I am not sure if I understand your question completely, but if you want to extract the RCS out of uncalibrated radar data you need the measurement of a reference object with a defined RCS. In addition to this you need the noise level of the system and range information of your targets. If you have all this you have to remove the noise power from the measurements, convert the remaining value of your reference object to the defined RCS and then correct the remaining data with the R4 law from the radar equation. But to be honest, I don't think that this is available for Sentinel 1 data. And it would be better to have several reference objects in different ranges.
So, I am a bit sorry for that answer, but I think it is not possible to extract the true RCS out of sentinel 1 data. Although I haven't tried and it's only a guess.