"Unbiased" coherence estimation

development
1

Hi everyone,

It is well known that the coherence is a measure of the phase stability. It is also a way to predict phase reliability.

In high fringe rates, one issue is that even if the fringes are perfect, the fringes are decreasing the coherence, because we observe a strong variation of the phase value inside the window of computation. It is interpretated as noise but that’s just a fast moving area.

One idea to solve the problem is to locally unwrap the phase inside the window, fitting a plane to the unwrapped values and remove the plane from the original values.

In that way, we remove the influence of beautiful high fringe rate but keep the noise that is translated in the coherence value. The processed was presented in 1995 in a SPIE conference.

image

Just to give you an idea : left is “unbiased” coherence and right is the coherence computed with SNAP
spiral_coherence

Just to show you that it’s not “cheating”. Here’s the high fringe rate in the top right of the previous image resulting in a low coherence in the biased coherence computation. As you can see, the phase is almost perfect. Super easy to unwrap. But the coherence is low.

image
image.png777×731 28.9 KB

That kind of algorithm would be super useful in SNAP in fast moving areas. Just a suggestion.

Thanks for reading

Quentin

2

There’s an option “Subtract topographic phase” in the Coherence estimation operator that should be doing exactly that. You of course need a good DEM to really benefit from the functionality.

3

Thanks for your response.

However it does not exactly that. Maybe I was not clear enough. In the illustrations above you only have displacement phase (no flat Earth nor topo). But the coherence of the displacement phase is decreased because of the high fringe rate (of the displacement), though the phase is unbelievable.

But with the aforementioned technique where you remove the unwrapped phase plane, you flatten the phase and make sure that decorrelation is not affected by the fringe pattern itself.

4

Ok got it, can you find a reference publication for the technique?

5

I can. But note that the technique is really poorly known. It was performed to remove topographic fringes (when the topo was unknown) but is now also used in our institute for differential interferometry.

Section 4.1 :

derauw1995 - Phase unwrapping using coherence measurements…pdf (607.1 KB)