Multistatic synthetic aperture radar (MuSAR) has the capabilities of short-time high resolution imaging and multi-angle target observation. The imaging quality of coherent MuSAR depends on the distribution of wavenumber spectrum (WS). In this paper, an imaging performance improvement method based on WS trimming is proposed to obtain high-quality imaging results when the WS distribution is not ideal. First, the echo signals of MuSAR are derived. Then, the distribution of the WS is analyzed, and the WS trimming problem is transformed into a constrained multiple objective optimization problem (CMOP), which is optimally solved by multi-objective particle swarm optimization (MOPSO) algorithm. Finally, numerical simulation are performed to verify the effectiveness of the proposed method.

Synthetic Aperture Radar (SAR), with its all-weather and all-day operation, is an effective tool for earth observation. However, with the continuous intensification of global changes, current earth observation methods face challenges in meeting the demands for global coverage and timeliness. Moon-based SAR (MBSAR) has the advantage of long observation time and wide coverage. However, the imaging capability of MBSAR is limited by the orbit characteristics of the moon. Using the moon as the illumination source and the high-earth orbit (HEO) satellite as the receiving station (MH-BISAR) not only allows for a wide imaging area and flexible viewing angle, but also can effectively reduce the signal transmission power. This paper first establishes the motion model of MH-BISAR in a unified coordinate system, then calculates the basic conditions such as the required transmit power for MH-BISAR and compares it with MBSAR. Next, the imaging capabilities of MH-BISAR and imaging time of global areas within one month are analyzed. Finally, it is concluded that MH-BISAR has the advantages of high resolution, long observable time, large imaging coverage, and low system requirements for earth observation. It can serve as a powerful means for earth observation.

In complex synthetic aperture radar (SAR) imaging configurations, such as bistatic SAR, time-domain algorithms are less constrained and more accurate than frequency-domain algorithms. But they have not be applied to real-time imaging well because of large computation. The back-projection algorithm based on wavenumber-domain spectral splicing (WFBP) that emerged recently can be used to resolve this contradiction. In this paper, an efficient implementation architecture of this algorithm is designed based on FPGA. Parallel structures are used for sub-aperture BP imaging and images fusion. The imaging results and speed of the system is verified by simulations and experiments. It runs WFBP over a image of size 1024×512 in 0.131s, significantly accelerating imaging while maintaining accuracy of time-domain algorithms.

Synthetic aperture radar (SAR) is capable of obtaining the high-resolution 2-D image of the interested target scene, which enables advanced remote sensing and military applications, such as missile terminal guidance. In this article, the terminal trajectory planning for SAR imaging guidance is first investigated. It is found that the guidance performance of an attack platform is determined by the adopted terminal trajectory. Therefore, the aim of the terminal trajectory planning is to generate a set of feasible flight paths to guide the attack platform toward the target and meanwhile obtain the optimized SAR imaging performance for enhanced guidance precision. The trajectory planning is then modeled as a constrained multiobjective optimization problem given a high-dimensional search space, where the trajectory control and SAR imaging performance are comprehensively considered. By utilizing the temporal-order-dependent property of the trajectory planning problem, a chronological iterative search framework (CISF) is proposed. The problem is decomposed into a series of subproblems, where the search space, objective functions, and constraints are reformulated in chronological order. The difficulty of solving the trajectory planning problem is thus significantly alleviated. Then, the search strategy of CISF is devised to solve the subproblems successively. The optimization results of the preceding subproblem can be utilized as the initial input of the subsequent subproblems to enhance the convergence and search performance. Finally, a trajectory planning method is put forward based on CISF. Experimental studies demonstrate the effectiveness and superiority of the proposed CISF compared with the state-of-the-art multiobjective evolutionary methods. The proposed trajectory planning method can generate a set of feasible terminal trajectories with optimized mission performance.

Bistatic synthetic aperture radar (BiSAR) has high geometric diversity and can obtain the target information from different observation angles. With the ability of azimuth beam steering for both platforms, different bistatic imaging modes can be implemented to achieve better cooperation of beam footprints for enhanced imaging performance. For the multimode general bistatic SAR with complex trajectory (MGCT-BiSAR), due to the different beam steering methods and the translational-variant geometry, the spatial variance of the Doppler centroid becomes complicated, which leads to a severe azimuth spectrum aliasing problem. Moreover, the complex trajectory of MGCT-BiSAR may introduce high-order range cell migration (RCM) and Doppler parameters, which leads to the 2-D coupling and the different Doppler characteristics for different beam steering modes. The existing imaging algorithm cannot uniformly process the multimode SAR data. This article proposes an improved polar formatting algorithm based on minimum azimuth spectrum width (minASW-PFA) to uniformly process the bistatic SAR data. The proposed method first calculates the unified deramping factors and proposes a minASW bulk deramping method to minimize the azimuth spectrum width of the different imaging modes. However, the process leads to a severe range-Doppler coupling, which cannot be fully eliminated by the traditional polar formatting and wavefront curvature compensation methods. Therefore, an improved polar format mapping and wavefront curvature compensation method are also proposed to solve the coupling problem and achieve high-order spatial variance compensation. Finally, numerical simulations verify the proposed algorithm to achieve unified imaging for multimode general bistatic SAR with complex trajectories.

Geosynchronous synthetic aperture radar (GEO-SAR) can provide long-duration and wide beam coverage over the interested target scene, which is an ideal illuminator for bistatic SAR acquisitions. As a particular system configuration, the GEO bistatic SAR with maneuvering platform as the receiver (GEO-MP-BiSAR) can achieve continuous observation of the interested target during the flight. The target recognition and tracking information can be generated from the updating images for enhanced guidance performance. However, the bistatic SAR imaging performance is dependent on the observation geometry, which in turn is determined by the trajectory of the receiver. Therefore, in this paper, the trajectory optimization for GEO-MP-BiSAR is firstly investigated. The goal of the method is to generate a set of feasible trajectories to guide the maneuvering platform towards the target, and meanwhile obtaining the optimized imaging performance during the whole flight. The trajectory optimization is then modeled as a multi-objective optimization problem with multiple constraints, where the trajectory control and SAR imaging performance are comprehensively considered. Then, a knee-guided multiobjective evolutionary algorithm is put forward to effectively solve the problem, where the knee solutions are utilized to guide the search process and improves convergence and diversity of the method. The proposed algorithm can generate the prescribed number of solutions with significant trade-offs between the performance metrics. The mission designer can then choose a solution from only a few optimized candidates for implementation, which greatly improves the efficiency of decision making. Experimental studies demonstrate the effectiveness of the proposed method.

Beam steering is widely applied in the airborne and space-borne SAR system to achieve a balance between large imaging area and high azimuth resolution. However, it leads to a linear variation of Doppler centroid in monostatic SAR, which is more complicated in bistatic SAR due to the contribution built by the transmitter and the receiver and results in Doppler spectrum aliasing. The spatial variance of the Doppler centroid of the beam steering bistatic SAR is analysed. And a modified preprocessing method for azimuth-variant bistatic SAR with beam steering mode is proposed to remove the Doppler spectrum aliasing by effectively increasing the sampling rate of azimuth time. The movement of the platforms is considered with cured trajectory.