Comments: Using a similar derivation as before (small angle approximation), it can be shown that the difference of the interferometric phase difference is proportional to the difference in range distance. A SAR interferogram of a flat terrain therefore shows a fringe pattern parallel to the flight direction, overlayed by the topographic phase. The "flat-earth" component can be estimated exactly from the geometry of the data acquisition, and has to be substracted if only the topographic fringes should be analysed.