A review of methods of remote sensing of sea-surface conditions by HF radar and design considerations for narrow-beam systems

Present achievements in HF ground-wave radar remote mapping of surface currents and wave characteristics are reviewed, and the features of the different systems that are in use are described. Particular emphasis is given to the problems remaining to ensure effective mapping of nondirectional wave parameters and wave directional spectra over a sector of sea extending up to a range of 150-200 km from a coastal or island site. As an example of current progress, measurements in the United Kingdom of first-moment wave period by single-site radar have achieved measurements to within 15 percent of buoy data over a range from 4 to 9 s. Measurements of significant wave height gave standard deviations of 15 and 20 percent for wave heights less than and greater than 2 m, respectively. In the latter case, a bias of 40 percent was found, which later work suggests is reducible to about 14 percent, using a model-fitting technique. To achieve directional wave spectrum measurement with a single radar requires some 40-dB ratio between the Bragg line return and the noise floor of the Doppler spectra. A two-site radar has been shown to yield advantages for directional spectrum measurement. The design of steerable narrow-beam radars in the 6-25-MHz band to achieve such performance is discussed. Suitable transmitted waveforms to achieve good rejection of interference and to be acceptable in the HF band are put forward. The operational experience in the United Kingdom, obtained with a particular FM interrupted continuous-wave radar system associated with a steerable array, is reported. Results of simulating the effect of the antenna radiation pattern on the observed Doppler spectra are illustrated. The principal effects are on the weaker Bragg line and the second-order continuum around it. This can result in errors in deduced surface wind direction and directional wave spectrum. Finally, some useful results on propagation monitoring of a skywave radar with the aid of a range-Doppler graphical output are reported.