Here are some of my observations about the Range Doppler-based Terrain Correction in the latest version of SNAP 9.0.0 using one Sentinel-1A SAR image and one ICEYE Spotlight SAR image:
- If I go with the option of “Copernicus 30m Global DEM (Auto Download)”, it works fine. The tool will download a copy of the original Copernicus 30m (in tiles) to a folder like this “auxdata\dem\Copernicus 30m Global DEM”. NOTE these DEMs are still with original vertical datum (i.e., with EGM2008).
- If I go with the option of external DEM and select a local Copernicus 30m DEM file already with WGS84 ellipsoidal height, without checking “Apply Earth Gravitational Model”, the outputs will have some geolocational offsets despite that the online help says the external DEM “must be with geographic coordinates (Plat, Plon, Ph) referred to global geodetic ellipsoid reference WGS84 (and height in meters).”. Also tried to checking “Apply Earth Gravitational Model”, which did not make any differences.
- If I go with the option of external DEM and select a local Copernicus 30m DEM file that is still with original vertical datum (i.e., EGM2008), without checking “Apply Earth Gravitational Model”, the outputs are the same as the ones from “Auto Download”.