The intensity image should be used for calibration and subsequent classification of geophysical features. To radiometrically calibrate the intensity, use the Calibrate tool in the Sentinel-1 Toolbox (SAR Processing > Radiometric > Calibrate). The S-1 Level 1 GRD product includes several Look-Up Tables (LUTs) to convert intensity values into sigma or gamma naught values. Use these LUTs as the processing parameters for the Calibration tool.
Using amplitude would produce a calibration result of a lower magnitude than using intensity because intensity equals amplitude squared. The calibration is applied using the equation
For more information about the calibration process see the ESA Sentinel-1 Technical Guide.
Intensity depends on the scattering properties of surfaces (if the spatial extent of the scattering medium is large relative to the signal wavelength) or objects (if the spatial extent of the scattering medium is smaller than the signal wavelength). The effectiveness of a scatterer is determined by the radar cross section (RCS). Many parameters influence the RCS: surface roughness, local geomorphology, scatterer density, 3-D distribution of scatterers, and the dielectric properties of the scene. These variables can help classify features in an intensity image.