Validation of the X-SAR SRTM DEM for ERS and JERS SAR geocoding and 2-pass differential interferometry in alpine regions

The X-SAR SRTM DEM of the alpine region in Switzerland was validated for SAR geocoding and 2-pass differential interferometry using ERS and JERS data. The absolute horizontal and vertical accuracies of the X-SAR SRTM DEM are in general in line with the nominal indications, but in very rugged areas we found horizontal offsets of up to 50 m in both northing and easting directions and some extreme absolute height errors up to even 1000 m. Also visible in the X-SAR SRTM DEM is an important noise for the slopes of SAR illumination. The ERS and JERS SAR images geocoded with the X-SAR SRTM DEM do not contain significant geometric artifacts, but, in parallel to what was discussed before, show in comparison to the map offsets in both easting and northing directions. The matching of geocoded products of descending and ascending modes is very good, i.e. on the order of 1 to 2 pixels. Considering that in our geocoding approach the geocoded SAR images are accurately co-registered to the DEM, the horizontal offsets have no influence for 2-pass differential interferometry. We found that the X-SAR SRTM DEM is well suited for ERS and JERS interferograms of the descending mode of baselines up to 100 m. For the differential interferometric analysis of ERS SAR data in ascending mode, however, attention has to be paid to the noise of the slopes facing away from the X-SAR.