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.