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The orbital angular momentum (OAM) of a vortex electromagnetic wave (VEW) has gained attention as a newly explored information carrier. OAM modes provide vortex azimuth resolution, which is a new degree of freedom (DOF) in radar application. Due to the special characteristics of the vortex azimuth domain, VEW shares compound Doppler information of two-dimensional (2D) speed. This paper proposes a 2D target velocity estimation method for VEW radar. The Doppler effect of VEW is first analyzed. Based on the relativity of tangential speed and OAM mode, a pulse-by-pulse OAM mode-changing strategy is designed. Then, a modified Radon–Fourier transformation (RFT) is proposed to estimate the compound Doppler frequency while range migration is compensated. In addition, decoupling and ambiguity-solving procedures are applied to the compound Doppler frequency estimation to obtain tangential and radial speed estimations separately. According to the simulation analyses, the effectiveness of the proposed method is verified.

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For future ultra-high resolution spaceborne SAR missions, large beam-steering capability and large antenna aperture are demanded to increase the azimuth resolution and to maintain the sensor sensitivity simultaneously. Thus, large-deployable antenna is a desirable option as compared with planar array antenna for these missions, since it is advantageous in terms of mass, size, and cost. Moreover, its antenna pattern will not have any distortion with mechanical beam-scanning approach. In this paper, a novel reflector SAR system, including its platform configuration, attitude maneuvering strategy as well as SAR payload electronics are presented in detail. Furthermore, the key technique of very-high resolution SAR-internal instrument calibration scheme is also provided so as to remove the imbalance among channels to guarantee coherency and then to extract the range replica consequently. In the end, a carried-out airborne flight campaign is described and its imaging results are presented to validate the effectiveness of our system as well as calibration approach. INDEX TERMS Synthetic aperture radar, very-high resolution, attitude maneuver, internal calibration, airborne campaign.

Distributed radar systems promise to significantly enhance target localization by virtue of the superiority of multi-view observations from widely separated radars, compared to their monostatic counterparts. Nevertheless, when the radar number is limited, performing target localization bears the brunt of the parameter identifiability requirement that the parameter number must be no less than the number of independent measurements. In this way, the canonical two-stage target localization method, as well as its developments, is no longer appropriate for direct application. Hence, in this paper, we propose a novel target localization method using time-difference-of-arrival (TDOA) measurements with the minimum number of radars under platform position uncertainties. The referred distributed system is a bistatic multi-receiver system, where the primary signal is transmitted by a geostationary Earth orbit (GEO) satellite while receivers are equipped on several unmanned aerial vehicles (UAVs). In the first stage, the reference range from the reference radar to the target is estimated by a quadratic function, and then the weighted least squares (WLS) solution of the target location is updated by substituting the range estimate back into it. In the second stage, we invoke the Taylor series approximation to further refine the target localization obtained by the first stage. It can be foreseen that the developed method is beneficial for scenarios with a limited number of radars, including engineering projects such as fire control, surveillance, and guidance, to support high-accuracy target localization. The simulation results show the superiority of the localization performance of the proposed method over other existing methods.