Hello everyone.
In Range-Doppler Terrain Correction process, if the radiometric normalization box was checked, the effects resulting from local incident angle variation are suppressed, thus we need DEM information to calculate local angle. That is a rough understanding about this processing tools.
Since the IW mode’s incidence angle range from 29 to 45 degree, I guess there must exist a fixed angle under which the sigma-naught as the reference for other incident angle.
I did find an equation which seems 23 deg is a standard for ERS PRI product.

I found a relative topic here Incidence angle normalization @johngan say the angle is about 35 deg although I cannot find any information from official handbook.
anyone can provide some clue what is the standard incidence angle if it exits? thks

and I also find a reference incidence angle for Envisat ASAR, it seems to be 30 deg according to Potential for High Resolution Systematic Global Surface Soil Moisture Retrieval via Change Detection Using Sentinel-1, is it correct?

I am working to a project for deriving soil moisture using SENTINEL-1 data. The equation above is familiar to me. It is used to derive the soil moisture. For this equation, we specifically need to know what the reference incidence angle is. In the example above, the reference incidence angle is 30. When we calibrate the SENTINEL-1 image (using SNAP), we know that this process provides correction for the incidence angle. Do we know though if the incidence angle has been normalized to 30 deg , 35 deg or any other value in SNAP?

Hello Johngan.
Actually, I find no detailed information about normalized angle in SNAP for Sentinel-1. But in SarScape, you can specify that angle to any positive value degree, and if value was negative, middle of angle is regarded as default reference (which quite make sense).