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Theories and concepts for utilization of HF sea echo are compared and tested against surface-wave measurements made from San Clemente Island in the Pacific in a joint NRL/ITS/NOAA experiment. The use of first-order sea echo as a reference target for calibration of HF over-the-horizon radars is established. Features of the higher order Doppler spectrum can be employed to deduce the principal parameters of the wave-height directional spectrum (i.e., sea state); and it is shown that significant wave height can be read from the spectral records. Finally, it is shown that surface currents and current (depth) gradients can be inferred from the same Doppler sea-echo records.