Circular Synthetic Aperture Radar Research Articles

Signal Characteristics Analysis and Intra-Pulse Motion Compensation for FMCW CSAR

The combination of frequency modulation continuous wave (FMCW) technology and circular synthetic aperture radar (CSAR) would pave the way for the development of the microminiature unmanned aerial vehicle (UAV) earth observation application. Regarding the FMCW CSAR imaging process, the stop-and-go approximation is no longer effective due to the longer pulse duration. In addition, the signal characteristics of FMCW CSAR with higher frequency band and higher resolution need to be further studied. Aiming at millimeter-wave FMCW CSAR imaging, the signal models of FMCW CSAR in the time domain and wavenumber domain are first derived in this work. Then, the signal characteristics including the two-dimensional (2-D) wavenumber spectrum, the spectrum support region, the point spread function, and the resolution are analyzed. Moreover, the effects of intra-pulse movement are analyzed, including the range offset error, the main lobe broadening, and the additional phase error in the range dimension, as well as the extra first-order and second-order phase errors in the 2-D wavenumber domain. Based on the intra-pulse motion analysis, two criterions are provided to determine whether the intra-pulse movement of FMCW CSAR is negligible. An improved back projection (BP) algorithm compensating the intra-pulse motion in the range domain and another algorithm combining the traditional BP and compensation the intra-pulse motion in the wavenumber domain are proposed. Simulation results verify the correctness of the FMCW CSAR signal characteristics analysis and the effectiveness of the intra-pulse movement compensation methods.

Analysis of the Azimuth Ambiguity and Imaging Area Restriction for Circular SAR Based on the Back-Projection Algorithm.

Circular synthetic aperture radar (CSAR) has a 360° observation capability on the central observation scenario. A typical way to process CSAR imaging is to cut data into small sub-apertures because most targets are only coherent at a very small observation angle. There are many sub-aperture imaging methods after development in recent years. The back-projection algorithm is widely used because it is simple and can be applied to an arbitrary trajectory. Because of the limitation of the Nyquist sampling frequency and influence of the antenna sidelobe, azimuth ambiguity is a phenomenon that may occur in the radar imaging process. The existing researches typically choose the back-projection (BP) imaging area according to the SAR platform flight path and the antenna beam width. The limitation of the CSAR imaging area and its azimuth ambiguity region are rarely analyzed theoretically. This paper focus on the sub-aperture imaging of CSAR, based on the BP algorithm, which derives the relationship of azimuth ambiguity with CSAR parameters such as the pause repeat frequency (PRF), slant range angle, velocity of radar platform, etc. This paper proposes an equation for the calculation of the azimuth ambiguity region and analyzes the limitations, which provides theoretical support for CSAR parameter design, imaging area selection, and azimuth ambiguity analysis.

Refocusing of Ground Moving Target in Circular Synthetic Aperture Radar

Due to the non-cooperative motion of ground moving targets, they appear smeared and dislocated in circular synthetic aperture radar (CSAR) imagery. Refocusing of ground moving target in CSAR is a challenging problem because of the circular trajectory of the radar platform and unknown motion parameters of ground moving targets. In this paper, an inverse SAR (ISAR) technique-based method with Legendre polynomial approximation for ground moving target refocusing in CSAR is proposed. Different from the conventional ISAR-based method, the method extracts range migration of the moving target in the range-frequency domain directly, which can avoid the range alignment procedure in conventional ISAR techniques. Furthermore, the Legendre polynomial approximation is applied to fit the extracted range migration, which can be carried out iteratively to fulfill the requirement of accuracy. The experimental CSAR data processing results show that the proposed method can achieve satisfied imaging quality of ground moving targets.

