Range Cell Migration Correction Research Articles

SAR Ground Moving Target Imaging Based on a New Range Model Using a Modified Keystone Transform

Imaging of a ground moving target (GMT) with synthetic aperture radar (SAR) is a challenging task, particularly when the Doppler ambiguity happens. This paper proposes a novel GMT range model and a novel Doppler ambiguity-tolerated GMT imaging algorithm based on modified keystone transform. In the proposed GMT range model, the range from radar antenna to the scattering center on the target is divided into two parts: the range from radar antenna to the target centroid and the projection length on the light of sight from the target centroid to the scattering center. Based on the new range model, a Doppler ambiguity-tolerated range cell migration correction (RCMC) method is proposed, in which keystone transform is modified to realize the differential RCMC by interpolation with nonzero phase sinc kernel. Finally, the GMT image is obtained in the range-Doppler domain by matched filtering in the slow-time domain and is restored into the 2-D space domain. The effectiveness of our proposed model and imaging method is demonstrated by both simulated and real airborne SAR data.

Range and Doppler Cell Migration in Wideband Automotive Radar

The automobile industry has a strategic intent to develop vehicles capable of higher and higher levels of autonomy. Radar is a key sensing component, being the only viable option for all weather, 24-hour-a-day use. Autonomy will require the radar to provide much greater levels of detailed information than is available from today's cruise control and collision avoidance systems. In particular, much wider bandwidths are needed for high range resolution. Furthermore, high Doppler resolution, requiring long integration times, is needed for moving object classification. These two opposing requirements create significant design and signal processing challenges. This paper examines the way in which choices in the key design parameters effect performance, showing that as both range and Doppler resolutions are enhanced, the potential for received energy to migrate across resolution cells increases and must be corrected to enable measurement of both focused high-resolution imagery and micro-Doppler signatures, as well as restoring system sensitivity. An advanced range and Doppler cell migration correction algorithm is introduced and performance evaluated using both theory and experimentation.

A Two-Step Nonlinear Chirp Scaling Method for Multichannel GEO Spaceborne–Airborne Bistatic SAR Spectrum Reconstructing and Focusing

Due to the high-altitude illumination and the separation of the receiver and transmitter, geosynchronous (GEO) spaceborne–airborne bistatic synthetic aperture radar (BiSAR) is more flexible and accessible in remote sensing applications. In this paper, the Doppler characteristics of GEO BiSAR with a squint receiver are analyzed. It is found that the Doppler spectrum is generally aliased in GEO BiSAR regarding the low pulse repetition frequency (PRF) adopted by the GEO SAR. In order to suppress the ambiguity without adjusting the PRF of GEO SAR, the azimuth multichannel receiving technique is applied to the receiver and then the multichannel transfer function for GEO BiSAR is derived. However, the whole bandwidth of the imaging scene is much larger than that of the center point, which requires extra receiving channels to suppress the ambiguity and thereby increasing the system complexity. A two-step nonlinear chirp scaling (NLCS) method is proposed to obtain the well-focused image with reduced receiving channels. First, a preprocessing step is conducted to achieve space-variant range cell migration correction. After that, the first-step NLCS processing is applied to equalize the 2-D space-variant Doppler centroid and thereby the Doppler bandwidth is decreased, i.e., the required number of receiving channels for reconstruction is reduced. Then, the unambiguous spectrum is reconstructed based on the proposed multichannel transfer function. Finally, the second-step NLCS processing is carried out to equalize the 2-D space-variant high-order Doppler parameters and obtain the well-focused image. The simulation results validate the effectiveness of the proposed method. With the proposed two-step NLCS method, the well-focused image for GEO BiSAR is obtained and the required number of receiving channels can be decreased, which is beneficial to reducing the system complexity and hardware cost.

Space‐variant RCMC method for squint beam‐steering SAR imaging on high‐speed manoeuvring platforms

The focusing of squint beam-steering synthetic aperture radar (SAR) data is a challenging task for the technical difficulty of space-variant range cell migration correction (RCMC). It is more complicated when considering the manoeuver of high-speed platforms. The conventional Doppler domain RCMC method is with low performance and requires a large amount of zero-padding. This Letter proposes a modified RCMC method which removes the space-variant range cell migration (RCM) components by deramp-keystone transform and then compensates the unified RCM terms in the time domain. Compared with the conventional method, the proposed one is with high performance and can avoid zero-padding. This method is validated by the simulated SAR data.

