Low-grazing-angle radar backscatter from the ocean surface

A series of simultaneous, low-grazing-angle (<0.1°) radar backscatter measurements were made with the L-, S- and C-band radars located on the Island of Roi-Namur, Kwajalein, MI. Using a variety of waveforms, coherent data were recorded in contiguous 1.26 km steps from 8 to 60 km ranges in sea state 3 and state 5 conditions. The author presents new, detailed spectral measurements at very low grazing angles to ranges well beyond the horizon. These measurements demonstrated the coherency of radar returns from beyond the horizon, and the existence and contribution of clear air turbulence to low-grazing-angle sea clutter data. Coherency was demonstrated in that meaningful Doppler spectra existed to 60 km ranges. The upwind spectrum was observed to be bimodal over the entire measurement range; one component moved at the wind speed and the other at approximately one-quarter wind speed. The principal and orthogonal polarisations of each component in the upwind direction were analysed separately