Squinted Synthetic Aperture Radar Research Articles

Improved reliable high-order SPECAN algorithm for highly squinted SAR imaging processing

AbstractFor the processing of highly squinted synthetic aperture radar (SAR) echo signals, three key challenges need to be considered: rectifying the nonzero Doppler centroid, compensating for azimuthal side-lobe defocusing (ASLD), and correcting the range cell migration (RCM). To address these three problems, we developed a reliable improved fourth-order spectral analysis (SPECAN) algorithm for highly squinted SAR imaging in this study. First, we present a fourth-order phase model (FoPM) that is more suitable for the highly squinted SAR system through a theoretical analysis. Second, based on the FoPM, we derive an improved fourth-order SPECAN algorithm in detail. In this derivation, the nonzero Doppler centroid, the ASLD, and the RCM caused by the high squint angle are corrected. Moreover, the whole simulation procedure of the improved algorithm only contains fast Fourier transform and complex multiplication, so the proposed algorithm can efficiently process highly squinted SAR echoes. Furthermore, the results of a comparison with the traditional SPECAN algorithm show the better performance of the proposed algorithm.

Modified Auto-Focusing Algorithm for High Squint Diving SAR Imaging Based on the Back-Projection Algorithm with Spectrum Alignment and Truncation

The study focuses on addressing the image defocusing issue caused by motion errors in highly squinted Synthetic Aperture Radar (SAR). The traditional auto-focusing algorithm, Phase Gradient Autofocus (PGA), is not effective in this mode due to difficulties in estimating the phase gradient accurately from strong point targets. Two main reasons contribute to this problem. Firstly, the direction of the energy-distributed lines in the Point Spread Function (PSF) does not align with the image’s azimuth direction in highly squinted mode. Secondly, the wavenumber spectrum of high squint SAR images obtained using the Back-Projection Algorithm (BPA) varies spatially, causing aliasing in the azimuth spectrum of all targets. In this paper, a new auto-focusing method is proposed for highly squinted SAR imaging. The modifications to the BP imaging grids have been implemented to address the first problem, while a novel wavenumber spectrum shifting and truncation method is proposed to accurately extract the phase gradient and tackle the spatial variation issue. The feasibility of the proposed algorithm is verified through simulations with point targets and processing of real data. The evaluation of the image shows an average improvement of four times in PSLR (Peak-Sidelobe-to-Noise Ratio).

Ultra-High-Resolution Spaceborne and Squint SAR Imaging for Height-Variant Geometry Using Polynomial Range Model

In this article, we propose an entire processing chain that addresses the geometrical defocusing problems caused by a large orbital arc, a high squint, and topography variations in ultra-high-resolution (UHR) synthetic aperture radar (SAR) operations. Slant-range histories are formulated using fifth-order polynomials to accurately express radar signals with a UHR spaceborne and squint SAR geometry. SAR focusing is first conducted with a constant height assumption. Thereafter, residual topography errors are compensated. Several beneficial contributions are included in the proposed processing chain. First, to achieve constant-height SAR focusing, we improve the typical least-square decomposition-based Stolt interpolations in terms of the image quality and computation time. Second, we clarify how illuminated areas are mapped in a two-dimensional imaging domain after polynomial-based focusing, which is completely different from the well-known relationship between the closest range and zero-Doppler time. Third, a generalized blockwise postfocusing method is devised to correct the range and azimuth defocusing effects induced by height variations. These contributions are verified using numerical simulations. Thus, we can conclude that the proposed method can provide focused SAR images for a height-variant geometry in UHR spaceborne and squint operations.

Extended Polar Format Algorithm (EPFA) for High-Resolution Highly Squinted SAR

The conventional polar format algorithm (CPFA) is widely used for synthetic aperture radar (SAR) because of its simple and efficient operations. However, due to its wavefront curvature assumption, the CPFA’s depth-of-focus (DOF) is extremely small, which greatly limits the scene size, especially for high-resolution and highly squinted (HRHS) SAR. To solve this problem, an extended PFA (EPFA) is proposed in this study, re-deriving mapping functions by expanding the range history into slant- and cross-range components according to the forms of real data storage. This allows the full use of storage data, which the CPFA cannot achieve due to the large approximations introduced by the projection of echo data onto the ground. The wavefront curvature error is then analyzed and eliminated using a space-variant phase compensation function. Due to the high accuracy of expansion in the slant range plane and the space-variant correction processing, the EPFA has a larger DOF than the CPFA. The EPFA is also more suitable for undulating terrains since it avoids the projection of real data onto the ground plane performed in the CPFA. Using comparative analyses of simulated data and real-world images, the results suggest that the proposed EPFA achieves better focusing effects than the CPFA and is particularly useful for HRHS SAR.

Focusing Highly Squinted SAR Data of a Short-Range Wide-Swath Region Using Azimuth-Dependent High-Order RCMC and Frequency Extended Nonlinear Chirp Scaling Based on Equidistant Sphere Model

Focusing highly squinted synthetic aperture radar (SAR) data of a short-range wide-swath (SRWS) region is a challenging task because the reference range linear range cell migration correction (LRCMC) and the inherent range-dependent squint angle produce two-dimensional (2-D) spatial-variant RCM and azimuth-dependent Doppler parameters. In this paper, a two-step imaging algorithm (strategy) based on a novel equidistant sphere model (ESM) is proposed to accommodate these issues. The ESM, which is used for depicting the spatial-variant property of the echo data, is established by an investigation into the property of the origin range after the keystone transform (KT) operation and the reference range RCMC. Based on the ESM, an azimuth-dependent high-order RCMC (ADH-RCMC) method is adopted in the first step to implement the residual azimuth-dependent RCMC. In the second step, a frequency extended nonlinear chirp scaling (FENLCS) algorithm is introduced to achieve the highly varying residual Doppler centroid correction and the azimuth-dependent high-order Doppler parameter equalization. Simulation results in the case of SRWS SAR demonstrate the superior performance of the proposed algorithm.

Frequency-Scaling-Based Spaceborne Squint SAR Sparse Imaging

In synthetic aperture radar (SAR), due to the characteristics of squint data in spaceborne imaging geometry, a more accurate range cell migration correction (RCMC) operation is necessary. Furthermore, imaging degradation due to ignoring range-variant filtering needed for secondary range compression (SRC) limits SAR imaging. In addition, due to enormous computational complexity required for squint SAR data processing, the existing imaging methods, for instance, chirp scaling algorithm (CSA) and range-Doppler algorithm (RDA) are no longer sufficient, especially for large-scale scenes. In order to solve above problems, this paper presents a novel spaceborne squint SAR sparse imaging method, which could not only eliminate the effects of squint to a certain extent, but also improve the performance of focused SAR image. Compared with matched filtering (MF) based squint imaging algorithm, the proposed method can obtain the SAR images with higher quality from full- or down-sampled echo data.

A Novel Algorithm for Hypersonic SAR Imaging With Large Squint Angle and Dive Trajectory

This letter proposes a modified polar format algorithm (PFA) applied to the highly squinted synthetic aperture radar (SAR) onboard an accelerating hypersonic platform. In conventional PFA, the range wavenumber is orthogonally decomposed to obtain the 2-D wavenumbers corresponding to the imaging Cartesian coordinates. This operation, however, is inadequate when facing with more complex motion trajectory. In this letter, the essence of the proposed method is to homogenize the spatially nonuniformly sampled echoes by combining interpolation with the generalized wavenumber definition. The imaging polar coordinate system in the slant plane provides new space-wavenumber Fourier transform pair, and the 1-D interpolation compensates for the range-variant phase errors. The proposed algorithm, thus, enables fast and effective imaging of hypersonic SAR in large squint angles and high maneuverability dive mode. Its performance is discussed and analyzed in this letter, including the resolution and the size of the imaging scene. The effectiveness, superiority, and application value of the proposed algorithm are verified by simulation.

