Evidence of Bragg scattering in microwave Doppler spectra of sea return

Abstract

Microwave signals backscattered from the ocean (and one lake) have been collected at many different windspeeds and fetches. Doppler spectra of some signals obtained at low microwave frequencies exhibit double peaks clearly indicating Bragg scattering. At higher microwave frequencies, high wind speeds, long fetches, or in the presence of substantial swell, these splittings disappear. A model of microwave Doppler spectra based on Bragg‐scattering, composite‐surface theory is developed and used to show that the results obtained in these field studies are compatible with the hypothesis that Bragg scattering dominates microwave backscatter from rough water surfaces under many wind speed and incidence angle conditions. In particular, the model shows double peaks of the proper separation which disappear under the same conditions as those of the actual spectra. Furthermore, Doppler bandwidths given by the model agree with those of the field data under a variety of conditions. A rough angular dependence of the amplitudes of Bragg waves traveling in different directions with respect to the wind is deduced from the measurements. Finally, the implications of these findings for synthetic aperture radar imagery of the ocean are briefly discussed.