A Novel Method for Single-Channel CSAR Ground Moving Target Imaging

Recently, the parameter estimation and focusing of ground moving target in the circular SAR (CSAR) attracts widely interest in researchers. In this paper, a sub-aperture-based frequency domain algorithm that divides the CSAR trajectory into sub-apertures is proposed, which achieves better focusing quality than that of the traditional straight-line fitting method. First, the CSAR sub-aperture is fitted by a parabola with special curvature, and the effectiveness of this fitting is analyzed. Next, a parameter estimation method is proposed to estimate the motion parameters of the ground moving target for each sub-aperture. Finally, the moving target is refocused based on the estimation results. Simulation as well as the real CSAR data processing results verified the theoretical analysis of the proposed method.

ULTRA-HIGH RESOLUTION IMAGING OF FACADES AND VERTICAL INFRASTRUCTURE BY CARBORNE SAR AND AIRBORNE CSAR

Abstract. In this paper we propose a strategy to focus ultra-high resolution single channel carborne SAR and airborne circular SAR (CSAR) data to image facades and vertical infrastructure. We illustrate the related theoretical background and the design of an optimal focusing geometry for carborne SAR applications while using backprojection focusing techniques. Of particular interest is thereby the determination of the minimum distance and orientation of the facade to the radar sensor. Potential image distortions due to a wrong choice of these parameters are illustrated. Effects on the final resolution of the data due to the rotation of the focusing geometry compared to typical airborne SAR are discussed. We validated the strategy by driving on conventional roads illuminating facades with an experimental mobile radar mapping (MRM) sensor operating at 300 GHz. We further present an adapted version of the proposed strategy to focus vertical infrastructure in CSAR data sets. By extracting the center coordinate and the principal orientation of an object from GiS data, the focusing plane is designed arbitrarily in the 3D space. For the CSAR data set, a radar sensor particularly designed for circular flight trajectories operating at 94 GHz was evaluated. An electrical pylon was chosen as potential target. In both applications, the final images show a high level of detail. The combination of proposed strategy and radar sensor with very high bandwidth is capable of subcentimeter imaging of facades. The height, shape and dimensions of objects can be extracted directly from the image geometry at very high accuracy.

Airborne Circular W-Band SAR for Multiple Aspect Urban Site Monitoring

This paper presents a strategy for urban site monitoring by very high-resolution circular synthetic aperture radar (CSAR) imaging of multiple aspects. We analytically derive the limits of coherent azimuth processing for nonplanar objects in CSAR if no digital surface model (DSM) is available. The result indicates the level of maximum achievable resolution of these objects in this geometry. The difficulty of constantly illuminating a specific scene in full aspect mode (360°) for such small wavelengths is solved by a hardware- and software-side integration of the radar in a mechanical tracking mode. This results in the first demonstration of full aspect airborne subaperture CSAR images collected with an active frequency-modulated continuous wave (FMCW) radar at W-band. We describe the geometry and the implementation of the real-time beam-steering mode and evaluate resulting effects in the CSAR processing chain. The physical properties in W-band allow the use of extremely short subapertures in length while generating high azimuthal bandwidths. We use this feature to generate full aspect image stacks for CSAR video monitoring in very high frame rates. This technique offers the capability of detecting and observing moving objects in single channel data by shadow tracking. Due to the relatively strong echo of roads, the shadows of moving cars are rich in contrast. The image stack is further evaluated to present wide angular anisotropic properties of targets and first results on multiple aspect image fusion. Both topics show huge potential for further investigations in terms of image analysis and scene classification.

Microwave Imaging of Breast Tumor Using Time-Domain UWB Circular-SAR Technique.

This paper explores the competency of the time domain ultra-wideband (UWB)-circular synthetic aperture radar (CSAR) to image the breast and detect tumors. The image reconstruction is performed using a time domain global back projection technique adapted to the circular trajectory data acquisition. This paper also proposes a sectional image reconstruction method to compensate for the group velocity changes in different layers of a multilayer medium. Experiments on an advanced breast phantom examines the suitability of this technique for breast tumor imaging. The advanced breast phantom is designed based on a MRI of a real patient, fabricated using 3D printing technology, and filled with liquids that emulate normal and cancerous tissues. The measurement results, compared with MRI imaging of the phantom, demonstrate the suitability of the UWB-CSAR method for breast tumor imaging. This method can be a tool for early diagnosis as well as for treatment monitoring during chemotherapy or radiotherapy.

