How to normalize the different incidence angle on different ASAR image

I have over 20 ASAR WSM data with different incidence angle (from about 20-40 degree). I want to normalize all data into the same incidence angle (23 degree) to compare the targets backscattering coefficients (sigma naught). But I do not know how to do that. So would you like to help me about that? Thanks

Apply the calibration operator

Sigma0 of distributed targets is a function of instrument parameters such as frequency, polarisation and incidence angle. So unless you have a model for the expected variation in sigma0 with incidence for a particular frequency and polarisation it is not possible to convert a sigma0 from one incidence angle to another.

I have two ALOS-2 images (one in SM2 mode and other in SM3 mode) with different incidence angles with a common overlapping area. I want to extract and compare the sigma-naughts of that common area. But since sigma-naught is a function of incidence angle, I cannot compare them.
How do I bring both the sigma-naught values to a common ref level for comparison??

Please help.

Thanks !!