GUNW/GOFF `mask` cannot distinguish geocoding fill

[nemo794]

For constant PRF GUNW and GOFF datasets, the mask layers do not provide a way to distinguish between pixels that are geocoding fill and pixels that are within the invalid region of the radar's swath.

This is required for computing metrics on the region of the radar swath.

NOTE: In GCOV mask layers, the geocoding fill pixels are populated with 255, which is the fill value. Recommend the same for InSAR.

[xhuang-jpl]

Thank you @nemo794 for reporting this. This is the similar issue I have opened 2 years ago (https://github-fn.jpl.nasa.gov/isce-3/isce/issues/1916).

The fill value should be 255 if we run it on the GPU, but for CPU, due to the design of its geocoding API, we cannot set the fill value. As you can see the code here that we

For CPU geocoding:

isce3/python/packages/nisar/workflows/geocode_insar.py

Line 781 in 7bb4eb8

cpu_geocode_rasters(geocode_obj, geo_datasets, desired, freq,

For GPU geocoding:

isce3/python/packages/nisar/workflows/geocode_insar.py

Line 1202 in 7bb4eb8

desired_geo_dataset_names = ["mask"]