I know that flat earth phase affects the unwrapping process and it does cause a series of straight parallel fringes as if the topography does not exist but I can’t quite understand what it is and how it is related to phase unwrapping. Plus, in differential InSAR, is it impossible to have the satellite to image a certain area at different dates from the exact same position? otherwise there would not be a topographic phase caused by the slight drift in satellite position each time? If someone could provide a simple explanation would be appreciated.

the “flat earth phase” tries to estimate the impact on the earth’s curvature on the change of phase return.

the “topographic phase” uses a DEM to additionally estimate the impact of topographic variations. Both are estimated and removed from the interferogram to minimize their impact on differential interferometry.

SNAP estimates the positions of the satellites (optimized by Apply Orbit file) to calculate their perpendicular baseline to estimate how much elevation difference is represented by one fringe cycle.

Therefore, the position does not need to be exactly the same, but the difference has to be known as good as possible.

I recommend these great resources: InSAR Principles: Guidelines for SAR Interferometry Processing and Interpretation (ESA TM-19)

I do appreciate the resources but just a question to correct my probable misconception; The horizontal distance between the target and the Radar nadir point “ground range” is not perfectly straight “assuming there’s no topography” . Is this the effect of earth curvature on the return phase?

where exactly is this from?

There it is

May you correct me if I misunderstood! I appreciate your cooperation.

I can recommend the short introduction to SAR & InSAR in my thesis as a starting point: