Wave Parameter Inversion for Shipboard Coherent S-Band Radar Under Shadow Modulation

For shipboard coherent S-band radar, the shadow modulation phenomenon happens at large sea surface undulations or small grazing angles. As the ship speed increases or the sea state rises, the shadow modulation leads to an increase in the peak value of the wave spectrum, which results in inaccurate wave parameter inversion. In order to achieve accurate wave detection in high sea state or high ship speed scenarios, a shadow modulation correction method based on velocity-fitting relationship and Hermite interpolation is presented. First, the variable range of radial velocity is computed using the simulated sea surface’s velocity fitting relation. The anomalous Doppler velocity is then rectified by applying the modified Akima piecewise cubic Hermite interpolation (MAPCHI) algorithm. The MAPCHI algorithm is appropriate for complex nonlinear curves and is not prone to abrupt fluctuations or flattening issues. The implementation of this interpolation algorithm can alleviate the problem of anomalous fluctuations in Doppler velocities induced by shadow modulation. Finally, the nondirectional wave spectrum and wave parameters are calculated. Numerical simulation experiments show that the proposed method can rectify the influence of shadow modulation at various speeds and sea state backdrops. Meanwhile, comparing the radar measurements in the South China Sea with the buoy results, the mean absolute error (MAE) of significant wave height and mean wave period are 0.21 m and 0.34 s, respectively. The results demonstrate that the proposed method can successfully remove shadow modulation interference with high robustness.