This article proposes a generalized compact polarimetry (GCP) mode along with two optimal polarization mode selection parameters to address the challenges of polarization mode selection in classification tasks across diverse scenarios. Theoretically, we conduct an in-depth analysis of the differences between circular and linear transmit polarizations, demonstrating their fundamental equivalence in terms of information content. For the first time, we propose that different classification tasks require different optimal polarization modes, and the optimal transmit polarization mode may lie in the elliptic polarization domain of synthetic aperture radar (SAR) systems rather than traditional circular compact polarimetric (CP) or linear dual-polarization (DP) modes. The proposed approach is validated using full-polarimetric SAR data from San Francisco and Hainan, showing that the optimal elliptical polarization mode achieves classification accuracies that are 2% to 42% higher than those of traditional CP or DP modes for certain categories, and performs comparably to full polarization. This improvement in accuracy stems from the interaction between the transmit polarization and the target scene, rather than advancements in classification algorithms. Using the two proposed parameters, the overall and category-specific classification performance of GCP modes can be effectively evaluated, enabling the identification of the optimal polarization mode for a given task. These findings provide significant insight into the design of future polarimetric SAR systems and offer new perspectives and directions for mission planning and mode selection for on-orbit satellites.

Range ambiguities severely degrade the performance of the synthetic aperture radar (SAR) system. To make an improvement without increasing the system complexity, antenna pattern synthesis is a feasible method. To this end, a novel algorithm that can shape the mainlobe and suppress the range ambiguities simultaneously by relaxing the problem as a semidefinite relaxation (SDR) programming problem is proposed here for the first time. Compared to the algorithm with global optimization, the presented algorithm could find the desired pattern without the large occupation of computing resources and has great potential in raising the performance of SAR system. In addition, some simulations are carried out, and the results prove that the proposed algorithm has the superior ability to suppress the range ambiguities while controlling the beamwidth and dynamic range ratio (DRR).

Synthetic aperture radar (SAR) is an advanced ground-observing remote sensing technology, and high-resolution wide-swath (HRWS) imaging has always been the goal of SAR. In general, an improved azimuth resolution requires a large pulse repetition frequency (PRF), resulting in high system requirements. To overcome this difficulty, azimuth multichannel technology has gradually developed, achieving HRWS imaging and decreasing the PRF by increasing the spatial sampling, i.e., increasing the number of antenna subapertures in the azimuth. This article proposes a theoretical architecture that generates multiple virtual receiving apertures in the azimuth rather than real apertures. The virtual receiving apertures are formed by multiplying the azimuth signals by linear phase histories provided by vortex beams carrying different orbital angular momentum (OAM) modes. Vortex beams with different OAM modes have different oblique phase wavefronts, so virtual receiving positions are generated in the along-track direction. This approach aims to reduce the PRF without increasing the azimuth real receiving aperture. Simulation results demonstrate the effectiveness and limitations of the method. Finally, to overcome the inherent limitations of the method, two possible implementation schemes are proposed.

The innovative spaceborne Earth observation mission LuTan-1 (LT-1) deploys advanced full polarimetric L-band synthetic aperture radar (SAR) to obtain high-precision, multidimensional ground feature information. As the quad-pol SAR system, LT-1 suffers from strong ambiguities that degrade the quality of the observation products. Antenna pattern synthesis algorithm can suppress the range ambiguities without the increase of the azimuth ambiguities and the system complexity and therefore has great potential to improve the overall ambiguity performance of the quad-pol SAR system. However, the previous research on this kind of method is generally limited to specific situations and lacks of the analysis of actual measurement results. Based on the application requirements of LT-1, this article proposes a novel pattern synthesis algorithm that suppresses the range ambiguities via sequential convex optimizations. In simulation and comparison, the proposed algorithm effectively suppresses the strong co-polarized range ambiguities and shows the flexibility, efficiency, and stability that are significantly better than the previous algorithms. What is more, the analysis of practical performance loss is performed based on the measurement result of the LT-1 antenna, and the practicality and validity of the algorithm are strongly verified.

Hybrid and$\pm \pi /4$quadrature-polarimetric (quad-pol) synthetic aperture radar (SAR) systems are established to simultaneously obtain all polarimetric components, including co-polarized (co-pol) and cross-polarized (cross-pol) components. However, the cross-pol components always suffer from severe azimuth ambiguities. In this article, the hybrid and$\pm \pi /4$quad-pol SAR systems with double receive channels are widely investigated to suppress the azimuth ambiguities of cross-pol components, in which the systems allow for a half pulse repetition frequency but at the cost of antenna size. We firstly provide a more thorough analysis for the double-channel (DC) hybrid and$\pm \pi /4$quad-pol SAR systems. Then, an improved reconstruction method is proposed to suppress the extremely severe azimuth ambiguity caused by the general reconstruction algorithms. However, the cross-pol signals still exist severe azimuth ambiguity. To this end, the displaced-phase-center antenna (DPCA) condition based on digital beamforming reflector antenna is employed, in which the undesired polarized signal can be greatly suppressed. Furthermore, numerical analysis is developed to demonstrate the excellent performance of the DC hybrid and$\pm \pi /4$quad-pol SAR systems with such DPCA condition. Finally, the distributed scene simulation results are presented to verify the advantage of the proposed approach.

In this paper, an advanced X-band airborne SAR is introduced for demonstrating two-dimensional Digital Beamforming (DBF) techniques. The advanced system consists of 64 channels split into 4 column in azimuth and 16 rows in elevation. This airborne experiment system serves as a test bed for the development, implementation and testing of digital beamforming techniques for future High-Resolution Wide-Swath (HRWS) SAR instruments. The set up and functions of this experiment system are presented in the paper. The flight campaign for this airborne SAR system will be undertaken by Institute of Electronics, Chinese Academy of Sciences (IECAS) in 2017.