Tomographic synthetic aperture radar (TomoSAR) shows great potential for high-quality 3-D mapping, especially in urban areas. As TomoSAR reconstruction methods advance into the deep learning (DL) era, current studies have demonstrated DL’s strengths in both precision and efficiency. However, for reconstructing urban areas with prominent spatial features from building structures, current studies focus on pixel-by-pixel reconstruction without leveraging the potential benefits of these features. In this context, an exploratory study to introduce spatial feature regularization in DL reconstruction is proposed for the first time, focusing on feature description, modeling, and regularization. Spatial features are analyzed and summarized by sharp edges and regular geometric shapes within the scene. To model these features, 2-D slices are used as the basic reconstruction units, and a general intraslice and interslice strategy is proposed to harness features within and between slices. Two-dimensional slices are fused into the entire 3-D scene. Two methods of fusion are designed: parallel and serial. To regularize these features, a new computational framework called light reconstruction and enhancement is designed, which includes two stages: light reconstruction with sparsity feature regularization and enhancement with spatial feature regularization. Finally, to evaluate performance, we design an extensive evaluation framework. A newly self-constructed compound urban building simulation dataset, combined with two public measured data, forms six different tests ranging from a classical close point resolution test to a diverse urban landscape challenge test. Evaluation results reveal the effectiveness of the designs and the boost provided by spatial feature regularization, resulting in higher reconstruction precision, more complete building spatial structure retrieval, and fewer outliers.

Tomographic synthetic aperture radar (tomoSAR) imaging is a powerful tool for urban 3D reconstruction. While recent deep learning methods have improved reconstruction quality, their reliance on simulated measurement-scene-image pairs for supervised training raises concerns over robustness in real-world scenarios due to the distribution shifts, such as varying observation geometry, observed scene distributions, and signal/noise levels. These concerns have received limited attention until now, motivating us to explore an alternative approach that leverages the strength of deep learning for tomoSAR reconstruction without requiring paired measurement and scene-image training data. Therefore, we propose the Unified Learning and Reconstruction (ULAR), a model-driven method for robust tomoSAR reconstruction, trained without such paired data. ULAR integrates physical-model consistency with scene feature regularization (capturing spatial structures) in a unified optimization process. Specifically, it jointly performs image reconstruction and spatial structure refinement by alternating between physics-guided updates and mainly self-supervised spatial-structure learning. The approach incorporates two complementary components for spatial-structure learning: a one-step self-supervised generative model for local spatial structures and a pretrained denoiser for nonlocal ones. And the denoiser is further enhanced with equivariance properties to improve its robustness. Experimental results on both simulated and real measured datasets demonstrate that ULAR achieves reconstruction accuracy comparable to supervised methods, and even surpasses them when distribution shifts exist, revealing strong robustness while not relying on simulated measurement-ground truth paired data. These results demonstrate the robustness of self-supervised learning for tomoSAR reconstruction, while highlighting its potential for better practicality and reliability in real-world applications.

Millimeter-wave (mmW) automotive radar imaging technology shows significant promise in advanced driver assistance systems (ADAS). Super-resolution imaging methods can be employed the limited aperture length of automotive radar to improve azimuth (angular) resolution. However, automotive radar images typically exhibit large dynamic range (LDR) and large scene (LS), leading to pay extensive computational complexity and storage demands when striving for higher image quality. To tackle this challenge, a fast$l_{q}$sparsity-driven imaging method with adaptive-focusing framework (FLSD-AF) for mmWave automotive radar super-resolution imaging in this article. First, in AF framework, a detect-before-imaging (DBI) is proposed to make echo data to adaptive focused on potential target area (PTR), thereby reducing the dimension of the effective data to reduce computational complexity and storage demands. Second, a subspace-phase-compensation (SPC) is proposed to reduces storage demands of the measurement matrix by addressing the imaging model mismatch in near-field under LS. Finally, a fast$l_{q}$sparsity-driven (FLSD) imaging method is proposed. It employs$l_{q}$-norm nonconvex penalty function to address the biased problem to improve imaging quality under LDR, meanwhile the computational complexity of the matrix operation is greatly reduced by utilizing joint Kailath-Variant (K-V) formula and Gohberg-Semencul (G-S) factorization. In summary, the proposed FLSD-AF not only substantially enhances the imaging performance, but also significantly diminishes the storage demands and computational complexity under LDR and LS. The results of simulations and experimental data all verify the proposed method.

