Radar sea-clutter at low grazing angles

Abstract

An analysis of sea-clutter data obtained in conditions typical of sea state 2 at X-, S-, L- bands, UHF and VHF is presented. Results show that sea-clutter exhibits very different spectral characteristics at higher frequencies compared to those at low frequencies. Experimental sea-clutter coefficients as a function of grazing angle at various frequency bands were obtained for upswell and cross-swell conditions. Most sea-clutter models use the radar look direction relative to the wind direction as an input parameter (e.g. upwind, crosswind, etc.). Hence a direct comparison of sea-clutter magnitude with values calculated from models under different wind conditions was not made. Instead, these results were contrasted with those calculated from two sea-clutter (the GIT and Sittrop) models, assuming an upwind and a crosswind condition. The data were quite dissimilar in the low-grazing-angle regions to those predicted by the two models. Further research on sea-clutter in the low-grazing-angle regions is needed to ascertain and explain the observed differences. Sea-clutter amplitude statistics were found to fit the K-distribution best in the important low probability of false alarm region. The distribution approached that of a Rayleigh model for vertical polarisation and low-resolution waveforms. Non-Rayleigh statistics were frequently observed even for relatively low resolution and vertical polarisation waveforms. For horizontal polarisation and/or high resolution waveforms, the statistics approached those of a lognormal model. The statistical results provided further evidence that the K-distribution can serve as a limiting distribution for sea-clutter. The results of this paper are pertinent to shipboard radar applications where the grazing angle is low. They also provide additional information for sea-clutter modelling in the low-angle region, particu larly at the lower frequency bands.