Measurements of ocean surface spectrum from an aircraft using the two-frequency microwave resonance technique

Abstract

This paper reports on the results of a two-frequency (Δκ) microwave radar experiment for measuring the spectral characteristics of the large-scale ocean surface gravity waves from an aircraft. The experiment was conducted as part of the 1979 Maritime Remote Sensing (MARSEN) project. A coherent Ku-band radar was used to study the Bragg type resonance matching of the electromagnetic wavelength at the difference frequency (Δƒ) to the length of the surface waves. The spatial spectrum of the surface reflectivity modulation was then computed from the measurements as the value of Δƒ was varied over a matching range of 15-300 m in surface wavelength. The strength and Doppler behaviour of individual resonances has been evaluated and modulation spectra have been produced from two data flights, with different surface conditions, at three altitudes, and for three flight paths—upwave, downwave and crosswave. Aircraft laser measurements of the one-dimensional surface profile spectrum were used in interpreting the radar data. In addition, a one-dimensional theoretical analysis of the Δκ resonance response that includes the effects of the antenna gain pattern and surface illumination parameters has been developed. The MARSEN results show strong resonances with consistent Doppler features and modulation spectra that agree well with the theory, surface truth and earlier stationary platform experimental results.