Interferogram with snap low coherence

Hi there,
I am new to InSAR and I am trying to create an interferogram in mountainous terrain in Iceland with one year in between using Envisat data. Whatever I do the coherence is really bad. Even in the flat areas. (when I use a shorter timeframe like one month then the flat areas are ok.) I used many different datasets.
Is there anything I can do to improve the coherence? I know that people have used this data before for DInSAR.
as an example I have used the following images:
Am I even using the right imagery or should I use a different Envisat product?

For processing I follow the steps I gathered in the forums here:

  • Apply orbit files (Doris Precise)
  • Coregister
  • subset
  • Interferogram formation (with external DEM for topographic phase subtraction)
    - -here I can already see that the coherence is very low.
  • Goldstein phase filter
  • snaphu export
  • unwrapping
  • snaphu import

I have spent so many days on trying to work this out and I would be really grateful for any advice.

Pretty much the only thing you can do is to process 30-day repeat pairs and even then the coherence will be spotty over vegetated areas.