The electromagnetic (EM) wave with orbital angular momentum (OAM) exhibits a spiral phase wavefront, thereby offering superior advantages in extracting features in the plane orthogonal to the radar line-of-sight (RLOS) compared to plane EM wave. The existing parameter estimation work based on the rotational Doppler (RD) effect assumes that the position of the rotational center is known, which usually does not occur in practice. Therefore, this paper designs an OAM multistation radar system specifically for extracting micromotion features of rotating target, solving the coupling problem of micromotion parameters in RD frequency parameters. In details, we first extract the RD frequency from the echoes of each OAM radar station. To this end, we propose an efficient parametric instantaneous frequency (IF) estimation method leveraging segmented sparse representation (SR) with quadratic frequency modulation (QFM) model and smoothed pseudo Wigner-Ville distribution (SPWVD). Then, we derive the least squares solution (LSS) for the RD frequency parameters. Subsequently, the micromotion parameters of the rotating target, including the position of the rotation center, the rotation attitude, and the rotation radius, are estimated based on the RD frequency parameters estimated with multiple OAM radar stations. Finally, we derive the Cramér-Rao Lower Bound (CRLB) for the estimation of the micromotion parameters. Both mathematical analyses and simulation results show that the proposed parameters estimation method can accurately estimate the micromotion parameters of the rotating target with the proposed OAM multistation radar system.

Micro-Doppler (m-D) analysis is the most effective mechanism for detecting rotating targets or components; however, it fails when the target rotation plane is perpendicular to the radar line of sight (LOS). The vortex electromagnetic wave (VEMW) provides a unconventional structure of wavefront phase modulation on the cross-plane of the radar LOS, on which the radial m-D vanishes while the rotational Doppler (RD) appears. In the absence of the position of rotation center, this paper focuses on the micromotion parameters estimation based on RD effect for rotating target, and then proposes an estimation procedure, referred to as the two-step method. The micromotion parameters of the rotating target include the rotation attitude, the rotation radius and the position of the rotation center while the latter is coupled to the former two. Firstly, the micromotion parameters are roughly estimated based on the RD curve parameters obtained from the time-frequency (TF) spectrum of the received signal. Secondly, the maximum likelihood estimation (MLE) is used to accurately estimate the micromotion parameters. In addition, the Cramér–Rao bound (CRB) of parameter estimation is derived. The simulation studies the influencing factors of estimation performance and verifies that the proposed estimation method can provide excellent estimation accuracy of the micromotion parameters.