We did this study a couple of years ago using multi-temporal composite data set of OSCAT2 at 12.5 km spatial resolution in concurrent with sigma0 data from AltiKa. Unfortunately, the OSCAT2 data that were used in this study were not made available public. However, I believe that there may be chances of getting enhanced resolution OSCAT2 data at the Brigham Young Univ webpage (scp.byu.edu) if you're interested but I havent checked it of late so I am not sure though. Please check there if you're looking for such data. And moreover, there may be chances for the availability of higher resolution data from ISRO too in near future.
I haven't done any data assimilation technique...only 'concurrent' use of data.
Please let me know if I have answered your questions or if you have more questions.