Dynamical Properties of Sea Surface Microwave Backscatter at Low-Incidence: Correlation Time and Doppler Shift

We investigate some temporal properties of the microwave backscattered field from the sea surface at low incidence, namely, the decorrelation time and the Doppler shift distribution. These quantities may have an important impact on the performances of the altimeter and synthetic aperture radar systems in Ku- and Ka-bands and must be accurately evaluated. In the framework of classical analytical scattering models and for realistic sea spectra, we obtain a simple expression for the decorrelation time with respect to the main sea state parameters and the scattering geometry. We further propose an original approach based on a time-domain estimator to evaluate the distribution of instantaneous Doppler shifts and the Doppler centroid. The evolution of the latter with the sea state and scattering angles is calculated and discussed. A procedure is proposed to recover the full two-sided Doppler spectrum. We discuss the use of the Doppler shift in view of the geophysical parameter retrieval at low incidence. We find that the surface wind vector can, in principle, be well estimated from the azimuthal variation of the Doppler shift, whereas the signature of the surface current is not sufficient to allow for its estimation.