Ground Moving Target Imaging for Highly Squint SAR by Modified Minimum Entropy Algorithm and Spectrum Rotation

Ground moving target (GMT) is displaced and defocused in conventional synthetic aperture radar (SAR) image due to the residual phase error of non-cooperative GMT motion. In this study, a GMT imaging (GMTIm) method is proposed for highly squint SAR. As the squint angle become large, the displace and defocus effect of the GMT image become severe and the geometry distortion of the GMT image cannot be ignored. The proposed method first deduced the two-dimensional (2-D) frequency domain signal of the GMT and the bulk compression function of the Range Migration Algorithm (RMA) in highly squint SAR. Then GMT ROI data are extracted and a modified minimum entropy algorithm (MMEA) is proposed to refocus the GMT image. MMEA introduces the idea of bisection into the iteration process to converge more efficiently than the previous minimum entropy method. To overcome the geometry distortion of the GMT image, an equivalent squint angle spectrum rotation method is proposed. Finally, to suppress the GMT image sidelobe, the sparse characteristic of GMT is considered and a sparse enhancement method is adopted. The proposed method can realize GMTIm in highly squint SAR where the squint angle reaches to 75 degrees. The PSNR and ISLR of point target in highly squint SAR is close to that in side-looking SAR. The simulated point target data and ship data are used to validate the effectiveness of the proposed method.

Refocusing of Moving Ships in Squint SAR Images Based on Spectrum Orthogonalization

Moving ship refocusing is challenging because the target motion parameters are unknown. Moreover, moving ships in squint synthetic aperture radar (SAR) images obtained by the back-projection (BP) algorithm usually suffer from geometric deformation and spectrum winding. Therefore, a spectrum-orthogonalization algorithm that refocuses moving ships in squint SAR images is presented. First, “squint minimization” is introduced to correct the spectrum by two spectrum compression functions: one to align the spectrum centers and another to translate the inclined spectrum into orthogonalized form. Then, the precise analytic function of the two-dimensional (2D) wavenumber spectrum is derived to obtain the phase error. Finally, motion compensation is performed in the two-dimensional wavenumber domain after the motion parameter is estimated by maximizing the image sharpness. This method has low computational complexity because it lacks interpolation and can be implemented by the inverse fast Fourier translation (IFFT) and fast Fourier translation (FFT). Processing results of simulation experiments and the GaoFen-3 squint SAR data validate the effectiveness of this method.

Improved Relative Range Equation in Squint SAR for Moving Targets Refocusing

Moving target detection is difficult for synthetic aperture radar (SAR). As SAR is designed for imaging of stationary ground scene, the moving targets would be blurred and displaced in conventional SAR imaging. To increase the signal clutter ratio, the moving targets should be refocused while detecting. Based on relative range equation, one can refocus and detect the moving targets simultaneously by searching the relative velocity. This method has been derived and applied for side looking SAR. In this paper, we extend the relative range equation to squint mode. The procedures of the refocusing method are also illustrated. By introducing a parameter of relative squint angle, the imaging position of the moving target is derived. The refocusing method is validated by simulations. The moving target can be optimally refocused, and the refocused position can be parametrized by the relative motion parameters.

A modified design method of pulse repetition frequency for synthetic aperture radar system based on the single point equivalent squint model

Pulse repetition frequency (PRF) is an essential parameter in synthetic aperture radar (SAR). However, a small PRF value will cause spectrum ambiguity and a large value will increase the computational complexity. In highly squinted SAR imaging with maneuvering platform, the traditional PRF design methods used are airborne and space borne. SAR systems perform with a low precision because of the complex geometric configuration and system deviations. In order to adapt to the maneuvering platform SAR system, a modified design method of PRF is proposed in this paper. First, the azimuth spectrum of echoes is analyzed by establishing the SAR geometry base on the single point equivalent squint (SPES) model. Subsequently, the calculation of azimuth bandwidth is transformed into an optimization problem, and the lower limit of PRF is obtained by the Golden Section Method. Besides, the criteria of spectrum unambiguity and time sequence constraints in PRF design is also discussed in detail. Simulation experiments demonstrate the feasibility and accuracy of the proposed method.

