Study of Doppler spectra of radar sea echo

Abstract

Doppler spectra of radar sea echo is investigated theoretically and experimentally. Electromagnetic scattering models using composite rough surfaces (i.e., Bragg resonant water waves superimposed on a carrier water wave) are developed by means of the basic principles in hydrodynamics applied to a gravity wave of finite height. The ideas are then carried over to the sea, where the assumption of “fully developed” yields a wave height dependence for the width of the spectra of radar sea echo of the form ∼H0.4–0.6, H being the height of the carrier water wave. New experimental results on spectra of radar sea echo with a multifrequency coherent pulsed radar (428, 1228, 4455, and 8910 MHz) for sea conditions, which ranged from 0.3- to 0.6-meter waves with 0.5- to 1-m/sec winds to 8-meter waves with 23- to 24-m/sec winds, show that the band width of radar sea echo is dependent on polarization and frequency, and the differential Doppler between the spectra for horizontal and vertical polarization is dependent on frequency and depression angle. Both the “noisiness” of the spectra and the greater width of the horizontal return indicate “spray” over the sea might be one of the parameters contributing to these dependencies. The simple models developed in terms of composite scattering surfaces are shown to yield the order of magnitude and the wave height dependence for the width of the spectra of radar sea echo, but they fail to give its polarization dependence.