Range Cell Migration Research Articles

Maneuvering platform high-squint SAR imaging method based on perturbation KT and subregion phase filtering

The imaging parameters of high-squint synthetic aperture radar (SAR) mounted on maneuvering platforms have obvious spatial variability, which cannot be effectively solved by traditional SAR imaging algorithms and limits the focus depth. To extend the focus depth of maneuvering SAR, an imaging method is proposed based on perturbation keystone transform (KT) and subregion phase filtering (SRPF). In the process of range cell migration correction, the azimuth-dependent range curvature is corrected by introducing the perturbation factor in two-dimensional (2-D) frequency domain, and then the linear range cell migration is corrected by conducting the azimuth KT. In the process of azimuth compression, after removing the negative influence generated by the 2-D frequency-domain perturbation, the precise azimuth time-domain phase is reconstructed. And then, the method of SRPF is proposed to extend the azimuth focus depth. This method divides the range cell into several subregions and carries out the phase filtering and deramp process in each subregion. Theoretical analysis and simulation results show that the proposed method has high efficiency and can effectively expand the focus depth of high-squint SAR mounted on maneuvering platforms.

Adaptive Detection of Dim Maneuvering Targets in Adjacent Range Cells

This letter addresses the detection problem of dim maneuvering targets in the presence of range cell migration. Specifically, it is assumed that the moving target can appear in more than one range cell within the transmitted pulse train. Then, the Bayesian information criterion and the generalized likelihood ratio test design procedure are jointly exploited to come up with six adaptive decision schemes capable of estimating the range indices related to the target migration. The computational complexity of the proposed detectors is also studied and suitably reduced. Simulation results show the effectiveness of the newly proposed solutions also for a limited set of training data and in comparison with suitable counterparts.

Focusing Nonparallel-Track Bistatic SAR Data Using Modified Frequency Extended Nonlinear Chirp Scaling

Unsynchronization of the separate transmit–receive beams makes it a challenging task to obtain high-quality images for nonparallel-track bistatic synthetic aperture radar (NP-BiSAR). To accommodate this issue, we propose an imaging configuration where the receiver’s beam follows the transmitter’s one actively by adjusting the squint angle of receiver. And a frequency extended nonlinear chirp scaling (FENLCS) algorithm is modified to cope with new effects introduced by the innovative configuration, which is based on an improved quadratic ellipse model. Based on the new model, some innovative improvements on image formation are made, including a residual azimuth-dependent high-order range cell migration correction (ADH-RCMC) and a rederived FENLCS that takes highly varying Doppler centroid into consideration, which contribute to better imaging quality. Simulation results validate the effectiveness of the proposed configuration and algorithm.

A Novel Compressive Sensing-Based Multichannel HRWS SAR Imaging Technique for Moving Targets

When high-resolution wide-swath (HRWS) multichannel synthetic aperture radar (MC-SAR) system is used for ocean observation, vast amount of redundant data is generated, significantly limiting its applications. Though compressive sensing (CS)-based method has been applied to the traditional single-channel or dual-channel SAR imaging system, it is no longer applicable for MC-SAR due to the existence of channel error when using the space-time equivalent sampling technique for azimuth signal reconstruction. By analyzing such periodic channel error, i.e., frequency-dependence phase mismatch (FD-PM), in this article, a novel dictionary is constructed for CS-based HRWS MC-SAR imaging after an improved range cell migration correction method is applied. As a result, a novel CS imaging mode is proposed for the ocean moving target based on the sparsity of the target scattering centers, by which the amount of data in MC-SAR can be reduced by sampling below the Nyquist sampling rate and the swath width can be further increased. Experimental results show that the proposed method clearly eliminates the azimuth defocus and blur caused by low sampling rate and FD-PM, and significantly reduces the amount of data to about one-third when compared to sampling at the Nyquist rate.

A Hybrid Integration Method for Moving Target Detection With GNSS-Based Passive Radar

Global navigation satellite system (GNSS) based passive radar has been applied in the detection of moving targets. However, the low signal power of GNSS on the earth's surface limits the application of this technology for the long-range or low-observable target detection. Increasing the observation time can effectively improve the detection capability. But the target motion involves the range cell migration (RCM) and the Doppler frequency migration (DFM) over the long observation time, which results in the integration gain loss and lower the detection performance. This article proposes a new hybrid coherent and noncoherent integration method named the keystone transform and Lv's distribution. The proposed method not only compensate the RCM and the DFM but also provide coherent and noncoherent integration gains to increase the signal-to-noise ratio. The simulated results and the field trial results demonstrate that the detection performance of the proposed method is superior to the other two known moving target detection methods. And the analysis of the computational complexity shows that the proposed method and the other two methods are in the same order of O(N <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> logN).

