Sparsity-based high-resolution analysis of mixed-mode over-the-horizon radar signals

Doppler frequency analysis is important in over-the-horizon radar (OTHR) in order to determine important target parameters such as the target altitude. In this paper, we analyze the Doppler signatures of OTHR signals propagated through both ordinary and extraordinary electromagnetic modes. As the signals following the two propagation modes undergo different group delays, the Doppler frequencies of a target associated with these two modes differ from each other. The frequency analysis of the resulting Doppler signatures becomes challenging when the Doppler components associated with these two modes are closely separated or even partially overlapping. In this paper, we develop a low-complexity sparsity-based method to resolve the Doppler signatures corresponding to the two propagation modes. It is based on a Lasso-based approach and the computational complexity is effectively reduced by employing a frequency focusing transform. To demonstrated the effectiveness of the proposed approach, simulation results are presented for challenging scenarios where the multipath Doppler signatures corresponding to the two propagation modes are interlaced or very close to each other.