venice Laguna

 

 

Split-Band SAR interferometry

This sub band splitting process may be applied to an interferometric pair of SAR images leading to a spectral interferometric decomposition. This technique know as Split Band Interferometric SAR (SBInSAR) allows measuring the "absolute" interferometric phase on a point by point basis, alleviating the well known phase unwrapping problem. Figure 5 shows an example of a DEM of the Uluru monolith (Australia) fully obtained using SBInSAR.

 

SBInSAR

 

 

 

 

CSL InSAR Suite

InSAR suite      

The CSL InSAR suite is a command line processor allowing to perform all here-above described processes. It can handle ERS, EnviSAT, ALOS PALSAR, CosmoSkymed and TerraSAR (TSX & TDX) data.

 

 

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Contact(s) : Dominique De Rauw dderauw@ulg.ac.be