New imaging method for airborne circular trace stripmap SAR

Circular trace stripmap synthetic aperture radar (SAR) (CTS-SAR) is a new operation mode presented recently. Compared with linear trace stripmap SAR and staring SAR, it is similar to them. Meanwhile, it shows some unique features. The geometry relationship and work principle of CTS-SAR are introduced and deduced. After analysing the moving characteristics of airborne platform, a novel imaging method with motion compensation for airborne CTS-SAR is proposed. The impact of motion error and the performance of this new method are analysed. Finally, the results of computer simulation and real data processing validate the analysed results and the proposed imaging method.

Circular SAR imaging algorithm based on polar format algorithm for moving target

For stationary targets, circular synthetic aperture radar (CSAR) imaging has tended to be mature and perfect. However, for moving targets, due to the long synthetic aperture and the variation of slant plane with the observation angle, the motion compensation is more complicated and has no effective approach. Based on the latest moving target trajectory reconstruction method in CSAR (2015), the polar format algorithm can be improved to image the moving targets when the motion parameters have been estimated. First, the authors placed the imaging reference point on the target to track it, and then compensated for the phase error caused by the incident angle changing and the wavefront curvature. The simulation results of point show that the algorithm is effective and accurate.

Parameter inversion algorithm of CSAR moving target based on PFA sub‐aperture images

Circular SAR (CSAR) as a new kind of SAR imaging mode has the advantage of long-time observation. For the moving target, the corresponding parameters of the moving target can be calculated by using the position information from the apparent coordinates of the moving target in these CSAR sub-aperture images. The CSAR sub-aperture imaging algorithms include BP and PFA, and the existing parameter estimation model is established on those well-focused and localised images. However, they account for a substantial computation. Here, an improved algorithm for parameter estimation based on CSAR sub-aperture images obtained by PFA is proposed. The wave-front phase error compensation is abandoned, and a new parameter estimation model is established based on PFA images with geometric positioning error. The computational complexity of the whole process of the algorithm is reduced, and the practicability is improved. The point target simulation shows that the method is effective and accurate.

A Novel Phase Calibration Method for Dual-Channel CSAR-GMTI Processing

Displaced phase center array (DPCA) and along-track interferometry (ATI) have been widely used for ground moving target indication (GMTI) in multi-channel circular synthetic aperture radar (CSAR) system. In ideal conditions, these methods can suppress background clutter effectively. However, the nonideal movement of platform may create a nonzero crab angle, which will induce a cross-track baseline offset and affect GMTI processing seriously. This letter analyzes the influence of crab angle on interferometric phase theoretically. Here, we propose a two-step phase calibration method to greatly improve the performance of CSAR-GMTI by eliminating the range-varying phase error. This method is independent of imaging and GMTI processing. Simulated and measured data results verify the correctness of the theoretical analysis and the validity of the proposed method.

Multi-aspect SAR target recognition based on space-fixed and space-varying scattering feature joint learning

ABSTRACTThe emerging synthetic aperture radar (SAR) multi-aspect imaging modes such as circular SAR (CSAR) bring new vitality and challenges to SAR imagery interpretation. In addition to the space-fixed features obtained from a single SAR image, multi-aspect SAR images can be used to extract the space-varying scattering features, which are beneficial to target recognition. However, due to the complexity of backscattering, the extraction of space-varying features has not yet been effectively explored. In this letter, a multi-aspect SAR target recognition framework based on a residual network (ResNet) and bidirectional long short-term memory (BiLSTM) network is proposed to address this. As a basis, the ResNet is used to extract the space-fixed scattering features from each single-aspect image, which is supervised by a joint loss. Then, the BiLSTM is employed to further extract the space-varying scattering features among the adjacent multi-aspect images through the recurrent learning. The experimental results show that the proposed method can achieve 100% accuracy for 10-class recognition, and 98.97% accuracy in case of a large depression angle. Compared with existing methods, it can implement high recognition accuracy and generalization with relatively simple combined neural networks.

