Numerical simulations of scattering from time-varying, randomly rough surfaces

Recent years have seen the development of robust and efficient numerical te chniques for exact calculations of rough surface scattering. We discuss how such methods, typically formulated for time-independent surfaces, can be e xtended to calculate scattering from time-evolving ocean-like surfaces. Est imates are provided for the choice of parameters in such time-varying simul ations. The method of ordered multiple interactions (MOMI) is used to calcu late time-varying scattering from surfaces generated according to linear an d nonlinear (Creamer) models for incidence angles ranging from normal to lo w grazing. We discuss the runtime considerations and demonstrate that combi ning the MOMI with a fast multipole method (FMM)-type acceleration techniqu e makes large-scale time-varying Monte Carlo simulations possible. The aver age Doppler spectra of backscattered signals obtained from such simulations are compared for different incident angles, polarizations, and surface mod els. In particular, the simulations show a broadening of the Doppler spectr a for nonlinear surfaces, especially at low grazing angles (LGA) and a sepa ration of the vertical and horizontal polarization spectra at LGA for nonli near surfaces. This spectral separation at LGA is not observed when the lin ear surfaces are used.