Modulation of Microwave Backscatter by Gravity Waves in a Wave Tank
Abstract
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 m/s 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 m/s), 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 cm/s 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.
Document Details
- Document Type
- Technical Report
- Publication Date
- Mar 29, 1976
- Accession Number
- ADA024048
Entities
People
- J. W. Wright
- W. C. Keller
Organizations
- United States Naval Research Laboratory