For dual-pol Sentinel-1 like data, RVI can be formed by 4*VH/(VV+VH). Here, VV and VH are in linear power scale, not in dB.
Be careful with the processing step (old SNAP7.0 version produces opposite channel c11==VV and c22==VH). However, latest version is working fine.
Read: Covariance Matrix C2 for Sentinel-1 --Diagonal Elements
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