Doppler spectra of the radar backscatter from the sea surface obtained from a three-scale composite surface model

The Doppler shift of the backscattered radar signal from the sea surface can be used for determining the line-of-sight velocity of the scatterers and thus for measuring surface currents and ocean wave spectra. The bandwidth of the instantaneous Doppler spectrum, which is associated with the distribution of the line-of-sight velocity within the radar resolution cell, is a measure for the scene coherence time which enters into the SAR imaging mechanism for ocean scenes. Experimental results indicate that the measured Doppler shift can significantly exceed the values expected from simple first-order considerations. Furthermore, the Doppler bandwidth has been found to vary along long ocean waves, and a difference between the Doppler bandwidths for upwind and downwind looking radars has been observed. The author presents a new model for the computation of Doppler spectra which includes the hydrodynamic modulation of short waves by longer waves. The proposed model can reproduce the experimental results at least qualitatively at the present stage.