Modulus Stretch-Based Circular SAR Imaging with Contour Thinning

This paper presents a modulus stretch-based circular Synthetic Aperture Radar (SAR) imaging method. This method improves the traditional backprojection algorithm for circular SAR imaging, and introduces the modulus stretch transformation function in the imaging process. By performing a modulus stretch transformation on the intermediate results, the target contour in the final imaging result is thinner and clearer. A thinner and clearer contour can help to increase the recognizability of the target and provide a basis for subsequent target recognition. The proposed method is demonstrated on the line target imaging simulations and Gothca dataset.

Echo Model Without Stop-and-Go Approximation for Bistatic SAR With Maneuvers

An echo model plays a pivotal role in the processing of synthetic aperture radar (SAR) data. It has been shown that the widely adopted echo models based on stop-and-go approximation (SAG) are unsuitable for use in many emerging radar systems with high resolution or high-speed platforms. More accurate echo models without SAG have been proposed for some special monostatic scenarios, such as low-Earth-orbit SAR and geosynchronous SAR. However, due to their inherent assumptions, such as linear and constant-speed/orbit-based motion, these conventional echo models cannot be applied to general cases, which involve bistatic configurations and maneuvers. Maneuvers or nonlinear/inconstant-speed motion are very common in cases, such as circular SAR and maneuvering-platform-borne SAR. In this letter, we propose a generalized echo model for bistatic SAR, which accounts for maneuvers of the platforms during pulse propagation. Moreover, we obtain a high-precision closed-form expression of the model by approximating high-order terms of the accurate range history of the receiver. Compared with the conventional models, the proposed echo model can be applied to a much greater number of cases, ranging from slow and constant-speed platforms to fast and maneuvering platforms. Theoretical analysis and backprojection-based image formation simulations confirm the validity of the proposed echo model.

A New Motion Parameter Estimation and Relocation Scheme for Airborne Three-Channel CSSAR-GMTI Systems

This paper proposes a new scheme of motion parameter estimation and relocation for airborne three-channel circular stripmap synthetic aperture radar (CSSAR)-ground moving target indication (GMTI) systems. Compared with the conventional straight-path SAR, the parameter estimation of a target is more challenging because the target’s range history and signal model are more complicated due to the complexity of the relative motion between CSSAR and ground moving target. In this paper, the signal model of a ground moving target and the expression for its along-track interferometric (ATI) phase from the environment of airborne three-channel CSSAR are derived. The coupling effect among the target’s motion and position parameters is also figured out. Then, a scheme of motion parameter estimation and relocation is proposed. The proposed scheme utilizes the ATI phase and the quadratic-term coefficient in the range equation to estimate the target’s motion and position parameters and utilizes an iterative strategy to address the coupling effect among these parameters. Numerical simulations are conducted to validate the satisfactory performance achieved by the proposed algorithm.

Time-Reversal SAR Imaging for Nondestructive Testing of Circular and Cylindrical Multilayered Dielectric Structures

In this paper, a synthetic aperture radar (SAR) approach, for imaging internal structures of generally lossy layered dielectric cylindrical objects, is presented. This method which properly accounts for different transmission and refraction paths at each boundary between different layers produces a properly focused image of embedded targets. This approach is also capable of addressing imaging needs for asymmetrical multilayered cylindrical bodies. Consequently, this approach overcomes the limitation associated with the conventional methodology, in which free-space propagation is assumed. The calculation method of angular sampling criterion for circular and cylindrical SARs (i.e., circumferential and in height) is also presented. Electromagnetic simulations are performed on a three-layer cylindrical object, symmetrical and asymmetrical, with embedded targets to validate the approach. In addition, representative measurements are conducted at X-band (8.2-12.4 GHz), demonstrating the effectiveness of the approach for practical nondestructive evaluation applications.