Multi-source fusion is an effective technical to improve the detection and sensing capability of near-field 3D-SAR. Among the fusion techniques, calibration has the advantages of high accuracy and high reliability compared to point cloud alignment, but due to the large differences in the imaging mechanisms of near-field 3D-SAR and Lidar, it is difficult to correspond to the calibration target. Therefore, we propose a multimodal fusion framework based on calibration for near-field 3D-SAR. Firstly, design calibration targets with high scattering and high reflectivity, Then, the plane fitting method is used to extract, Finally, point set registration technology is used for alignment to achieve joint calibration of near-field 3D-SAR and Lidar. Furthermore, the three-source fusion is completed by combining the joint calibration method of Lidar and camera. The experimental results verify the effectiveness of the proposed method and improve the scattering diagnosis and detection and identification ability of near-field 3D-SAR.

The near-field array synthetic aperture radar (SAR) imaging method that’s based on deep neural networks has significantly advanced imaging accuracy and efficiency compared to traditional techniques like matched filtering and sparse reconstruction. However, it currently relies on supervised learning, which is affected by differences between simulated and real data. To address the issue, we introduces an unsupervised approach based on generative models of latent variables for near-field array SAR imaging. By focusing on generating target image distributions, this method bypasses the need for simulated training data. Instead, it leverages the concept of generative models with latent variables, using a prior auxiliary variable to construct a decoding neural network that transforms these latent variables into target images. In addition, a model-driven loss function is designed based on physical priors related to the linear correspondence between echoes and target images in the SAR measurement process.To enhance image quality further, sparse constraints (L1) loss function is integrated into the approach’s final loss function. Experimental validation using actual millimeter-wave near-field array SAR data demonstrates the effectiveness of this unsupervised imaging method. It offers advantages such as not relying on simulated data for training, suitability for diverse target types, superior imaging accuracy compared to traditional methods, and the ability to maintain high accuracy even at low sampling rates (10%).

Millimeter-wave (mmW) automotive synthetic aperture radar (Auto-SAR) technology holds significant promise for advanced driver assistance systems (ADASs). Sparse imaging methods can improve the quality of Auto-SAR images, such as suppressing sidelobes and noise. However, the$l_{1}$convex regularization-based sparse imaging methods suffer from the bias estimation, which reduces the target amplitude and ignores the association between scatterers, weakening the target structure. To address these issues, we proposed joint generalized$l_{q}$and convolutional (Glq-Con) regularization to enhance mmW Auto-SAR sparse imaging in this article. First, to improve the target amplitude, we propose utilizing the nonconvexity of Glq to reduce the bias effect; meanwhile, the global convergence of Glq ensures the imaging accuracy. Then, considering the continuity of the imaging target in driving scenes, we propose to utilize convolution regularization to modify the previously reconstructed amplitude of Glq to improve the target structure. Besides, to reduce computational complexity, we establish an efficient sparse imaging model. In this model, the fast Fourier transform (FFT) operator is employed to approximate complex matrix operation in the iterative process. We also use an efficient optimizer to solve the imaging model. Finally, both simulations and measured typical driving scenario experiments demonstrate that the proposed method significantly enhanced the Auto-SAR image, especially for the targets of weak scatterers.

