Wave Parameter Inversion From Motion-Affected Echoes Using Shipboard Coherent Microwave Radar

Abstract

Shipboard coherent microwave radar has been a rapidly developing tool for ocean wave measurements. However, the radial velocity estimated from echoes collected with a shipboard radar is significantly affected by the forward speed and six-degrees-of-freedom (six-DOF) motion of the ship. Accordingly, the performance of ocean wave parameter inversion using such radar degrades. To address this problem, the distribution of energy modulated by the ship motion in the wavenumber-frequency spectrum is illustrated, and wave parameters inversion method based on shipboard coherent S-band radar is proposed. An adaptive filter is designed according to the distribution of the energy components modulated by the ship motion in the wavenumber-frequency spectrum. The proposed method filters out the non-wave components and preserves the wave field components. A two-dimensional inverse Fourier transform is applied to the filtered wavenumber-frequency spectrum to obtain the spatial-temporal radial velocities. Then the wave height spectrum is estimated from the radial velocities based on the direct relationship between the radial velocity spectrum and the wave height spectrum. Later, the significant wave height and mean wave period can be derived from the wave height spectrum. A dataset collected with a shipboard coherent S-band radar in the South China Sea in December 2020 is analyzed. The ship deployed with the radar sailed around a wave buoy according to a route plan for two days in that experiment. Comparisons between the radar-estimated and buoy-measured measurements in cases of ship motion are conducted. The results indicate that the proposed method can invert wave parameters well.