Analytical Study of Along-Track InSAR Imaging of a Distributed Evolving Target With Application to Phase and Coherence Signatures of Breakers and Whitecaps

Airborne along-track interferometric synthetic aperture radar (AT-InSAR) observations of the ocean surface often exhibit peculiar drops in coherence that can be quite compact (a few meters in size) and seem to be associated with breaking waves and whitecaps. Intriguingly, these signatures in the coherence map can be quite sharp and rather conspicuous, whereas the corresponding SAR image intensity in the same area may either suffer from apparent motion-related defocusing or lack any recognizable corresponding feature altogether. Such coherence drops are often accompanied by manifest perturbations in the interferometric phase. While there have been suggestions to use these details for breaker detection and even for breaker velocity retrieval, a basic understanding of why and how the patterns form needs to be developed first. To gain some insight in these phenomena, we consider AT-InSAR imaging of a simplified model feature that is a moving distributed target with a certain decorrelation time. The approach yields closed-form expressions that can be analyzed to identify possible mechanisms for forming such signatures. It appears that the intuitive explanation linking the drop in coherence to faster temporal decorrelation (something expected for a turbulent breaker) has its limitations when the feature is azimuthally small. Interference of the defocused and/or displaced image with the background may be the mechanism in the latter case.