I have run into similar problems when trying to use ALOS-2 images with level 1.5 processing (also in CEOS format). What I managed to gather from the SNAP source code and the images' metadata, the geocoding problem seems to stem from the following.
- The provided georeferencing metadata does not account for elevation and is quite inaccurate and not too useful. The image should really be reprojected with the Terrain-Correction operator instead, but...
- The image, which has been reprojected to ground range, does not have the slant-range-to-ground-range coefficients included in its metadata, making accurate geometric terrain correction currently impossible. SNAP does calculate approximate SRGR coefficients from the near-, mid- and far-range values provided in the metadata but this is not enough.
I resorted to using a trial version of Geomatica to take care of geometric terrain correction, which worked surprisingly well. I guess they use the provided georeferencing metadata to calculate the correct SRGR coefficients before proceeding with terrain correction.
@lveci Is my understanding of the situation correct and are there any plans to improve the geocoding accuracy of ALOS level 1.5 products in SNAP?