**Comments: **SAR
interferometry introduces also another parameter: the interferometric
coherence. It is defined as the normalized complex cross-correlation
between the two images (above formula). As only one measurement of
the interferogram is available, the expectation value has to be
replaced by a spatial average. The coherence describes the amount of
phase noise in an interferogram, a value of 1.0 comes from a perfect
smooth phase, a value of 0.0 means only noise.

The interferometric phase is noisy, because the assumption that the object phase is identical in both SAR images is not always correct. There are several reasons for a change.