A Study On Differential Interferometry In Subsidence
3. Data Processing
The data processing is taken from single look complex image for each scene, and each was identified by a location in latitude and longitude and by date with ground areas of approximately 100 Km x 100 Km with pixel spacing of 7.9 m at slant range and 3.9 m at azimuth. The data are in complex format, with 16-bits representations for each of the real and imaginary parts. One complex scene has 4900 range elements and 26000 azimuth lines with a total size of about 500 MB.
For purpose of demonstration, we performed a three-pass differential interferometry, using three SAR images from ERS1 and ERS2, dated Jan. 31, 1996, Feb. 1, 1996, and May 16, 1996, respectively. The first two images are taken on two consecutive days in tandem mode, thus having relatively high coherence. For urban area the average coherence is about 0.4 and displays clear fringe (Figure2). A DEM was firstly generated from this pair. Fairly good accuracy can be obtained except for high relief areas. The third image was taken much later, 105 days apart, and was used to produce the displacement filed during that period. For urban area the average coherence is about 0.2 with observable fringe pattern. Because the satellites are very close to each other, the base line is only 2 meters (Figure 3).
Figure 2 Interferogram and Coherence map of pair 01.31/02.01
Figure 3 Interferogram and Coherence map of pair 01.31/05.16
Figure 4 Subsidence from Differential Interferometry
Figure 5 Subsidence from GPSs