Retrieval of Polarimetric Azimuthal Angular Characteristics via the Application of Target Decomposition to Spectral Domain Circular SAR Images

In this paper, we discuss the application of polarimetric target decomposition (TD) theorems to circular synthetic aperture radar (CSAR) images to retrieve the polarimetric azimuthal angular characteristics of the targets. In CSAR systems, radar-carrying aircraft moves along a circular path. The antenna beam is pointed toward the center of the circular path during the data acquisition to irradiate the spotlighted area from various azimuthal or aspect angles. Therefore, the resultant polarimetric CSAR images contain information about the polarimetric scattering mechanisms of the targets at each aspect angles. To recover this information, we propose the use of spectral decomposition of CSAR images in combination with several TD algorithms. The spectral domain CSAR images can be viewed as the azimuthal angular spectrum images. Thus, the application of TD algorithms to the spectral domain images is expected to be able to obtain the angular dependence and polarimetric scattering properties of the targets simultaneously. We carried out a simple numerical simulation and an indoor laboratory experiment to validate the proposed TD scheme.

Moving Target Tracking Based on Improved GMPHD Filter in Circular SAR System

A new moving target tracking method based on an improved Gaussian mixture probability hypothesis density (GMPHD) filter in a circular synthetic aperture radar (SAR) system is proposed. In order to obtain the temporal dynamic behavior of moving targets in a SAR scene, a multiple-target observation is first extracted from a sequence of clutter-suppressed subaperture circular SAR images. Then, a new multiple-target tracking model for the circular SAR system is established based on the dynamic behavior of moving targets. An improved labeled GMPHD filter with an adaptive target birth intensity is proposed to realize the moving target detection and tracking in a strong ground clutter environment, which is suitable for the circular SAR system. This method can not only provide accurate estimates for the number and states of moving targets but also can extract their trajectories. The effectiveness of the proposed method is verified through simulations and experimental Gotcha data.

Resolution Analysis of Circular Synthetic Aperture Radar Noncoherent Imaging

Circular synthetic aperture radar (CSAR) has attracted much attention in the field of high-resolution SAR imaging. The spatial resolution is usually considered as the most significant parameter for SAR image quality evaluation and has been widely applied for different monostatic/bistatic SAR operation modes. However, it is hard to get an analytical solution to calculate exactly the value of spatial resolution, especially for the circumstance of CSAR noncoherent imaging. In this paper, based on general impulse response function (IRF) of CSAR noncoherent imaging, a method to obtain the quasi-analytical result of the spatial resolution is proposed, which takes the fractional bandwidth and the subaperture angle into consideration. First, the IRF for CSAR is introduced. Second, based on the IRF, the spatial resolution of CSAR noncoherent imaging is derived. Moreover, the half-power beam-width factor is presented, which allows obtaining the form of spatial resolution in an easier way. Finally, the effectiveness of the proposed method is verified with the simulated and experimental CSAR data.

A modified Omega-K algorithm for squint circular trace scanning SAR using improved range model

Abstract For squint circular trace scanning SAR (CTSSAR), the circular flight trajectory and squint angle make the conventional hyperbolic range model (CHRM) hard to accurately represent the true range history. Traditional frequency domain algorithms cannot accomplish precise imaging directly. To address this problem, a modified Omega-K algorithm with improved range model is proposed. The algorithm can be divided into two main steps: Range-independent error compensation and range-dependent error compensation. By employing the improved range model, the complex coarse range-independent range phase error is compensated. Then the omega-K algorithm is implemented to remove the residual range-dependent phase error for target focusing. Simulation results validate the effectiveness of the proposed range model together with the imaging algorithm.