Orbit files are only provided for a selection of satellites. Kompsat does not require the application of orbit files, because it already has good orbit state vectors int he metadata.
Radiometric Terrain Flattening corrects radiometric distortions caused by the topography. Examples are given here: S1 radiometric correction
If the topography of your area is extreme and causes many of the slopes facing the sensor to be very bright in the radar image, you can consider applying terran flattening (using SRTM 1Sec at best)