I think SNAP actually does. Flattening to Gamma corrects radiometric distortions caused by topography. It is based on this work:
http://www.tandfonline.com/doi/abs/10.5589/m11-059?mobileUi=0&journalCode=ujrs20
https://www.geo.uzh.ch/microsite/rsl-documents/research/publications/peer-reviewed-articles/201110-CJRS-Small-RSAT2-RTC-2669677312/201110-CJRS-Small-RSAT2-RTC.pdf