In this study, we address the challenges associated with Video Synthetic Aperture Radar (Video SAR) shadow tracking, a technique used for continuous monitoring of ground moving targets. Due to challenges such as changes in shadow appearance, low contrast between shadow and background, and scene occlusion in Video SAR, existing methods often encounter extensive matching errors in the data association process, resulting in unsatisfactory tracking performance. To overcome these issues, we propose a novel method, GNN-JFL, which is based on joint motion-appearance feature extraction and graph neural data association. This method uses the detector as a flexible plugin and introduces two key improvements in the tracker section to enhance tracking accuracy. Firstly, we introduce joint feature learning to extract the complementary appearance and motion features from shadow shapes and positions, obtaining more robust feature representations to improve tracking performance under intricate challenges. Secondly, by organically integrating Multi-object Tracking (MOT) problems and Graph Neural Networks (GNN), we propose a novel GNN-based shadow tracking architecture, which utilizes graph relationships to learn the associations between shadows for more accurate tracking predictions. Our method is validated using two measured datasets and demonstrate superior performance in terms of multi-object tracking accuracy (MOTA). It outperforms the suboptimal method by 4.2% and 3.6% in the two datasets, respectively. This research contributes to the advancement of continuous monitoring techniques employing Video SAR shadow tracking.

TomoSAR imaging is a classical inverse problem. Learning to optimize is a newly emerging technique for solving such problems in a deep-learning way. This technique may facilitate the efficiency and accuracy of the inverse process. In this research, we apply this framework to TomoSAR imaging and aim to enhance spatial structures within the framework. We establish a two-part imaging optimization model. One part is a regularization term for the forward observation process, and the other part is for constraining the sparsity of structures in the gradient domain. Using the methodology of learning to optimize, we design basic neural network modules and stack them in a cascaded manner to solve the model. We validate the proposed network using a public TomoSAR dataset. The results show that the proposed method obtains buildings with much more complete overall surfaces and more apparent edges.

The Synthetic Aperture Radar (SAR) system has undergone significant advancements, transitioning into a multi-functional platform with various modes. This study introduces a novel imaging mode that enables the simultaneous acquisition of height and intensity features, addressing the diverse requirements of different regions. Referred to as non-common band imaging, this mode optimizes bandwidth allocation within the limited illumination time, enhancing efficiency and flexibility. However, the interferometric phase retrieval problem arises in this mode. To address this challenge, we establish a forward model for the master-slave images and propose an optimization-solving model that incorporates wavelet sparsity regularization to mitigate noise interference. Furthermore, we introduce a physics-aware spectrum fusion network, combining proximal gradient descent methodology with the innovative deep unfolding technique, to restore the interferometric phase. Extensive experiments conducted on simulated and real measured data validate the effectiveness of the proposed network in terms of efficiency, accuracy, and noise reduction capabilities.

The tomographic Synthetic Aperture Radar (SAR) reconstruction based on deep learning (DL) can achieve high precision and efficiency, making it a promising method for various applications. However, current deep learning-based methods perform reconstruction by separately processing each range-azimuth resolution unit, which limits the utilization of features between resolution units and ignores the three-dimensional characteristics of the target. To address this limitation, we propose an iterative multidimensional feature reconstruction network that improves the accuracy of reconstruction by introducing correlation constraints between resolution units. Specifically, the proposed method slices the pulse-compressed data along the azimuth direction and treats each range-elevation slice as a calculation unit for reconstruction processing. And a multidimensional feature regularization module is designed to iteratively process two-dimensional features within the slice, taking into account the correlation between resolution units. Real-data experiments demonstrate that our proposed method outperforms existing network that utilize L1-norm sparse regularization in terms of achieving higher completeness.