Structure-Aided 2-D Autofocus for Airborne Bistatic Synthetic Aperture Radar

In this article, a new interpretation of the polar format algorithm (PFA) for general bistatic spotlight synthetic aperture radar (SAR) imaging is presented. From the viewpoint of 2-D decoupling, we examine the commonly adopted implementation of the PFA, i.e., the separable 1-D range and azimuth resampling procedures for their roles in range cell migration (RCM) correction, respectively. Utilizing this new formulation, we analyze the effect of range and azimuth resampling on the residual 2-D phase error and reveal the inherent structure characteristics of the residual 2-D phase error in the wavenumber domain. By exploiting the available a priori knowledge on the phase error structure, a structure-aided 2-D autofocus approach to refocus the defocused PFA imagery is proposed. The proposed approach fully exploits the potentiality of the available data and the a priori knowledge about the phase error that need to estimate, so the accuracy of the residual 2-D phase error estimation and correction can be greatly improved. Finally, experimental results are presented to show the effectiveness of the proposed approach.

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.

Moving Target Imaging Using GNSS-Based Passive Bistatic Synthetic Aperture Radar

Current studies of global navigation satellite systems (GNSS)-based bistatic synthetic aperture radar (GNSS-SAR) is focused on static objects on land. However, moving target imaging is also very significant for modern SAR systems. Imaging a moving target has two main problems. One is the unknown range cell migration; the other is the motion parameter estimation, such as the target’s velocity. This paper proposes a moving target imaging formation algorithm for GNSS-SAR. First, an approximate bistatic range history is derived to describe the phase variation of the target signal along the azimuth time. Then, a keystone transform is employed to correct the range cell migration. To address the motion parameter estimation, a chirp rate estimation method based on short-time Fourier transform and random sample consensus is proposed with high processing efficiency and robust estimation errors in low signal-to-noise ratio scenes. The estimated chirp rate can calculate the target’s velocity. Finally, azimuth compression derivation is performed to accomplish GNSS-SAR imaging. A maritime experimental campaign is conducted to validate the effectiveness of the proposed algorithm. The two cargo ships in the SAR images have good accordance with the ground truth in terms of the target-to-receiver vertical distances along the range and the ships’ length along the cross-range.

Migration Correction Algorithm for Coherent Integration of Low-Observable Target With Uniform Radial Acceleration

Range and Doppler frequency cell migration induced by the movement of the low-observable target would bring about serious integration loss, which may deteriorate the target detection or imaging performance. Hence, this article proposes a migration correction algorithm based on keystone transform and modified generalized radon Fourier transform (KT-MGRFT). More specifically, KT is first utilized to eliminate the linear range cell migration (RCM) induced by the target unambiguous velocity. Then, MGRFT is presented to estimate the fold factor and acceleration. After that, the residual RCM and Doppler frequency cell migration (DFCM) are compensated by multiplying a reference signal constructed by the estimated fold factor and acceleration. Finally, the coherent integration can be achieved via the inverse Fourier transform (IFT) and the Fourier transform (FT) with respect to range frequency and scaled slow time, respectively. Compared with standard GRFT, the proposed method can remove the searching procedure of unambiguous velocity, and thus, it has lower computational complexity. In addition, through taking the maximum value in searching the range-Doppler frequency domain, the blind speed sidelobe (BSSL) effect is suppressed. Both the numerical simulations and the real experiment with the X-band radar are given to verify the effectiveness of the proposed method.

Fast 3-D Image Reconstruction on Nonregular UWB Sparse MIMO Planar Array Using Scaling Techniques

This article proposes a novel multistatic scaling algorithm for fast 3-D imaging with an ultrawideband (UWB) multiple-input-multiple-output (MIMO) planar array architecture. The proposed imaging method well addresses the weaknesses of the state-of-the-art imaging techniques, such as heavy computation burden and requirement of regular MIMO topologies. The key range cell migration in MIMO data set is corrected by a multistatic scaling operation. Only the fast Fourier transform and multiplications are employed during the entire imaging procedure that is easy to implement. For an arbitrary MIMO planar array, the computation complexity of the algorithm is O(N <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> log <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> N), which is lower than the optimum backpropagation and ω-K techniques. We have designed a 16 × 16 UWB sparse MIMO planar array with nonregular topology in our experiments. Results with both synthetic and practical data demonstrate the accuracy and efficiency of the algorithm, i.e., it obtains images of the same quality with much less time compared with the state-of-the-art other imaging algorithms.

