Use of the coherence of synthetic aperture radar cross spectra for ocean wave measurements

A noise model for synthetic aperture radar (SAR) look cross spectra (LCS) acquired over the ocean is proposed. The study is meant to contribute to the improvement of algorithms for retrieval of two dimensional ocean wave spectra from LCS. Error bars for the LCS phase, which contains information on the ocean wave phase speed and propagation direction are derived. The error estimates depend on the respective LCS coherence and the amount of smoothing applied in the LCS estimation process. A model for the LCS coherence is introduced. The first part of the model describes the dependence of the coherence on system parameters like spatial resolution. The second part is associated with the motion of the imaged ocean wave field. Decorrelation is shown to be caused by the coupling of dispersive ocean wave components in the SAR image formation process. Forward simulations for the cross spectrum coherence are carried out using parameterized swell and wind sea spectra. Coherence is estimated from a global data set of reprocessed ERS-2 wave mode data and compared to theory.