This paper proposes to solve the extralarge scale Electromagnetic(EM) scattering problem on a wind-driven sea surface at Low-grazing-angle(LGA) directed against a shore-based radar with an Iterative physical optics(IPO) model. Using Monte Carlo simulations,it compares the average Doppler spectra of backscattered signals at different angles and sea states. The methods are: the forward-backward methodology and its modification with under-relaxation iteration which improves convergence and stability of the IPO; the local iteration methodology and the Fast far-field approximation(Fa FFA)in the matrix-vector product which reduce the computational complexity based on the scattering characteristics of sea surface. The simulations show a broadening effect of the Doppler spectra in a more complicated sea state at LGA. This paper proposes to solve the extralarge scale Electromagnetic (EM) scattering problem on a wind-driven sea surface at Low-grazing-angle (LGA) directed against a shore-based radar with an Iterative physical optics (IPO) model. Using Monte Carlo simulations, it compares the average Doppler spectra of backscattered signals at different angles and sea states. The methods are: the forward-backward methodology and its modification with under-relaxation iteration which improves convergence and stability of the IPO; the local iteration methodology and the Fast far-field approximation (Fa FFA) in the matrix-vector product which reduce the computational complexity based on the scattering characteristics of sea surface. The simulations show a broadening effect of the Doppler spectra in a more complicated sea state at LGA.