Automatic extraction of directional ocean wave spectra from marine radar imagery recorded on moving vessels

The use of time sequences of marine radar imagery to estimate directional ocean wave spectra is a well documented procedure. When the imagery is collected from a moving vessel, the standard spectral estimation procedure can misinterpret the data in the frequency domain and hence produce incorrect estimations of the wave propagation direction. Since wavenumber and frequency information are collected independently, radar imagery may also be used to estimate the dispersion relation for ocean waves, even if it is significantly distorted and aliased by relative vessel motion. From this observed dispersion relation, vessel velocity may be extracted, and hence the zero velocity dispersion relation and the true directional wave spectrum may be computed. The author´s research group has developed a technique for automatically correcting radar estimated spectra for these directional errors. The author computes the directional wavenumber spectrum in the usual fashion, and then extract (k.ω) slices from the three dimensional transform cube in the indicated directions of each significant peak in the spectrum. Concentrations of energy in these slices lie along the velocity modified, aliased dispersion relation. The author locates the centroid of each peak and determine the slope of the principal axis of the energy through this centroid. These two parameters uniquely determine both the relative vessel velocity and the branch of the aliased dispersion relation, allowing for estimation of correct wave spectra. This procedure is demonstrated using data collected from the HMCS Nipigon operating on the Grand Banks of Newfoundland in the Fall of 1997