Variations of interferometric phase difference

Why even if there were no variations in elevations there will still be variations of interferometric phase difference? I tried experimenting it to areas with flat elevations but still the output has variations in its interferometric phase difference.

besides topography, an interferogram consists of many different phase contributions, e.g.

  • surface displacement
  • atmospheric disturbance
  • noise due to phase decorrelation
  • ionospheric effects
  • orbit inaccuracies

It is explained in this tutorial: Sentinel-1 TOPS interferometry and also here: InSAR Principles (notes on noise on page 26)

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