Doppler spectrum of microwave SAR signals from two-dimensional time-varying sea surface

Within the framework of the two-scale scattering model, Doppler shift and bandwidth of microwave scattering field from two-dimensional (2-D) sea surface have been simulated. And the comparisons of the simulated results with the Doppler anomaly measurements from Envisat ASAR demonstrate the rationality of the formulas proposed by us. Meanwhile, the influences of the tilt and the hydrodynamic modulations due to large-scale underlying waves on Doppler shift and bandwidth have also been discussed in detail. Numerical results show that the Doppler shifts for HH-and VV-polarizations are both observably affected by the tilt and the hydrodynamic modulations and the values of the predicted Doppler shifts are always larger than Bragg frequency. Because the radar scattering coefficient is more sensitive to sea surface slope in HH polarization than in VV, the predicted Doppler shifts in HH polarization are always somewhat larger than those in VV. Moreover, we also find that there is a remarkable difference between the Doppler shift predicted in upwind and downwind direction due to the hydrodynamic modulations. Relative to Doppler shift, bandwidth of Doppler spectrum is mainly affected by orbit velocity of large-scale waves and is insensitive to the modulations and radar polarizations.