Coherence

Concept

Coherence is a measure of correlation in an interferogram.

Coherence ranges from 0.0, where there is no useful information in the interferogram; to 1.0, where there is no noise in the interferogram (a perfect interferogram). Both extremes are rarely seen-- most images lie somewhere in between.

Coherence is affected by, in approximate order:

Local slope (steep slopes lead to low coherence)
Properties of the surface being imaged (vegetated or moving surfaces have low coherence).
Time lag between the passes in an interferogram (long lags lead to low coherence)
The baseline (large baselines lead to low coherence)
Technical details of the generation of the interferogram (poor co-registration or resampling leads to low coherence)

Application

Coherence can serve as a measure of the quality of an interferogram; tell you more about the surface type (vegetated vs. rock); or tell you when a tiny, otherwise invisible change has occured in the image.

A British group was actually able to detect the trail of a group of tanks on a British military base by the change in coherence.

Example

Coherence is only visible in the phase image of an interferogram.

High coherence makes for attractive, not-noisy interferograms. Here is a coherent phase image of a mountain.

Low coherence makes unattractive, noisy interferograms. Often, these interferograms are difficult to phase unwrap. Here is an incoherent phase image of a mountain.

Formula

Coherence is the magnitude of an interferogram's pixels divided by the product of the magnitudes of the original image's pixels. It is usually calculated on a small "window" of pixels at a time, from the complex interferogram and images.

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Last Updated: September 1, 1998
If you have any questions, please feel free to email olawlor@acm.org