An efficient and accurate three-dimensional imaging algorithm for forward-looking linear-array sar with constant acceleration based on FrFT

Linear-array synthetic aperture radar can break through the limitations of traditional SAR on 3-D forward looking imaging, which continues to attract the worldwide attention, especially on 3-D topographic mapping and aircraft blind landing. To overcome the large range migration and strong space-variant characteristic caused by the uniformly accelerated SAR platform, a new 3-D fractional chirp scaling algorithm (FrCSA) for forward looking linear-array SAR with constant acceleration is proposed in this paper. Based on the novel geometry configuration and the equivalent hyperbolic signal model, a chirp scaling operation is employed to compensate the range dependent range cell migration (RCM). Meanwhile, the FrFT tool is introduced to achieve much more accurate focusing. The 3-D imaging results of 3 × 3 × 3 array point scatterers indicate that the proposed method can achieve better focusing capabilities, greater peak side-lobe ratio (PSLR) and integrated side-lobe ratio (ISLR) comparing with the traditional CS algorithm based on FFT.

Using an Equivalence-Based Approach to Derive 2-D Spectrum of BiSAR Data and Implementation Into an RDA Processor

An equivalent range equation with a monostatic equivalent component and a bistatic synthetic aperture radar (BiSAR) compensation component was proposed. There were five monostatic equivalent parameters (MEPs) in the both components. With the MEPs, a BiSAR range equation can be expressed in a form similar to the monostatic SAR (MoSAR) expression. Then, by using the range equation and the principle of stationary phase (POSP), we analytically derive the 2-D spectrum of a point target, and this 2-D spectrum is implemented into the range Doppler algorithm (RDA) that processes the translational invariant (TI) BiSAR data. In our RDA, spatially variant range cell migration (RCM) correction and azimuth compression (AC), both of which are related to the spatially variant MEPs, are required to focus all targets in the whole scene of different range cells. Simulations under a wide range of imaging parameters are conducted to assess the equivalent range equation and the 2-D spectrums. Satisfactory results are obtained. In addition, traditional methods for motion compensation can be directly applied to our equivalent range equation. Finally, the RDA is evaluated through the analysis of acquired raw BiSAR data. Well-focused images are obtained. Therefore, the proposed equivalent range equation, derived 2-D spectrum, and RDA have been validated for forming imagery using raw BiSAR data.

A Novel Azimuth Spectrum Reconstruction and Imaging Method for Moving Targets in Geosynchronous Spaceborne–Airborne Bistatic Multichannel SAR

Geosynchronous spaceborne–airborne bistatic multichannel synthetic aperture radar (GEO-SABM SAR) is a form of bistatic SAR constructed by a GEO transmitter and an airborne multichannel receiver. The frequent coverage of earth observations in GEO-SABM SAR provides a great advantage in SAR-ground moving target indication (SAR-GMTI). However, the bandwidth of the azimuth spectrum in this configuration, which is larger than the pulse repetition frequency (PRF), makes the whole spectrum fold into several subsegments, i.e., azimuth spectrum aliasing (ASA) occurs. In addition, azimuth Doppler ambiguity (ADA) caused by the larger radial velocity of the moving target may occur simultaneously. To address these problems, an improved velocity SAR (VSAR)-based azimuth spectrum reconstruction method for moving targets is proposed. The influence of ASA and ADA on the signal of the moving targets is analyzed, and a signal model after coarse imaging with static target parameters is deduced. From the signal model, each subsegment has different Doppler frequencies and can be extracted based on improved VSAR. Meanwhile, the Doppler ambiguous integer has a definite relationship with the slope of the range migration of the subsegments after a range cell migration correction (RCMC) based on the static target parameters and can be estimated by this relationship. Furthermore, based on the extracted spectrum subsegments and estimated Doppler ambiguous integer, the azimuth spectrum can be reconstructed. Finally, the effectiveness of the proposed method is verified by the numerical experiments.

Integration of Rotation Estimation and High-Order Compensation for Ultrahigh-Resolution Microwave Photonic ISAR Imagery

The microwave photonic (MWP) radar technique is capable of providing ultrawide frequency bandwidth waveforms to generate ultrahigh-resolution (UHR) inverse synthetic aperture radar (ISAR) imagery. Nevertheless, conventional ISAR imaging algorithms have limitations in focusing UHR MWP-ISAR imagery, where high-precision high-order range cell migration (RCM) and phase correction are crucially necessary. In this article, a UHR MWP-ISAR imaging algorithm integrating rotation estimation and high-order motion terms compensation is proposed. By establishing the relationship between parametric ISAR rotation model and high-order motion terms, an average range profile sharpness maximization (ARPSM) is developed to obtain rotation velocity by using nonuniform fast Fourier transform (NUFFT). Second-order range-dependent RCM is corrected with parametric compensation model by using the rotation velocity estimation. Furthermore, the spatial-variant high-order phase error is extracted to compensation by the entire image sharpness maximization (EISM). A new imaging framework is established with two one-dimensional (1-D) parameter estimations: ARPSM and EISM. Extensive experiments demonstrate that the proposed algorithm outperforms traditional ISAR imaging strategies in high-order RCM correction and azimuth focusing performance.