The near-field array three-dimensional SAR can obtain the three-dimensional electromagnetic scattering characteristics of targets and present the imaging results in the form of point cloud. In recent years, it has been gradually used in the field of target RCS measurement. However, there are some problems in near-field array three-dimensional SAR images, such as interference, sidelobe and missing of target shape. Lidar has the characteristics of high positioning accuracy and strong target shape description. The fusion of Lidar with the near-field array three-dimensional SAR can effectively assist the scattering characteristic diagnosis of SAR images, in which the calibration plays an important role. This paper presents a calibration method for Lidar and near-field array three-dimensional SAR. The method is divided into three steps of calibration target design, extraction and alignment. By designing a special calibration target, correcting the imaging position deviation of Lidar, and converting the problem of calibration corresponding point selection into a problem of matching target set, the coordinate system of Lidar and SAR can be aligned. The effectiveness of the proposed method is verified by the experimental results of measured data.

Millimeter-wave (mmW) automotive radar imaging technology has great potential in advanced driver assistance systems (ADAS). Existing super-resolution imaging methods can improve angular (azimuth) resolution for automotive radar with limited aperture. However, these super-resolution methods have high computational complexity meanwhile have poor imaging performance in single-snapshot. In this paper, combined with CS based on ℓ1-norm regularization, we proposed a fast super-resolution imaging method via subspace detection and near-field phase compensation. Frist, the range-pulse-compression (RPC) data in the near and far field is formed by using range FFT. Then, the subspace detection is presented to reduce the size of the measurement matrix by using angle FFT to obtain the potential angle subspace of the target with all RPC data. After, the near-field phase compensation is utilized to make the measurement matrix applicable to all RPC data. Finally, the iterative shrinkage-thresholding algorithm (ISTA) algorithm is utilized to form the super-resolution images based on the measurement matrices and all RPC data. The simulation results show the proposed method can significantly improve imaging resolution with lower computational complexity than other imaging methods.

Ship detection in synthetic aperture radar (SAR) images is a hot pot. However, most existing convolution neural network (CNN)-based research is limited to single polarization ship detection and neglects the utilize of the rich polarization information to further improve detection performance. Thus, to address the problem, in this paper, a group-wise shuffle attention R-CNN (GWSA R-CNN) is proposed for ship detection in dual-polarization SAR images. Based on the raw Faster R-CNN, GWSA R-CNN embeds a group-wise shuffle attention module (GWSA module) in the detection subnetwork to capture enriched organic fusion polarization information. Finally, the experimental results on the dual-polarization SAR ship detection dataset (DSSDD) show the state-of-the-art (SOTA) performance of our GWSA R-CNN, outperforming than other 7 competitive models. Specifically, GWSA R-CNN surpasses the second-best model 1.82% average precision (AP).

Near-field three-dimensional synthetic aperture radar (Near-field 3D-SAR) is a powerful imaging technique with diverse applications in scattering diagnosis, person/parcel imaging, building monitoring, forest monitoring, and more. This paper provides an overview of the imaging methods employed in Near-field 3D-SAR, focusing on the underlying methodologies, imaging models, and solving flowcharts. By analyzing and summa-rizing these methodologies and flowcharts, valuable insights are uncovered. Additionally, potential research directions are identified for further exploration.

This work focuses on 3D Radar imaging inverse problems. Current methods obtain undifferentiated results that suffer task-depended information retrieval loss and thus don’t meet the task’s specific demands well. For example, biased scattering energy may be acceptable for screen imaging but not for scattering diagnosis. To address this issue, we propose a new task-oriented imaging framework. The imaging principle is task-oriented through an analysis phase to obtain task's demands. The imaging model is multi-cognition regularized to embed and fulfill demands. The imaging method is designed to be generalized, where couplings between cognitions are decoupled and solved individually with approximation and variable-splitting techniques. Tasks include scattering diagnosis, person screen imaging, and parcel screening imaging are given as examples. Experiments on data from two systems indicate that the proposed framework outperforms the current ones in task-depended information retrieval.

