I would like to calculate Radar Vegetation Index using Sentinel-1 data. The formula is: 8HV/(HH+VV+2HV) (all of them are sigma).
Sentinel-1 doesn’t have HH polarization. So, I have a question - how to calculate such index using Sentinel-1? Is there some modified formula may me or something?

Thank you!
I actually did VV/VH and it works very well for me for an Ascending orbit and not so well for Descending. So I fought, to try something else may be.

Does anyone have an idea how this formula should look like if i have already db units? I first thought replacing the division by a subtraction would be correct according to the log laws but the result was strange.

Sorry, I’ve little bit misinformed you. I used that formula to convert from linear units to ratio (how many times backscatter is weaker than initial signal). I did 1/linear units.
For db I used a little bit more complicated formula to convert it to the ratio mentioned above
=10^(ABS(Value in db)/10)
I believe that this formula is good, but the results are slightly different from 1/linear units.
In any case I would like to use linear units which SNAP provides. It looks easier than tricky works wiht dBs.

Yes, it would be like you’ve written.
If Sentinel-1 had HH polarization, I would use RVI formula too.
May be it would be useful for you, we used VV/VH polarization ratio formula for our researches. And after that, we found a good relationship with vegetation indices.

Thank you for your prompt reply.
According to the above-mentioned paper where it assumed that ϭHH ≈ ϭVV and since Sentinel-1 has VH and VV polarizations so can we modify the equation as follows?

Yes, this is an option. Different scatterers might be targeted by VV but in general, the index is about the contribution of volume scattering which is indicated by cross-polarized response.