Range- and Aperture-Dependent Motion Compensation Based on Precise Frequency Division and Chirp Scaling for Synthetic Aperture Radar

The complicated vibrations of the radar sensor, referred to as motion error, generally produce extra range cell migration (RCM) along the range direction as well as spectral replicas along the azimuth direction, which severely impairs the quality of a focused synthetic aperture radar (SAR) image. This paper concerns the compensation of motion error that is deemed to be range- and aperture-dependent. To this end, two pairs of relations, such as shift range and angle frequency, are analyzed. Based on the derived relations, we propose a chirp scaling-based range envelope correction to cope with the range-dependent position shifts (i.e., RCM) and a precise frequency division approach to account for the residual aperture-dependent motion error. The proposed algorithm can compensate bulk and complicated motion error completely before RCM correction and ensure the focusing quality of an SAR image without autofocus. Both simulated and real data experiments validate the effectiveness of the proposed algorithm.

A Novel Azimuth Doppler Signal Reconstruction Approach for the GEO-LEO Bi-Static Multi-Channel HRWS SAR System

For the geosynchronous-low earth orbit (GEO-LEO) bi-static multi-channel high-resolution and wide-swath synthetic aperture radar (HRWS SAR) system, a novel azimuth Doppler signal reconstruction algorithm is developed based on an equivalent baseline approximating in the azimuth Doppler chirp Fourier transform domain. The main difficulty for the GEO-LEO bistatic multi-channel HRWS SAR system is that the baseline is variant with the scene in the conventional Doppler Fourier transform domain during azimuth signal reconstruction. The developed approach first employs the linear range cell migration correction and chirp Fourier transform to obtain the coarse-focused ambiguity SAR image. More importantly, the baseline component can be approximated a uniform baseline length for the whole image scene and the ambiguous Doppler spectrum can be reconstructed using the available Doppler signal reconstruction approach. Finally, the experimental results for a simulation GEO-LEO bi-static multi-channel HRWS SAR system are employed to validate the theoretical investigations and the proposed approach.

Imaging for Small UAV-Borne FMCW SAR.

Unmanned aerial vehicle borne frequency modulated continuous wave synthetic aperture radars are attracting more and more attention due to their low cost and flexible operation capacity, including the ability to capture images at different elevation angles for precise target identification. However, small unmanned aerial vehicles suffer from large trajectory deviation and severe range-azimuth coupling due to their simple navigational control and susceptibility to air turbulence. In this paper, we utilize the squint minimization technique to reduce this coupling while simultaneously eliminating intra-pulse motion-induced effects with an additional spectrum scaling. After which, the modified range doppler algorithm is derived for second order range compression and block-wise range cell migration correction. Raw data-based motion compensation is carried out with a doppler tracker. Squinted azimuth dependent phase gradient algorithm is employed to deal with azimuth dependent parameters and inexact deramping, with minimum entropy-based autofocusing algorithms. Finally, azimuth nonlinear chirp scaling is used for azimuth compression. Simulation and real data experiment results presented verify the effectiveness of the above signal processing approach.

Squint Mode GEO SAR Imaging Using Bulk Range Walk Correction on Received Signals

Geosynchronous synthetic aperture radar (GEO SAR) has the potential for conducting long-term observation of target zones, which is essential for remote sensing applications such as disaster monitoring and vegetation measurements. The squint imaging mode is crucial for long-term observation using GEO SAR. However, this type of SAR imaging is problematic because the squint mode introduces a nonzero range cell walk, which increases the prevalence of invalid data in echoes and intensifies the coupling between the azimuth and range. Therefore, this paper proposes a novel squint mode GEO SAR imaging method based on the correction of the bulk range walk of received signals. Adjusting the starting time of the receiving window significantly reduces the redundancy in echoes. Then, first-order filtering, range cell migration correction, range compression, partial dechirp, and azimuth compression are used to obtain the imaging result. Simulation results for the GEO SAR imaging of Wenchuan County in China demonstrate that the proposed algorithm can achieve a resolution of 5 m within a 30 × 30 km swath over 48% of the orbital period.

