Differential SAR-Interferometry

Contact Persons

Dr. David Small

Dipl. El.-Ing. ETH Arnold Barmettler

Thomas Egli

Keywords

Synthetic Aperture Radar, Multipass-Interferometry, Digital Height Model

Quick Reference

Differential SAR-Interferometry is a technique stemming from SAR-Interferometry. Its goal is to detect changes in topography (e.g. caused by an earthquake). Theses changes correlate with the interferometric phase as the topography itself does too. It's important to note that with differential SAR-Interferometry it's only possible to detect movement differences in the surface and not absolute movements. In the same way this technique is only sensitive to movement differences in range direction. To detect movement differences, one must eliminate the influence of the topography. Three different techniques to solve this problem will be briefly introduced. Also, the advantages and disadvantages of each method will be discussed.

Some Results

2-Pass Method:
In the 2-pass method two SAR-scenes (including orbit data) and a digital elevation model (DEM) are required. The height-values of the DEM have to be converted to synthetic phase-values and must be represented in the geometry corresponding to the real interferogram. The result is the so-called synthetic interferogram. This is the fastest method, because the two interferograms are in the same geometry. That's why a scaling of the fringe-frequency is not needed, nor is the phase-unwrapping. The phase in the synthetic interferogram is only dependant on the topography, but the phase in the real interferogram is influenced by the movement. In this way the phase of these two interferograms must be subtracted, such that the influence of the topography will be eliminated. The resulting interferogram is called a differential interferogram. Illustration 1 gives an overview of the 2-pass method. For reasons of simplicity, the SAR-scenes and the orbit data (needed for the generation of the synthetic interferogram) are labelled with "orbit x".

 

3-Pass Method:
For these technique there are three SAR-scenes needed, but no DEM. With these three scenes two real interferograms have to be generated. Because their master scenes are the same, both are in the same geometry. But their baselines are not identical. That's why a scaling of the fringe frequency is needed. Otherwise, the two interferograms would not be comparable, due to their different fringe frequencies. In the phase of one of the interferograms only the topographic influence is included, in the phase of the other one also the surface changes effects. To get the differential interferogram, again the phases of both interferograms have to be subtracted (Illustration 2).

4-Pass Method:
This method requires four SAR-scenes. As in the 3-pass method two real interferograms have to be generated. But because their master scenes are not identical, their geometries are different. This is why as a first step, the coregistration of the two interferograms is necessary. The subsequent steps are analogous to the 3-pass method (Illustration 3).

Comparison of the Presented Methods:
The 2-pass method is the most direct way to generate a differential interferogram. The rather complex steps of coregistration, fringe frequency scaling and phase-unwrapping are not needed. The only possible problem might be the absence of a DEM.

The 3-pass method compared with the 4-pass method has the advantage that no scene-coregistration is necessary. For its succesful application, three SAR-scenes are required, each having been taken with a short time difference relative to the event of interest (e.g. earthquake). If this is not the case, high coherence loss may prevent the accomplishment of this method. At this point the 4-pass method may have certain advantages. Because two interferograms with no dependancies on each other are generated, the time difference between the first and the second interferogram plays an insignificant role. That means, one can acquire the SAR-scenes for the second interferogram (the one without any surface changes effects) from when it's most suitable.

Time Frame

Phase investigations with differential SAR-Interferometry are done at RSL since 1995. Subject of investigation were the crustal dynamics (earthquakes) in Greece. For further informations see: Monitoring Crustal Dynamics Using Differential SAR-Interferometry.

Publications

• BIEGGER, S. [1998]:
  Untersuchungen zur Generierung eines DHM's von Kefalonia (Griechenland) mit Multi-Baseline SAR-Interferometrie, Diplomarbeit Geographisches Institut, Universität Zürich