Collocation fails for mountain region

I am trying to collocate a coherence image with a sentinel-2 image, the region that I am trying to collocate is in the High mountain Asia. For some reason, there is a major noticeable shift in the final result. My steps were as follows :

  1. Create a coregistered stack
  2. Calculate coherence using the Coregistration tool
  3. Subtract flat-earth phase and subtract the topographic phase by using external DEM (ALOS world 3D)
  4. Perform collocation using the collocation tool.

Result :-

RGB of the sentinel data

collocation for the coherence image

The Collocation tool will work only if your two images cover the same area, that is your AOI must appear complete in the two images to be collocated,

In this case it is covered, yet there is an issue.

Could you show the SAR image?

The SAR image is

The Optical image is

Well, it is difficult to say. I understand your master is the OLCI scene. Check the accuracy of the geocoding in OLCI and S1. In SNAP S3TBX preferences check the box use “per-pixel geocoding instead of tie-points”. I am not sure how your coherence band is accurate, you could also check that,