The near-surface current velocity determined from image sequences of the sea surface

A method to measure the ocean's near-surface current velocity vector based on the analysis of remote sea-surface image sequences was developed. The spatial and temporal records were transformed to the wavenunber-frequency domain, resulting in a three-dimensional (3-D) image power spectrum. In the spectrum, the signal energy of the waves is localized on a shell defined by the dispersion relation of surface waves. The sum of the sensor's velocity and the near-surface current profile deforms the dispersion shell due to the Doppler-frequency shift. An iterative least-squares fitting technique and an error-estimation model was implemented. To improve the method's accuracy, spectral wave energy found in higher harmonics of the dispersion shell and aliasing effects are taken into account. The most important nonlinear mechanism leading to higher harmonics is explained as resulting from wave shadowing due to the low grazing angles typical for ground-or ship-based radars. The low rotation time of nautical radar antennas causes temporal undersampling, which leads to aliasing in the frequency domain. In this paper, the improved method is examined analytically and is tested with Monte Carlo simulations. The variation of the shape of the measured or simulated 3-D image spectra, especially the peak wavenumber, the directional spread, and the main travel direction, controls the behavior and accuracy of the technique. A comparison of velocities acquired by nautical radar and independent Doppler Log current measurements is presented. The technique's accuracy, its limits, and its adaptability are discussed. Additional improvements are proposed, which lead to higher accuracy considering the shape of the spectral background noise and the technique's applicability to high ship speeds. The presented method is an important step toward operationalization of a commercial wave-monitoring system based on nautical radars.