Spatial-Variant SAR Range Cell Migration Correction Using Subaperture Strategy

The coupling between range and azimuth dimensions is the main obstacle for highly squinted synthetic aperture radar (SAR) data focusing. Range walk correction (RWC) processing is effective to remove the linear coupling term, but the residual high order range cell migration (RCM) parts are spatial-variant in both range and azimuth dimensions. In this paper, we propose a precise spatial-variant range cell migration correction (RCMC) method with subaperture processing. The method contains two stages. Firstly, the main component of range-variant RCM is corrected in the coarse RCMC stage. Secondly, data are derived into azimuth subapertures (SAs), an SA-image-domain RCMC is developed by interp correction, where the SA image is obtained using a modified spectrum analysis (SPECAN) algorithm by establishing the relationship between Doppler frequency and residual spatial-variant RCM. In the proposed algorithm, precise compensation of space-variant RCM is implemented by SA processing, which is designed for a better practicality in real-time processing system. Simulated and real measured data experiments are designed to validate the effectiveness of the proposed approach for highly squinted SAR imaging.

Continuous pulse repetition interval variation for highly squint high-resolution spotlight SAR imaging

ABSTRACT Highly squint Synthetic Aperture Radar (SAR) is a competitive candidate for intelligence surveillance and disaster monitoring because of its capacity for multi-angle information acquisition and high spatial resolution. However, large Range Cell Migration (RCM) results in a considerably long receive window and an extremely large volume of invalid data, which burdens the computational load and severely reduces the efficiency of image processing. The existing method is designed to remove range walks using continuous Pulse Repetition Interval (PRI) variation. When a high-resolution system is required, the range curvature is apparent. This paper proposes a novel strategy to remove the RCM of a reference point accurately using continuous PRI variation for highly squint high-resolution spotlight spaceborne SAR. Additionally, the proposed strategy avoids contamination by transmit and nadir interferences. The simulation results show that the proposed method achieves improved data volume reduction performance compared with the existing method.

Microwave Correlation Forward-Looking Super-Resolution Imaging Based on Compressed Sensing

Forward-looking correlated imaging plays an increasingly important role in modern radar imaging systems. It overcomes disadvantages of traditional side or squint synthetic aperture radar (SAR) which is dependent on specific relative motion between the radar and target scene. A new microwave forward-looking correlated 3-D imaging method based on random radiation field combined with sparse reconstruction is proposed in this article. Firstly, phased array radar (PAR) is adopted to form different and random antenna patterns. Then, combined with the compressed sensing (CS) theory, the target image can be recovered with very few samples which can break through Rayleigh resolution limitation. Furthermore, the proposed method can achieve resolution at least 5.5 times higher than real aperture imaging. To raise computation efficiency of sparse reconstruction, an improved quasi-Newton iteration method based on graphics processing unit (GPU) platform is developed. Meanwhile, a GPU-based (NVIDIA Tesla K40c) accelerated computing method can significantly reduce the processing time compared with the time given by a personal computer (PC). Both simulation and field experiment verify the validity of the proposed method.

Squinted Airborne Synthetic Aperture Radar Imaging with Unknown Curved Trajectory.

The imagery of airborne highly squinted synthetic aperture radar (SAR) with curved trajectory is a challenging task due to the translational-variant range cell migration (RCM) and azimuth modulation. However, in most cases of practical application, the curved trajectory cannot be accurately known, which brings greater difficulties to the imaging problem. To accommodate these issues, we propose a novel motion modelling and optimisation based imaging algorithm for the highly squinted SAR with unknown curved trajectory. First, to correct the translational-variant RCM, a coarse-to-fine RCM correction scheme as well as a range perturbation approach is applied. Afterwards, an optimisation model of motion information under the criterion of minimum entropy is built during the azimuth processing by nonlinear chirp scaling (NLCS). Correspondingly, a differential evolution (DE) optimisation strategy is proposed to estimate the motion information in an iterative manner. We empirically compare the proposed algorithms with several state-of-the-art highly squinted curved SAR imaging algorithms. Numerical results show the effectiveness of the proposed method in the case without any prior information of the curved trajectory.

