Experimental Observations at Very Low Grazing Angles of High Range Resolution Microwave Backscatter from the Sea

Abstract : X-band (9.5-10.0 GHz) backscatter at near grazing incidence (0.2 degrees) from the sea off the coast of Kauai, Hawaii, was measured with a radar characterized by a high spatial resolution in range (0.3 meters) and a high temporal resolution (2000 Hz PRF). Extensive amounts (over 1200 seconds per measurement) of vertically and horizontally polarized sea clutter data were taken with upwind and crosswind transmit geometries. The clutter was statistically and phenomenologically analyzed over time scales varying from long (200 seconds), to intermediate (5 seconds), to short (50 milliseconds), and over range swaths varying from full (160 meters), to partial (30 meters), to a single range cell (0.3 meters). Each type of clutter exhibited the characteristic spiky behavior which has come to be expected from microwave sea backscatter observed at low grazing angles and high range resolutions, while showing, between themselves, marked transmit geometry and polarization dependent contrasts, with the horizontally polarized clutter, measured with an upwind transmit geometry, being especially notable for its frequently occurring, significant high frequency spectral content. The extent of the high frequency spectral content appears to increase with the overall magnitude of a spatially and temporally extended spiking event, and the distribution of radar cross sections appears to vary between extended spiking events of the same clutter type.