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ERS-2 Zero-Gyro-Mode Data Application Showcases

Urs Wegmuller(1), Maurizio Santoro(1), Charles Werner(1), Tazio Strozzi(1), Andreas Wiesmann(1) and Wolfgang Lengert(2)

(1) Gamma Remote Sensing AG, Worbstrasse 225, CH-3073 Guemligen, Switzerland
(2) ESA-ESRIN, Via Galileo Galilei, 00044 Frascati, Italy

Abstract

Since 2001 ERS-2 operates in Zero-Gyro Mode (ZGM), which does not guarantee the very high stability of the satellite’s orbital attitude as defined in the mission requirements. The consequence is a continuous variations of the Doppler Centroid for repeated acquisitions over a certain area. In spite of this degradation of the system the ERS-2 data can still be used for many applications, including interferometry., Over the last years little use was made of the ERS-2 data acquired after 2001, this in spite of excellent temporal coverage over most of Europe, as well as parts of North America and Asia. An important reason for this „under-use“ of the ERS-2 data is that the archives did not provide comprehensive information on the Doppler Centroids, which implied that it was not known beforehand if the Doppler spectra of a selected pair overlapped, which is a pre-requisite for interferometry. This lacking information is currently being filled in by ESA to valorize the ERS-2 archive.

The scope of this contribution is to demonstrate with a number of showcases that ERS-2 ZGM data still allow InSAR applications. Archived ERS-2 data of the period 2001 – 2004 are of particular interest for surface deformation monitoring. For many sites very few consistent ASAR acquisitions exist for this period. ERS-2 ZGM data are therefore of high interest to close this temporal gap, in particular in cases where the deformation pattern changes over time.

Another unique opportunity offered by ERS-2 ZGM data is ERS-2 – ASAR cross-interferometry. The almost simultaneous acquisition of SAR images by the ERS-2 and the ENVISAT satellites (in ERS-like mode, i.e. ASAR IS2 VV-polarization) allows the generation of a new type of interferogram characterized by a short 28 minutes repeat-pass interval. However, because of the slightly different sensor frequency, cross-interferograms show coherence only under particular conditions. Besides the requirement concerning an at least partial Doppler spectra overlap, only at perpendicular baselines of approximately 2 kilometers can the look-angle effect on the reflectivity spectrum compensate for the carrier frequency difference effect. Given the large baseline and short time interval ERS-ENVISAT cross-interferometry is expected to have a good potential for the generation of precise DEMs in relatively flat areas. There is interest in such application e.g. in coastal zones with low relief and wetlands. Furthermore, ERS2-ENVISAT cross-interferograms are of interest to derive flow rates for fast moving Arctic glaciers.

For the purpose of assuring many more acquisitions suited for ERS2-ENVISAT cross-interferometry ESA is currently operating ERS-2 and ENVISAT in a constellation.

 

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry