Maybe this question might be more physics related, but I've read about mostly in context of space science:

What is a delay-doppler map (DDM)?

I can find it

I know about Doppler and also about delay in the sense of Time-of-Flight, so I only can imagine that a DDM has something to do with measuring both of them for GNSS reflectometry, although I have problems working it out. In the end it comes down, that I'm not sure how such a figure has been achieved and how to read: Delay Doppler map (DDM)Delay Doppler map (DDM) from the CYGNSS mission


The DDM measures both the delay that a reflected signal takes to get back to you and the doppler (frequency) shift of that signal.

In your left most figure, what you are looking at is a picture of how a GPS signal reflected off of the ocean. The bright red spot is the point of "maximum specular reflection", that is, the point directly underneath the spacecraft that also happens to be the most reflective.

As the ocean waves make the ocean more...wavy...the reflection becomes less and less specular. Wind speeds also have something to do with it, among other effects that change both the frequency and the amount of signal reflected, and in which directions.

Since we know how the signal propagates (through an atmosphere or through space), we know the orbital parameters and transmitting powers/receiving antenna properties of the satellites generating/receiving we can create a theoretical model for what the surface should reflect like, given the unknowns like wind speed, wave height etc. (in asteroids, it would be things like albedo and surface roughness). One can then, through a LOT of signal processing, find those unknown parameters that best fit the reflected actual data. This can be used then to infer surface roughness (and really whatever else, like wind speed, that you have a model for).


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