A Frequency Domain Backprojection Algorithm Based on Local Cartesian Coordinate and Subregion Range Migration Correction for High-Squint SAR Mounted on Maneuvering Platforms

Accurate range modeling, cross-range-dependent range migration, and space-variant Doppler parameter are main issues to be solved in processing high-squint synthetic aperture radar (SAR) data acquired from maneuvering platforms. A frequency domain backprojection algorithm, based on local Cartesian coordinate (LCC) and subregion range cell migration correction, is proposed to deal with these problems. With the proposed algorithm, the range model is built in an LCC system to accurately match the signal characteristics after range walk correction. Then, the compensation of cross-range-dependent range migration is implemented based on properly divided subregions after azimuth spectrum filtering. Finally, the space-variant Doppler parameter and higher order phase terms are coherently integrated in range-Doppler domain to get the focused subregion images with full resolution of the synthetic aperture. The final image of the entire scene is obtained by directly connecting all subregion images. The results of simulated and real SAR data validate the proposed algorithm.

Nonsystematic Range Cell Migration Analysis and Autofocus Correction for Bistatic Forward-looking SAR

In general, autofocus methods integrated with frequency-domain imaging algorithms are instrumental to obtain a well-focused bistatic forward-looking synthetic aperture radar (BFSAR) image in the presence of motion errors. Nevertheless, before applying autofocus methods to correct the azimuth phase errors, range cell migration (RCM) should be eliminated by the RCM correction (RCMC) procedure in frequency-domain imaging algorithms. With motion errors being taken into account, there always exists some residual nonsystematic RCM (NsRCM), which refers to the residual migration components after the RCMC procedure. For the conventional side-looking SAR, NsRCM is caused by motion errors. On the other hand, NsRCM of BFSAR is originated from motion errors before RCMC and the NsRCM amplified by the RCMC procedure. In this paper, we analyze the different types of NsRCM in BFSAR imaging and their relationship. Based on the analyses, we propose an autofocus NsRCM correction scheme for BFSAR imagery using frequency-domain imaging algorithms that can eliminate the range-dependent NsRCM. The proposed scheme consists of three steps. First, for the BFSAR data after range compression and RCMC, a division procedure is carried out in the azimuth direction. The subblocks with the highest signal-to-clutter ratio along the range direction are selected after the azimuth segmentation procedure. Second, for the selected subblocks, the total NsRCM is estimated based on the minimum-entropy criterion. Based on the estimation results, different parts of the NsRCM are obtained by solving an ordinary differential equation. Third, a two-step compensation of the NsRCM is executed to reach the spatially variant correction. Simulations and experimental results are provided to demonstrate that our proposed scheme is effective for BFSAR imaging.

Fast imaging algorithm for the multiple receiver synthetic aperture sonars

For the multiple receiver synthetic aperture sonar, traditional fast imaging algorithms are limited by the approximation of the point target reference spectrum (PTRS). To solve this problem, a novel imaging algorithm based on a new PTRS is proposed in this study. With the presented method, the matched filtering function in the azimuth dimension must be first derived. Then, the range–azimuth coupling phase is obtained by using the difference between the PTRS and the matched filtering function in the azimuth dimension. To perform the decoupling between the range dimension and the azimuth dimension, the bulk range cell migration correction (RCMC) and the differential RCMC based on the range-dependent sub-block processing method are used. Compared to traditional methods, the new method avoids the complicated series expansion. Furthermore, it can handle the general case with the explicit point of the stationary phase. Processing results of the simulated data and the real data indicate that the high-performance results can be got by using the presented method. Moreover, it further shows that the presented method is very suitable for the wide-swath imagery.

Space-variant motion compensation for synthetic aperture radar under complicated flight path

Motion compensation (MOCO) is an essential procedure for synthetic aperture radar under a complicated flight path. Wide swath and low-precision navigation data limit traditional MOCO algorithms. We present a space-variant MOCO method to solve two problems: (1) estimation of the deviations of radar during data acquisition and (2) range envelope correction (REC) before range cell migration correction. In the first problem, we apply phase-gradient autofocus with correlate input and weighted total least square jointly. In the second one, the chirp-scaling-based (CS) REC is proposed. Finally, both simulation and real data experiments validate the proposed method.

