Second-Order Scattering from the Sea: Ten-Meter Radar Observations of the Doppler Continuum

Ten-meter radar observations of the sea have been used to study second-order interactions between waves in electromagnetic scattering from the sea. Techniques of coherent, pulsed radar provide echo frequency spectra from several range intervals. The echo spectra are resolved with an analysis window of a few millihertz. These spectra show a clear second-order echo continuum which appears as sidebands about the first-order Bragg scattering lines. Up to one-half of the total echo power has been observed in these sidebands. The principal characteristics of these sidebands vary with time, apparently in response to the sea state. The form of the echo spectra is consistent with the results of perturbation theory computations based on Rice's method.