Data-Driven Motion Compensation for Airborne Bistatic SAR Imagery Under Fast Factorized Back Projection Framework

Due to the independence of azimuth invariance and high implementing efficiency, a fast time-domain algorithm has significant advantages for airborne bistatic synthetic aperture radar (BiSAR) data process with general geometric configuration. In this article, the practical problem of unexpected motion errors of the airborne platform is carefully analyzed under a fast factorized back-projection (FFBP) framework for a general BiSAR process and a coherent data-driven motion compensation (MOCO) algorithm integrated with FFBP is proposed. By utilizing wavenumber decomposition, the analytical spectrum of a polar grid image is obtained where the motion error can be conveniently investigated in image spectrum domain and the coherence between azimuthal phase error (APE) and motion-induced nonsystematic range cell migration (NsRCM) can be perfectly revealed. Then, a new data-driven MOCO method for both APE and NsRCM correction is developed with the FFBP process. Different from the data-driven MOCO in most frequency-domain algorithms, the residual NsRCM introduced by the FFBP process is particularly analyzed and addressed in the MOCO, which significantly improves the image quality in focusing. Promising results from both simulation and raw data experiments are presented and analyzed to validate the advantages of the proposed algorithm for the airborne BiSAR process.

An Improved Imaging Algorithm for High-Resolution and Highly Squinted One-Stationary Bistatic SAR Using Extended Nonlinear Chirp Scaling Based on Equi-Sum of Bistatic Ranges

The linear range walk correction (LRWC) and the inherent azimuth-variant geometric configuration in the case of one-stationary bistatic synthetic aperture radar (OS-BiSAR) imaging produce 2-D variant range cell migrations (RCMs) and azimuth-dependent Doppler parameters, which makes it more difficult to obtain high-quality image for high-resolution and highly squinted OS-BiSAR. To accommodate these issues, an improved extended nonlinear chirp scaling (ENLCS) algorithm is developed in this letter. An analytic model based on the equi-sum of bistatic ranges (ESBR) after the RCM correction is proposed to reveal the azimuth-variant property of the azimuth distributed targets. Based on this innovative model, analytic coefficients of the ENLCS are derived, and high-quality image formation for OS-BiSAR is accomplished. Simulations are conducted to demonstrate the validity of the proposed algorithm.

Azimuth Multichannel Reconstruction for Moving Targets in Geosynchronous Spaceborne–Airborne Bistatic SAR

In a multichannel geosynchronous spaceborne–airborne bistatic synthetic aperture radar (GEO-SA-BiSAR) system, the airborne receiver can obtain high-resolution microwave images with good signal-to-noise ratios (SNRs) by passively receiving echoes from the desired area. Since the Doppler modulation and range history of a moving target are obviously different from a stationary target, a signal geometry model for moving targets in multichannel GEO-SA-BiSAR is established in this paper. According to simulation results, the along track velocity introduces target defocusing in azimuth, and the slant range velocity mainly causes multiple false targets. To resolve these problems, a modified multichannel reconstruction method in azimuth channel GEO-SA-BiSAR is proposed according to the azimuth multichannel impulse response of the imaged moving target. Before azimuth multichannel raw data combination, both spatial-variant range cell migration correction (RCMC) and azimuth nonlinear chirp scaling (ANLCS) should be performed to reduce the influence of the range offset and lower the Doppler bandwidth of the whole raw data, respectively. Afterward, a novel azimuth multichannel reconstruction algorithm is carried out via the modified reconstruction matrix based on the estimated target velocity. The target slant range velocity estimation is implemented by introducing the signal intensity ratio (SIR). Compared with the conventional method for the stationary target to handle the raw data of the moving target, the false targets could be obviously suppressed by using the proposed approach. Imaging results on both simulated point and distributed scene targets validate the proposed multichannel reconstruction approach.

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.

Azimuth Migration-Corrected Phase Gradient Autofocus for Bistatic SAR Polar Format Imaging

A polar format algorithm (PFA) as a high-efficient imaging method is widely used in bistatic spotlight synthetic aperture radar (SAR). In cases where motion error causes great defocusing in PFA images, phase gradient autofocus (PGA) is suitable for motion error compensation due to its robustness. However, a significant coupling of range-azimuth directions in bistatic SAR (BiSAR) makes conventional PGA unapplicable in PFA images. For this limitation, this letter gives the solution to the adaptability of PGA in BiSAR based on the azimuth migration correction in the wavenumber domain. The bistatic motion error is modeled, and the caused phase error is derived and analyzed. Through the wavenumber-domain analysis, the 2-D coupling is indicated still existing after polar format imaging, unlike other algorithms that utilize range cell migration correction (RCMC). Then, a phase compensation for the migration of azimuth direction is proposed to remove the 2-D coupling. Based on this, the new autofocus arithmetic flow for bistatic PFA is designed. The effectiveness of the proposed method is verified by numerical simulations.

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°.