In recent years, electromagnetic (EM) vortex imaging has emerged as a novel technique in the field of radar super-resolution imaging. This approach fundamentally differs from conventional methods by the relative lateral motion between the target and the platform. Although the super-resolution azimuthal imaging can be achieved, different orbital angular momentum (OAM) modes should be modulated in different signal pulses. In contrast, a new OAM modulation and monopulse imaging method is proposed in this paper, which involves multiplexing OAM modes based on frequency diversity in one pulse and can considerably improve the imaging efficiency. By introducing tiny frequency offsets among the elements of a uniform circular frequency diverse array (UC-FDA), the OAM multiplexing beams are generated. Subsequently, the EM vortex imaging model for coherent UC-FDA is established, and the imaging method is proposed. The theoretical performance analyses of spatial azimuthal resolution and efficiency are derived. Finally, the results demonstrate that the proposed method exhibits comparable imaging performance to the conventional EM vortex imaging method, even when only a single pulse is transmitted.

Due to the high-precision angle measurement performance, the monopulse technique plays a key role in fields such as remote sensing and space surveillance. The accuracy of monopulse angle measurement depends on the received amplitude and phase information, which is sensitive to the polarization component. Previous research has demonstrated that the performance of monopulse radar equipped with a parabolic antenna suffers from the cross-polarization component. However, it is not clear whether phased arrays (PAs) with higher degrees of freedom will also be affected by the cross-polarization component, and the parameter tolerance for performance degradation remains uncertain. In this paper, we establish a mathematical model of monopulse angle measurement in PA radar, which provides a comprehensive consideration of the cross-polarization component. Then, the received amplitude and phase patterns of PA radar are analyzed, and the theoretical angle errors caused by the cross-polarization jamming are derived. The experiments are conducted based on the measured amplitude-phase patterns of both co-polarization and cross-polarization. Experimental results are consistent with the theoretical analysis: the angle errors caused by cross-polarization jamming can reach half of the beamwidth in both azimuth and elevation dimensions, provided that the power of the cross-polarization and co-polarization components at the receiver is equal.

Compared with conventional radar systems, orbital angular momentum (OAM)-based radar can expand the degrees of freedom for object detection. Previous studies mainly focus on the azimuth measurement but ignore the elevation. The key challenge lies in the simultaneous acquisition of the azimuth-elevation two-dimensional (2D) angle from vortex electromagnetic (EM) waves carrying a limited number of OAM modes. In this paper, the principle of range and azimuth-elevation angle measurement is derived from the mathematical model of radar transmitting vortex EM waves. Furthermore, based on three OAM modes and a single receiver, a closed-form solution of elevation measurement is introduced, and an unambiguous 2D angle measurement method is provided. Compared with the traditional monopulse method, the proposed method ensures 360° full coverage and high precision. Finally, the effectiveness of the proposed method is demonstrated by both simulation and experimental results.

Cross-polarization (X-Pol) jammming covertly degrades the performance of monopulse angle estimation. This letter outlines a more general model for monopulse angle measurement on the dualpolarization array in the presence of X-Pol jamming. It introduces the mismatched factors between two subarrays to elucidate the theoretical monopulse error induced by X-Pol jamming. To mitigate distortion, a calibration method is proposed that dynamically combines the estimation results from the two polarization channels. In the experiments, the radiation patterns and mismatched factors of the dual-polarization array are measured, and the simulations based on these radiation patterns are conducted. The results validate the accuracy of the proposed model and the effectiveness of the calibration method in practical.