Doppler Frequency Components Estimation From Range-Doppler Spectrum Using Shipboard Coherent Microwave Radar

Shipboard coherent microwave radar is an emerging tool for ocean observation, which utilizes the direct relationship between the orbital wave velocity and the wave height spectrum to retrieve wave parameters. However, the ship's motion and broken waves would introduce extra Doppler components into the Doppler spectrum of the sea echo, and accordingly degrade the performance of wave measurements. Consequently, the Doppler components should be estimated before the inversion of ocean wave parameters. To address this problem, a Doppler frequency components estimation method, which describes the problem as an optimization problem with constraints to fit the raw Doppler spectrum, is proposed. First, the Doppler spectrum model for shipboard coherent microwave radar is parameterized and simplified. Then, an objective function and the constraints are established based on the generation mechanism of frequency components in the Doppler spectrum to make the reconstructed Doppler spectrum fit the raw Doppler spectrum. Subsequently, the particle swarm optimization (PSO) algorithm is used to find the optimal coefficient to solve the optimal solution. At last, frequency components, which are produced by ship motion, broken waves, and orbital modulation of gravity waves, are estimated. To validate the proposed method, the simulation data and the experimental data collected with a shipboard S-band radar in the South China Sea in December 2020 are analyzed. The estimated frequency components from radar data are compared with the MTi-G-measured and buoy-measured data. The results indicate that the proposed method is effective for estimating the frequency components and could improve the performance of wave measurements.