I try to extract the water area from Sentinel-1 radar images, but I do not know why the backscatter coefficient of the sea in VV polarization is more than that of VH polarization. “VH polarization is more sensitive than other(VV, HH, HV) polarizarion”, isn’t it ?
Cross-polarization (VH) is lower than co-polarization (VV) over flat surfaces because only a small part of the signal returns polarized (vertical turning horizontal) to the sensor.
VH is not generally more sensitive than VV. But it is often more suitable to describe variations in volume scattering (from vegetation or snow).
Some notes on this: Difference between radar channels
Thank you @ABraun
I wonder why the difference of backscatter coefficient between VV and VH polarization on 17 March is so greater than that on 29 March?
there could be so many reasons, depending on landcover, atmospheric conditions and characteristics of the imaged surfaces. Nice overviews are given here: https://servirglobal.net/Global/Articles/Article/2674/sar-handbook-comprehensive-methodologies-for-forest-monitoring-and-biomass-estimation