Coherence improvement of cross-interferometric pair by a block azimuth filtering

ERS-ENVISAT cross-interferometry with a small height ambiguity is useful to measure the topography in the regions of low slope. Azimuth common-band filtering is a key pre-processing for the cross-interferometry. An adaptive block azimuth filtering was designed and applied to the ERS-ENVISAT tandem pair. As range bin increases, the Doppler centroid of the ERS increases while that of the ENVISAT decreases. It results in systematic increment of the Doppler centroid differences as range bin increases. The coherence of 0.7 in the near range reduced to 0.2 in the far ranges. The block azimuth filter estimates Doppler centroids of at each block, and adjusts band width of the filter. The coherence degradation caused by range-dependent Doppler centroid can be compensated by this method. The coherence is better than conventional azimuth filtering about 0.1 in far range. The resulting interferogram shows improved coherences between 0.8 to 0.4 in near and far range, respectively. However there is still gap of coherence in near and far ranges, it is resulted from inherent problem of less overlapped bandwidth at far range. Although the total improvement was not significant, it is valuable for selection of permanent scatterers in the far range. Test results are discussed in this paper.