An improved ENLCS algorithm based on a quadratic ellipse model for high-resolution one-stationary bistatic SAR

ABSTRACTHigh-resolution one-stationary bistatic synthetic aperture radar (BiSAR) data focusing is a challenging task due to the large range cell migration (RCM) and the azimuth-variant Doppler phases. In this letter, an extended nonlinear chirp scaling (ENLCS) based on a quadratic ellipse model is investigated for the one-stationary BiSAR configuration. An analytical relationship of iso-equivalent-sum-of-bistatic-range (iso-ESBR) of azimuth distributed targets is derived after range cell migration correction (RCMC) which is combined by linear range walk correction (LRWC) and keystone transform (KT). Following which, a new quadratic ellipse model is constructed to analyze the azimuth-variant property of the Doppler phases. Based on the new quadratic ellipse model, an improved ENLCS algorithm with new coefficients is derived to accomplish the azimuth equalization for azimuth compression. Simulation results validate the effectiveness of the proposed algorithm.

Focusing of Medium-Earth-Orbit SAR Using an ASE-Velocity Model Based on MOCO Principle

The available focusing algorithms for medium-Earth-orbit (MEO) SAR are all based on the complex nonhyperbolic range equation, which may make it more difficult in imaging processing. In this paper, we model the range equation as the standard hyperbolic form based on the motion compensation (MOCO) principle. However, the conventional two-step MOCO may introduce azimuth spectrum expansion due to the potential large motion error, which can lead to severe azimuth ambiguity. To resolve this problem, we develop an omega-K algorithm based on a modified two-step MOCO and an adaptively straight equivalent (ASE)-velocity model. The algorithm is implemented through three-step processing: 1) the modified two-step MOCO does not compensate for the quadratic motion error (the main factor for the spectrum expansion); 2) an ASE-velocity model is introduced to compensate for the quadratic motion error; and 3) an extended Stolt mapping is proposed to perform the accurate range cell migration correction, and the tandem singular value decomposition-nonlinear chirp scaling algorithm is to correct the azimuth-variant phase error and to perform the azimuth compression. Processing of simulated data and airborne SAR real data validates the effectiveness of the proposed algorithm.

Focus High-Resolution Highly Squint SAR Data Using Azimuth-Variant Residual RCMC and Extended Nonlinear Chirp Scaling Based on a New Circle Model

The combination of linear range walk correction and keystone transform is a good choice to focus high-resolution highly squint synthetic aperture radar (SAR) data because it is an effective way to remove linear range cell migration (RCM) completely and mitigate range–azimuth coupling. However, the results of this kind of imaging algorithm produce 2-D-variant residual RCM and variant-dependence Doppler phases. To obtain high-quality SAR image, an improved imaging algorithm using an azimuth-variant residual RCM correction (RCMC) and an extended nonlinear chirp scaling (ENLCS) is proposed in this letter. A new circle model is constructed to analyze the azimuth-variant properties of the residual high-order RCM and the Doppler phases. Based on this circle model, an azimuth-variant residual RCMC is implemented by multiplying a fourth-order phase function, and an improved ENLCS is derived to accomplish the azimuth equalization for azimuth compression. Simulation results validate the excellent performance of the proposed algorithm.

A Nonlinear Chirp Scaling Algorithm for Processing Multi-Receiver SAS Data with Small Squint Angle

Squint multi-receiver synthetic aperture sonar (SAS) data is more challenging to process than data about its common side-looking counterpart because the geometry is complicated. While traditional multi-receiver algorithms mainly handle general side-looking cases, they do not work well for squint cases. Up to now, no multi-receiver algorithm for handling squint multi- receiver SAS data has been reported. This paper focuses on an imaging algorithm for the squint multi-receiver SAS. Firstly, the “non-stop-go-stop” signal model is established for squint multi- receiver SAS. Then a squint multi-receiver nonlinear chirp scaling algorithm (SMANLCSA) is proposed that is suited to process multi-receiver SAS data with small squint angle and SMANLCSA involves three key steps. The first step is to turn squint multi-receiver data into squint single-receiver data. The second step is a Doppler centroid frequency shift. The third step is range cell migration correction using nonlinear chirp scaling. Simulations have shown that SMANLCSA can handle squint multi-receiver data more accurately and effectively than the previous algorithm..