The near-field SAR technology has shown great application value in many fields, such as security inspection, and radar cross section (RCS) measurement. However, due to side-lobe crosstalk and near-field spherical wave effect, the near-field SAR image has high clutter and side-lobe, which leads to serious image degradation. Complex image degradation results in the loss of target structure and contour, which limits the further application of near-field SAR technology. Due to the complex degradation, current restoration methods are not effective enough in terms of weak scattering center and target shape (geometry and structure) restoration. In this article, we first analyze near-field SAR image degradation. Then, utilizing the recent promising deep learning, we propose a novel near-field SAR image restoration framework. In this framework, we construct model-driven 2D CNN for 2D image restoration and 3D CNN for 3D image restoration, respectively. To validate the proposed framework, we construct experiments on simulated 2D and 3D test set, respectively. The experimental results prove the effectiveness of the proposed framework for both 2D and 3D situations.

Video synthetic aperture radar (Video SAR) has drawn much attention because it can continuously observe and track the moving target. Rather than tracking the target directly, it is better to track its shadow because the shadow has no location shift, and the back-scattering characteristic is stable. However, most current shadow tracking methods not only suffer from false alarms because their discrimination capacities are not good enough but also suffer from missed detection because the feature extraction capacities are limited under complicated environment. Therefore, we propose a shadow-enhanced self-attention and anchor-adaptive network (SE-SA-AAN) to achieve accurate moving target tracking for Video SAR. Firstly, the pre-processing technique sparse low-rank noise decomposition (SLRND) is proposed for enhancing shadows’ salience to facilitate subsequent feature extraction. Secondly, the transformer self-attention mechanism (TSAM) is embedded in the parameters-shared backbone in the feature extraction network to concentrate on regions of interests for suppressing clutter interference. Then, the representative features are input to the detector and tracker. The detector adds the semantic guided anchor-adaptive mechanism (SGAAM) to obtain optimized anchors that match the shadows’ location and shape in each frame. Meanwhile, the tracker applies a Siamese network to achieve trajectory tracking for each shadow. Based on the detection and tracking results, a data association is applied to achieve moving targets tracking. Finally, experiments on Sandia National Laboratories (SNL) data demonstrate that SE-SA-AAN outperforms the state-of-the-art methods FairMOT, TransTrack and Centertrack by 6.4%, 7.8% and 8.3% multiple object tracking accuracy (MOTA) separately.

Millimeter-wave (mmW) automotive radar imaging technology has shown significant potential in autopilot assistance systems. The automotive radar with limited aperture can achieve high-resolution image by synthetic aperture technology. However, conventional imaging methods result in strong background clutter and high sidelobe interferences. To solve these problems, we propose a target-oriented Bayesian compressive sensing imaging method with region-adaptive extractor (TO-BCS-RAE) for mmW automotive radar imaging. (1) First, to extract the potential-target-regions (PTR) as well as subtracting the background clutters outside the PTR in a high-resolution initial image (by synthetic aperture), a region-adaptive extractor (RAE) is developed with utilizing 2D CFAR, isolated-point removing, and imaging clustering. Meanwhile, a more accurate prior distribution of target scattering points can be obtained in the PTR. (2) Then, to suppress the background clutters while enhancing the smooth structure of targets in the PTR, a target-oriented Bayesian compressive sensing (TO-BCS) imaging method is proposed by combining the prior probability distributions and inherent continuity of the target scattering points. It can also effectively reduce the high sidelobes. (3) Finally, to verify the effectiveness of TO-BCS-RAE, we conduct experiments on real data collected from an automotive radar with a vehicle platform in three typical driving scenarios. Both simulated and experimental results show the imaging quality of the proposed imaging method over conventional BP, OMP and ISTA methods.

Light field (LF) cameras often have significant limitations in spatial and angular resolutions due to their design. Many techniques that attempt to reconstruct LF images at a higher resolution only consider either spatial or angular resolution, but not both. We propose a generative network using high-dimensional convolution to improve both aspects. Our experimental results on both synthetic and real-world data demonstrate that the proposed model outperforms existing state-of-the-art methods in terms of both peak signal-to-noise ratio (PSNR) and visual quality. The proposed method can also generate more realistic spatial details with better fidelity.