Dear @jun_lu,
thank you for your reply.
Indeed, that’s the problem. If I open in SNAP the DEM downloaded from 30-Meter SRTM Elevation Data Downloader, I can see for example that for the pixels belonging to the sea the elevation is 0 (in according to the EGM96 reference). On the other side if I use the option “add elevation band” in the terrain correction tool, considering SRTM1sec as a reference, the band “elevation” that represents the used DEM, shows completely different values from the DEM downloaded: for example, looking again to the sea, all the pixels are not anymore 0 but a variable value around 55m.
Now my question is: when I use “add elevation band”, the SRT1sec DEM in EGM96 reference is in some way transformed in a new reference system (ellipsoid I guess)? Why the sea pixels are so different in elevation values, considering that the DEM is the same (SRTM1sec)?
Thank you again for your help.
PS Just to be sure, it is the same happened in this post
S Savastano