**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.