FILTRANGE

In this chapter the processing of step FILTRANGE is described. This optional step filters the spectra in range direction of master and slave to reduce noise in the interferogram. The noise reduction results from filtering out non overlapping parts of the spectrum. This spectral non overlap in range between master and slave is caused by a slightly different viewing angle of both sensors. The longer the perpendicular baseline, the smaller the overlapping part. Eventually a baseline of about 1100 m results in no overlap at all (the critical baseline for ERS). (Assuming no local terrain slope.) A reduction of typically 10-20% in the number of residues can be achieved.

Method porbits filters based on the orbits (perpendicular baseline) for a constant (given) terrain slope. Perform this step after coarse coregistration, since the approximate overlap is used to filter both images. The output images are cropped to this overlap. To filter 'on the save side', i.e., not to filter out too much, use a negative terrain slope of, e.g., 10 degrees.
This step is not recommended, except perhaps to improve the coregistration polynomial for long baseline pairs. After the resampling the range filtering then could be repeated on the original data with the adaptive algorithm.

Method adaptive should be performed after the resampling of the slave to the master grid, because the fringe frequency is estimated from the interferogram (that is temporary computed). It is performed simulataneous for the master and slave image.