Modeling of the electromagnetic (EM) scattering mechanisms from two-dimensional (2-D) time-evolving sea surfaces is
a particularly complicated problem. The intricate structure of surface waves and the scattering models noticeably
influence the simulated radar signatures. Scattering calculation and Doppler spectra from the sea surfaces have been
intensively studied, experimentally as well as theoretically in the past decades. However, to author's knowledge, very
few results can be found in literatures for Doppler spectra from two-dimensional time-evolving nonlinear sea surfaces.
In this work we focus on the Doppler spectral characteristics from 2-D time-evolving nonlinear sea surfaces. Based on
Creamer's sea surface model, the first-order small slope approximation (SSA) method is applied to solve the 3-D
scattering problem. The Doppler spectra of the backscattered signals from 2-D time-evolving sea surfaces are studied for
different incident angles (from normal to grazing) as well as wind directions (from upwind to crosswind). The impacts of
the nonlinearity on the Doppler shifts and spectral widths of backscattered signals are analyzed.