A Fast and High-Resolution Imaging Method for Electromagnetic Vortex Radar Using Uniform Concentric Circular Arrays

Electromagnetic (EM) vortex waves carrying orbital angular momentum (OAM) have a helical wavefront structure, which allows the echo signal to contain the azimuthal information of the target when performing radar detection tasks, thus they show great potential in the field of staring imaging. It has been demonstrated that EM vortex radar (EMVR) based on the uniform concentric circular arrays (UCCAs) achieves equal divergence angles, so that the target can be simultaneously illuminated by multimode OAM beams. However, the imaging performance of EMVR is affected by the Bessel function term, which makes it difficult to achieve focused imaging results under limited OAM modes and apertures. To address this issue, a fast and high-resolution method based on accelerated 2-D alternating direction method of multipliers (2-D-ADMM) is proposed. First, an imaging model based on beam steering is established to effectively improve the detection performance, and the elevation resolution and azimuth resolution for EMVR are derived through the calculation of correlation function. Then, we propose a novel method based on the nonuniform fast Fourier transform (NUFFT) to speed up the matrix-vector multiplication by means of the fast Gaussian gridding NUFFT method. Afterward, EMVR imaging problem is transformed to a target estimation problem, and hence, the proposed 2-D-ADMM is directly applied to the EMVR model in matrix form with high reconstructing efficiency. Numerical simulation and EM calculation simulation have demonstrated the effectiveness of the proposed method.