I am currently facing an issue related to orbit correction in SNAP.
According to the post linked below, orbit correction merely replaces the orbit state vectors, so it should not matter at which stage it is applied in the processing chain.
However, in my experience, when I apply orbit correction before performing terrain correction on Sentinel-1 IW SLC data, everything works fine. But if I reverse the order—applying terrain correction first and then orbit correction—the orbit correction does not seem to take effect.
The orbit state vectors in the SAR product metadata can be inaccurate. The Apply Orbit File operator refines the orbit state vectors using the precise orbit file downloaded from the ESA GNSS Hub. The accuracy of the orbit state vectors is crucial for precise geolocation and geometric correction of SAR images. For example, the accuracy of Terrain Correction (and many other operators) significantly depends on the accuracy of the orbit state vectors. That’s why orbit correction is generally performed at the first. For the thermal noise example you mentioned, the thermal noise removal operator does not use the orbit information. That’s why it can be applied before the orbit correction in this case. To play safe, always apply the orbit correction first.