Interpretation and application of second-order HF sea-echo doppler spectrum

The purpose of the present research is to demonstrate that sea state-as expressed via the waveheight directional spectrum-can be determined with a long-range sky-wave HF radar system by a proper interpretation of the higher-order signal-echo Doppler spectrum. Sea-echo spectra measured at many HF frequencies in an experimental surface-wave radar system agree with theoretical predictions to first and second order, indicating that gross initial estimates of the waveheight directional spectrum can be obtained quickly from inspection of the radar records. Those initial estimates-along with an understanding of the general behavior of the integrand-will permit the numerical inversion of an integral equation for second-order scatter to obtain a considerably more refined calculation of the waveheight directional spectrum. Such techniques should prove useful in a future operational system for the routine monitoring of sea conditions out to 1500 nmi from the U.S. coasts.