LABORATORY STUDIES OF RADAR SEA SPIKES AT LOW GRAZING ANGLES

We report on measurements of low grazing angle radar sea spikes in a laboratory wave tank. Using an X band radar with 10-cm range resolution and multiple-range bin capability, the sea spikes were observed to have a horizontal/vertical polarization ratio much larger than unity. This is similar to observations in earlier field experiments, but is in contrast to a ratio of unity recently observed for moderate grazing angles (Jessup et al., 1990). Our experiment was designed to validate the low grazing field results and to establish design criteria for spatial-temporal measurement instrumentation; single-point measurements currently used as surface truth cannot provide water surface profiles needed for modeling of radar scatter from deterministic surface features. Surface scattering features were generated using a paddle-generated wave, combined with wind waves for a series of wind speeds. The surface feature responsible for the sea spike echo was visually identified as a short wave crest, occurring on the windward side of the paddle wave, driven to extreme steepness by the passage of the faster paddle wave crest. Long decorrelation times observed in the field are also seen in these studies, of the order of 500 ms for horizontal polarization and less than 5 ms for vertical. We suggest that rather than Bragg scatter, the occurrence of similar ubiquitous features on the ocean are responsible for horizontally polarized low grazing angle sea scatter at the X band for low to moderate winds, and perhaps at other frequencies as well.