Within the framework of the higher-order Kirchhoff approximation, the properties of the electromagnetic scattering from sinusoidal water waves are presented, and the theoretical formulas up to third-order for describing the scattering field and its spectrum are derived. It shows that not only the spectral peaks which correspond to phase velocity of the water wave but also other discrete harmonic peaks can be found from the theoretical spectrum model. And the Doppler shifts of the spectral peaks are all integral multiple of the sinusoidal wave's frequency. For the backscattering field from a sinusoidal wave, the higher-order resonant peaks would also be found at different scattering angles, and the values of these peaks decrease with the scattering angle. On the other hand, the comparisons with the MoM demonstrate that the contributions of the slope-dependent terms can be generally neglected if the amplitude of the sinusoidal wave is small. However, if the waves slope is larger, the impact of the second order scattering becomes obvious and cannot be omitted.
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