Algorithms of sea surface remote sensing are based on changes of Doppler shifts, which are measured by a Doppler radar. Microwave irradiation width on the sea surface and time taken to collect data for frequency analysis influence Doppler spectra. In order to evaluate the influences of these parameters in observing algorithms, a simulation of microwave backscattering from numerical sea surface was done in time domain to obtain Doppler spectra. Doppler spectra have been simulated in the case of various numerical regular waves. In the case of the microwave irradiation width is larger than the wavelength of the numerical regular wave or the Fourier transform time for the frequency analysis is longer than the period of the numerical regular wave, the peak value of Doppler spectra shows the phase velocity of the Bragg resonance wave. The results show the principle of measuring sea surface current. In the case of the microwave irradiation width is smaller than the wavelength of the numerical regular wave or the Fourier transform time is shorter than the period of the numerical regular wave, Doppler spectra vary with the orbital motions of the regular wave. As the result, when the sea surface wavelength is five times or more as long as the microwave irradiation width, the time fluctuations of Doppler velocity which shows a mean value of Doppler spectrum are good agreement with the orbital motions of the numerical regular wave. Also in such condition, the wave height of the sea surface waves can be observed accurately by analyzing the changes of Doppler velocity.

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