Modulation of Microwave Backscatter by Gravity Waves in a Wave Tank
NAVAL RESEARCH LAB WASHINGTON DC
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Modulation of coherent microwave backscatter at 9.375 GHz, vertical polization, and at a depression angle of 45 deg has been measured in a wind wave tank as a function of windspeed, modulating wave frequency, and amplitude. A relaxation time model can be used to describe the phase and magnitude of the modulation for windspeeds of less than about 7 to 8 ms and wave slopes of less than about 0.1 the magnitude of the modulation depends strongly on windspeed and direction. Marked deviations from the model occur at the highest wind utilized 10 ms, and the small-scale wind-generated waves coalesce near or behind the crests of the modulating wave. The orbital velocity of the large waves is deduced to an accuracy of a 3 cms or 10 from the measurements of modulated doppler shift, though the phase of the modulated doppler shift exhibits windspeed dependence apparently due to the wind drift. Mean scattering cross sections show a substantial dependence on modulating wave amplitude for intermediate windspeeds. The implication of the measurements for radar imaging of ocean waves is discussed. It is concluded that the modulation of line-of- sight scatterer speed is a much more reliable measure of ocean wave amplitude than the amplitude modulation of backscattered power.
- Atmospheric Physics
- Physical and Dynamic Oceanography
- Active and Passive Radar Detection and Equipment