I have ALOS-2 quad-pol data over the moderate topographical region. I am facing problem in calculating accurate sigma naught (backscatter coefficient). I have performed following processing steps. Kindly guide what’s the correct progressing sequence and where I have gone wrong in my processing. In my case, sigma naught values are very low ranging from ~-28 to -55 db. Line profile of sigma naught for HV is shown in the picture below.
- Slant Range to Ground Range Conversion
- Computing Sigma naught using 10*log10(I^2+Q^2)-83
- Ranger Doppler Terrain Correction
SNAP has a built-in calibration for ALOS-2, so I would suggest you don’t change the pixel values before this (by multi-looking and resampling) and apply the Radiometric calibration as the very first step.
Additionally, you have to consider that the amout of signal which was transmitted horizontally and returns vertically is extremely low. Both HV and VH should be clearly smaller than HH and VV respectively.
I wonder what is the Observation Mode of your data?
I had my L1.1 data in WBD (Scan SAR nominal [14MHz] mode Dual polarization) mode and was not able to open them in the first place.