- Since the DN is roughly just the square root of the backscatter coefficient (aka normalized radar cross section or sigma nought), which is a dimensionless quantity, the DN itself is dimensionless as well.
- There calibration equations for Sentinel-1 are described in detail in this document and in the SNAP help as well.
There is, of course, no need to do the calibration yourself. You can use the Calibration operator in SNAP (Radar→Radiometric→Calibrate) to convert DN to sigma/beta/gamma nought. You might want to add the Radiometric Terrain Flattening operator in conjunction to it, too, to further improve the radiometric calibration by removing some topographic effects that are not compensated for in the Calibration step.