**Comments: **A dominant frequency can be
estimated by a Fourier-transform. The Fourier-transform of the
complex interferogram therefore shows a maximum at the place of the
dominant flat-earth fringe frequency. An obtained number of ffour
means that four fringe cycles are present, i.e. that the flat-earth
corresponds to a phase ramp from zero to four time two pi.

Also in azimuth a certain fringe frequency can occur. This is due to non-parallel orbits.