what does the “APPLY orbit file” for？is it necessary？
This is a SNAP inline step for acquiring the satellite orbit file. Usually the satellite flying orbit track is being detected by many sensors, like mounted gyro, GPS, and also ground observations. To calculate out the precise orbit data takes time, so the precise orbit data are not included in many SAR satellite data bundles. SNAP is trying to get more precise orbit data to help to improve the geocoding and other SAR processing results.
Suggestion: try to do this before every other SAR preprocessing steps.
thank you very much，and you mean we should take apply orbit file as the first step before all pre-processing？
The preprocessing steps are very tricky. See this post:
But in my point of view, do “apply orbit” before everything is totally OK.
thanks a lot