A Novel Approach for 3-D Imaging of Ships With SAR Systems Based on 3-DOF Micro-Doppler Frequencies

To address the urgent need to improve the ship type discrimination capability in maritime target detection, this article explores synthetic aperture radar (SAR) three-dimensional (3-D) imaging of moving ships, which can provide high robustness and stability of 3-D structural characteristics for ship type discrimination. This paper defines the three-degree-of-freedom (3-DOF) micro-Doppler frequency of ship scatterers, specifically referring to the Doppler frequency caused by the three-axis rotation of the ship when the motion of the radar platform and the ship's planar motion are both compensated. The 3-DOF micro-Doppler frequency of the ship scatterers is closely related to the roll, pitch, and yaw three-axis rotation parameters of the ship and contains the 3-D coordinate information of the scatterers. Building upon the high-precision extraction of the 3-DOF micro-Doppler frequency of ship scatterers, this paper proposes an SAR 3-D imaging method for moving ships based on this frequency. The proposed method describes the ship's rotational characteristics using 27 parameters and converts the variation in the 3-DOF micro-Doppler frequency of the scatterer into a set of weighted coefficients of B-spline basis functions. Two long short-term memory networks are constructed to estimate the 27 parameters describing the ship's rotation and invert the 3-D coordinates of the scatterers. Finally, through the processing of simulated and experimental data, the feasibility of using the 3-DOF micro-Doppler frequency of ship scatterers to achieve high-precision estimation of the ship's three-axis rotation parameters and high-precision inversion of 3-D coordinates was verified.