Wide-scene airborne ladar imaging exploiting the synthetic aperture technique with terrain observation by progressive scans

The image scene is limited by the small size of the spot, which is linked with the short wavelength of the laser in synthetic aperture ladar (SAL). The common wide-scene imaging mode in synthetic aperture radar, i.e., the terrain observation by progressive scans imaging mode, is extended and applied in SAL. A suitable imaging algorithm is also proposed according to the characteristics of the signal. First, the azimuth prefiltering based on the spectrum analysis is adopted to eliminate the azimuth spectrum aliasing caused by the steering of the antenna beam. Then, the improved frequency scaling algorithm is applied to complete the range compression and range cell migration correction. Meanwhile, the Doppler frequency shift induced by the continuous moving of the platform is compensated. Finally, the azimuth compression is completed in the Doppler domain, and the SAL image without ambiguity is obtained with the application of the dechirp technique. Simulation results provide the evidence on the effectiveness of the proposed method.

An Optimal 2-D Spectrum Matching Method for SAR Ground Moving Target Imaging

In synthetic aperture radar (SAR) imagery, images of ground moving targets (GMTs) are smeared, distorted, and shifted. Current GMT imaging methods are mostly based on range-Doppler algorithms, which have two main drawbacks: 1) coupling between range cell migration correction (RCMC) and Doppler parameter estimation and 2) cross terms degrade the performance of the nonlinear estimation methods. In this paper, an optimal 2-D spectrum matching method for SAR GMT imaging is proposed. The main innovation or advantage of this method is that the GMT imaging problem is transformed into a constrained optimization problem, and differential evolution is applied to guarantee a high-processing efficiency. As they are associated with the Doppler centroid variation compensation and range shift compensation processing, all GMT point scatterers can be well focused and well located. Compared with the current methods, the improvements exhibited by this method include three main benefits: 1) RCMC and Doppler parameter estimation can be simultaneously accomplished; 2) both along- and cross-track GMT velocities can be simultaneously estimated; and 3) this method can be applied to both monostatic and bistatic SARs. Numerical simulations and experimental data processing have verified the effectiveness and robustness of the proposed method.

A Novel Focus Approach for Squint Mode Multi-Channel in Azimuth High-Resolution and Wide-Swath SAR Imaging Processing

An imaging processing for the squint mode multi-channel in azimuth high-resolution and wide-swath (MC-HRWS) synthetic aperture radar (SAR) system is presented in this paper. First, the linear range cell migration correction (RCMC) is employed to correct the skew Doppler spectrum for each channel echo. Then, a robust Doppler spectrum reconstruction method is proposed, which is based on the robust Capon beamforming and multi-Doppler-direction restriction technique. After the reconstruction, a two-step focus approach is utilized to focus the image, where the first step is utilized to deal with the azimuth-invariance components and the second addresses the azimuth-variance component introduced by the linear RCMC. Since the Doppler centroid estimation is crucial to squint mode SAR imaging, this paper also presents a robust entropy-based Doppler centroid estimation algorithm for an MC-HRWS SAR system, where the correlation function approach is employed to estimate the baseband Doppler centroid. More importantly, an entropy-based Doppler ambiguity number resolving approach is proposed and the bisection algorithm is utilized to accelerate the searching process. Some simulation experiments are conducted to verify our proposal. Besides, the results of some real MC-HRWS SAR data also show that the proposed method works well.

Near-Field Three-Dimensional Planar Millimeter-Wave Holographic Imaging by Using Frequency Scaling Algorithm

In this paper, a fast three-dimensional (3-D) frequency scaling algorithm (FSA) with large depth of focus is presented for near-field planar millimeter-wave (MMW) holographic imaging. Considering the cross-range range coupling term which is neglected in the conventional range migration algorithm (RMA), we propose an algorithm performing the range cell migration correction for de-chirped signals without interpolation by using a 3-D frequency scaling operation. First, to deal with the cross-range range coupling term, a 3-D frequency scaling operator is derived to eliminate the space variation of range cell migration. Then, a range migration correction factor is performed to compensate for the residual range cell migration. Finally, the imaging results are obtained by matched filtering in the cross-range direction. Compared with the conventional RMA, the proposed algorithm is comparable in accuracy but more efficient by using only chirp multiplications and fast Fourier transforms (FFTs). The algorithm has been tested with satisfying results by both simulation and experiment.