Coherent marine radar measurements of directional wave spectra using vertically polarized antennas in moderate and shoaling depths

A coherent marine radar (COHrad), and modified standard marine radar with coherent-on-receive capability (CORrad), have been developed for radial Doppler measurements. Each measures the radial component of the radar echo, and can be used for determination of both the echo intensity and the orbital wave velocity spatial maps of ocean waves, as well as the mean radial ocean surface velocity. These radars allow direct measurement of the ocean wave spectrum by means of 3D-FFT processing of a sequence of coherent radar images of orbital wave spatial patterns. COHrad now has a 2.5-s rotation rate, so 256 rotations take just over ten minutes, with frequency alias at 0.2 Hz, while CORrad uses 1.25-s, 512 images, and 0.4 H alias. Radial current maps are obtained by summing all 256 radial velocity images, forcing orbital wave patterns to blend to the mean, and gives a map of mean radial current at the very surface. We present results of radar measurements of radial current and RMS waveheight for both COH and COR radars, using both horizontally and vertically polarized antennas. The latter are found to mitigate against errors in rms wave height retrieval for high wave conditions, when breaking features generate errors for horizontal polarization. Results are presented of time series of 16 frequency components of omni-directional spectra and compared with data from a pressure sensor array at the U.S. Army Field Research Facility in Duck, NC.