Satellite Imaging

Ikonos Satellite Imaging
Thermal Satellite Imaging
Differential Radar Interferometry (DInSAR)

Differential Radar Interferometry (DInSAR)

Differential interferometry (DInSAR) applied to a digital elevation models (DEM) proves to be a powerful tool to detect surface displacements on the order of a few centimetres.

Differential Radar Interferometry (DInSAR) was applied on Nisyros to study the regional deformation of the area in conjunction with the GPS observations existing from the area.

Suitable interferometric pairs of SAR images were selected after searching the European Space Agency FRINGE database for images satisfying the necessary technical parameters, such as the baseline and temporal separation. Suitable data were selected according to the following criteria: Bp <100 m; SAR acquisitions of the same season to avoid seasonal land use changes; and temporal separation including the period 1996 to 1997 (maximum of seismic activity).

The two-pass differential interferometry method (or DEM-elimination method) was applied. This method employs two SAR images, producing one interferogram. To perform the differential, another interferogram has to be synthesized.

The synthesized interferogram is generated from an existing digital elevation model (DEM). It is then subtracted from the original interferogram, thereby removing all fringes that relate to ground elevation, leaving only fringes that represent surface displacements.

The phase differences that remain as fringes in the differential interferogram are a result of range changes of any displaced point on the ground from one interferogram to the next. In the differential interferogram, each fringe is directly related to the radar wavelength (56 mm for ERS satellites) and represents a displacement relative to the satellite of only half the above wavelength (28 mm). Results are displayed in Figure 23.

The main steps of the interferometric processing include:

• External DEM production.
• Coregistration analysis to validate the input master/slave interferometric pair for spatial and spectral overlap.
• Coregistration of the external DEM to master at a sub-pixel level accuracy.
• Interferogram generation. Range and azimuth spectral filtering should be applied to reduce the decorrelation of baseline and azimuth spectral shift. Flat earth phase and optionally topographic phase should also be subtracted.
• Phase-coherence imagery generation for evaluating the quality of interferogram for phase unwrapping and random changes of land surface.
• Interferogram phase unwrap using up-to-date algorithms.

The above processing generates two InSAR geo-coded images for the production of a differential interferogram for deformation estimates. The differential interferogram (change map) and the coherence imagery is analyzed.

[Fig.23 subset] one fringe cycle = 28 mm displacement

Fig. 23 The produced interferometric image from the first interferometric pair, covering the period 22 May 1995 to 22 January 1996, coinciding with the beginning of the seismic unrest period in Nisyros Island (Fig. 17). One interferometric fringe, equivalent to 28 mm of ground deformation along the slant direction, was recognized and evaluated. Most of the island is covered by a circular shaped fringe, which extends from the NW part, covers the southern island and dies out (due to poor coherence) on the NE part of Nisyros. A second fringe located on the SW extremity of the island can also be recognized. National Kapodistrian University of Athens (NKUA)
(Click on image to enlarge).

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