스퀸트 SAR 비선형 신호변환특성 분석결과 기반 시스템 운용범위 예측기법

스퀸트 SAR 비선형 신호변환특성 분석결과 기반 시스템 운용범위 예측기법

2-D Beam Steering Method for Squinted High-Orbit SAR Imaging

Since path curvature becomes severer for higher orbit synthetic aperture radar (SAR), the stripmap mode may not provide a reliable azimuth resolution under different look angles or at different positions. Beam steering is especially valuable herein for adjusting the azimuth resolution under different observation conditions by designing the antenna steering rate. Moreover, considering that the large range migration and center range variation in the squint mode may increase the echo length and reduce the achievable scene width, we proposed a novel 2-D beam steering (TDBS) method, which promises not only a required azimuth resolution but also a wide swath (or shortened echo length) at squint when cooperated with the variable interpulse time (VIPT) technique. The simulation results obtained under different look directions are shown to validate the effectiveness of the proposed beam controlling method.

High-Resolution Squinted SAR Imaging Algorithm Applied in the Near-Field Environment

In the squinted synthetic aperture radar (SAR) imaging of the near-field environment, range-dependent characteristic of squint angle cannot be ignored, which causes azimuth-dependent range cell migration (RCM) after linear range walk correction (LRWC). In this study, an efficient SAR imaging algorithm applied in the near-field environment is proposed. In the processing of the range focusing, LRWC is firstly used to remove the linear RCM. Then, the residual LRCM is expanded into azimuth-invariant and azimuth-variant terms in consideration of the residual LRCM of azimuth-dependent. Range cell migration azimuth scaling (RCMAS) is designed to remove the azimuth-variant term before secondary range compression (SRC) and range compression (RC). In the azimuth focusing, azimuth distortion compensation (ADC) is performed to compensate the azimuth distortion, following which azimuth nonlinear chirp scaling (ANCS) is applied to equalize the frequency modulation (FM) rate for azimuth compression (AC). The simulated results show that more accurate and improved imaging result can be obtained with the proposed algorithm.

스퀸트 SAR 시스템을 위한 최적화된 RCM 궤적의 기하학적 분석

In this study, an optimized characteristic function of the range cell migration(RCM) trajectory for a squint synthetic-aperture radar(SAR) system is analyzed using a geometric approach, and its performance is validated through the reconstruction of an SAR image. To achieve this, a range equation of a squint SAR system is developed in a rotational transformed coordinate, and the characteristic function of the RCM trajectory is transformed to the Doppler domain using a frequency-modulated continuous wave-SAR signal model and Fourier-transform-based signal transformation process. This formula developed through geometric analysis is applied in RCM correction and matched filtering for squint SAR image processing. Its optimized performance is compared and validated in the same process. The analysis results indicate that the resolution included an error of approximately 5 %, compared with the predicted result, in the analysis range within a squint angle of 45°.

A Modified Range Model and Doppler Resampling Based Imaging Algorithm for High Squint SAR on Maneuvering Platforms

There are two technical difficulties to overcome before obtaining a well-focused image from high squint (HS) synthetic aperture radar (SAR) with constant acceleration. One is effective range modeling and the other is the correction of space-variant (SV) Doppler parameters. Based on the imaging characteristics analysis, an orthogonal expansion range model (OERM) is proposed which can handle the coordinate rotation caused by range walk correction (RWC). Then a modified spectral analysis (SPECAN) with the Doppler resampling method is designed to correct the SV Doppler parameters. Finally, the proposed algorithm is verified by both simulated and real SAR data. Meanwhile, it shows an improvement in azimuth focusing quality over the reference one.