Abstract
A shipboard continuous wave coherent dual‐polarized focused radar operating at 9.23 GHz, an optical device which detects the occurrence of specular reflection at the radar incidence angle, and a laser wave slope gauge have been used to investigate mechanisms governing radar backscatter from surface water waves as part of the Joint Canada‐U.S. Ocean Wave Investigation Project ( also known as the Georgia Strait Experiment ). The relative contributions of Bragg and specular backscatter mechanisms have been identified using the output of the optical specular detector and the radar backscatter polarization ratios. Analysis of the radar data together with data from the laser wave slope gauge ( Hughes and Dawson, this issue ) has been used to further identify the mechanisms governing radar backscatter and modulations of radar backscatter from surface water waves. It is found that specular reflection is very significant at 20°incidence angle and not negligible at 40°. It is found that when specular contributions are absent, the relationship between the surface wave slope and the radar backscatter is well described by Bragg theory. Specular contributions, when they occur, appear as large spikes on top of the Bragg return. Analysis of the wave slope data shows that the wave slope modulations at “ X band ” and “L band ” Bragg wave frequencies are comparable, in contradiction to simple Bragg wave modulation theories which predict that “ X band ” waves should exhibit much weaker modulations. Modulations of the radar backscatter in low seas are due primarily to Bragg scattering. As the wind speed increases, the hydrodynamic modulations tend to decrease, but the radar modulations can be enhanced by specular reflection. Possible additional contributions due to wedge diffraction type scattering at higher wind speeds may also be important but have not been addressed in this experiment. Exceptionally low radar returns were observed in some cases and may be due to the presence of organic films on the surface.
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