Dual-channel Synthetic Aperture Radar Research Articles

Blocked Azimuth Spectrum Reconstruction Algorithm for Onboard Real-Time Dual-Channel SAR Imaging

Dual-channel synthetic aperture radar (SAR) is a widely used technology to achieve high-resolution and wide-swath imaging in current SAR system. Due to the fact of nonuniform sampling and spectrum ambiguity, the Doppler spectrum must be reconstructed before focusing. However, the traditional spectrum reconstruction algorithms require large onboard storage memory and computational resources and hence, make it hard to realize real-time processing. To solve the problem, this letter proposes a blocked spectrum reconstruction algorithm for onboard real-time imaging. Different from the traditional ones, the new method updates parameters block by block in azimuth, and hence, reducing the storage and computational resources at the cost of sacrificing the accuracy of the spectrum reconstruction to some extent. Fortunately, the accuracy loss is acceptable under properly set focusing depth, i.e., azimuth block width. The simulation and real data experiments verify the proposed approach.

New Channel Errors Estimation Method for Multichannel SAR Based on Virtual Calibration Source

The multichannel synthetic aperture radar (SAR) system can effectively overcome the fundamental limitation between high-resolution and wide-swath. However, the unavoidable channel errors will result in a mismatch of the reconstruction filter and false targets in pairs. To address this issue, a novel channel errors calibration method is proposed based on the idea of minimizing the mean square error (MMSE) between the signal subspace and the space spanned by the practical steering vectors. The practical steering matrix of each Doppler bin can be constructed according to the Doppler spectrum. Compared with the time-domain correlation method, the proposed method no longer depends on the accuracy of the Doppler centroid estimation. Besides, compared with the orthogonal subspace method, the proposed method has the advantage of robustness under the condition of large samples by using the diagonal loading technique. To evaluate the performance, the results of simulation data and the real data acquired by the GF-3 dual-channel SAR system demonstrate that the proposed method has higher accuracy and more robustness than the conventional methods, especially in the case of low SNRs and high non-uniformity.

Multi-Channel SAR System Design for HRWS and GMTI in Low PRF

High-resolution and wide-swath (HRWS) is an important challenge in synthetic aperture radar (SAR). Combined SAR with array, the azimuth dual-channel SAR configuration achieves high azimuth resolution and HRWS in low Pulse repetition frequency (PRF) by doppler band synthesizing technology. However, low PRF introduces ambiguity to ground moving target indication (GMTI). Interferometry is performed between two look data in the range frequency domain to resolve the ambiguity by decreasing the interferometry phase. As a result, the phase becomes insensitive to the radial velocity of moving targets. This condition brings difficultly in moving target detection. Two channels together with two equivalent channels are exploited fully to solve these problems. The four-channel interferometry is also conducted to improve the GMTI performance. Parameters, including system and motion parameters, are estimated unambiguously to complete HRWS imaging and GMTI. System parameter design is also considered to satisfy the requirements of HRWS and GMTI simultaneously. Simulations demonstrate the validity of the proposed methods.

Channel Error Effect Analysis for Reconstruction Algorithm in Dual-Channel SAR Imaging

Dual-channel synthetic aperture radar (SAR) system is promising in high-resolution wide-swath (HRWS) imaging. However, gain and phase unbalance in the dual-channel SAR system will severely degrade the performance. To make clear the channel error effect on the image of the reconstruction algorithm, a precise analysis of the channel error effect is proposed in this letter. Based on the channel error effect analysis, the amplitude and position of the ambiguity caused by the channel error can be calculated, which is significant for understanding the image quality degradation caused by channel unbalance. Finally, based on the Chinese first dual-channel spaceborne SAR system—GF-3, the validity of this letter is verified by the simulated and real data, respectively.

Distributed Compressed Sensing Based Ground Moving Target Indication for Dual-Channel SAR System.

The dual-channel synthetic aperture radar (SAR) system is widely applied in the field of ground moving-target indication (GMTI). With the increase of the imaging resolution, the resulting substantial raw data samples increase the transmission and storage burden. We tackle the problem by adopting the joint sparsity model 1 (JSM-1) in distributed compressed sensing (DCS) to exploit the correlation between the two channels of the dual-channel SAR system. We propose a novel algorithm, namely the hierarchical variational Bayesian based distributed compressed sensing (HVB-DCS) algorithm for the JSM-1 model, which decouples the common component from the innovation components by applying variational Bayesian approximation. Using the proposed HVB-DCS algorithm in the dual-channel SAR based GMTI (SAR-GMTI) system, we can jointly reconstruct the dual-channel signals, and simultaneously detect the moving targets and stationary clutter, which enables sampling at a further lower rate in azimuth as well as improves the reconstruction accuracy. The simulation and experimental results show that the proposed HVB-DCS algorithm is capable of detecting multiple moving targets while suppressing the clutter at a much lower data rate in azimuth compared with the compressed sensing (CS) and range-Doppler (RD) algorithms.

Effective baseline estimation in dual-channel synthetic aperture radar for moving target imaging and relocation

In a dual-channel synthetic aperture radar (SAR) system with the phase centers being positioned along the track, the motion parameters can be obtained by the interferometric phase. When the phase centers of a dual-channel system misalign with the moving track, the interferometric phase depends on the effective baseline length, defined as the projection of the physical baseline on the moving track of the platform. Therefore, the accurate estimation of the effective baseline is crucial for the motion parameter estimation of moving targets in a dual-channel SAR system. This paper proposes a robust model for the estimation of the effective baseline, which considers the motion errors brought by the yaw and rolling of the platform. It also employs an average-median filter to improve the estimated accuracy of the effective baseline in range-compressed domain by considering the error of interferometric phase, such as Taylor approximate expansion, clutter pollution and noise influence. Furthermore, the approach for estimating the along-track and radial velocities of moving targets is presented in detail. The accuracy of the proposed method for effective baseline estimation will be analyzed by the root-mean-square-error (RMSE) compared with the Cramer–Rao bound (CRB). The results by applying the proposed method into a set of real SAR data are consistent with the analysis presented in this paper.

Vibration Target Imaging Based on the DPCA Technique in Dual-channel SAR

This paper studies the vibration target imaging methods in dual-channel synthetic aperture radar (SAR). Firstly, the echo model of vibration target has been established in dual-channel SAR. According to the analysis of the echo characteristics, an imaging method in original signal domain has been proposed to make vibration target refocused in SAR images. In the meanwhile, due to the problem that when there are static targets in the range cell of vibration targets, the imaging method in original signal domain can’t focus on these two kinds of targets at the same time, another vibration target imaging method based on the displaced phase center antenna (DPCA) technique has been presented as a solution. The validity of the two imaging methods has been verified by simulation, which also points out that compared with the imaging method in original signal domain, the imaging method in DPCA signal domain has better anti-noise performance.

Demonstration of Dual-Channel TOPS SAR Imaging With Airborne C-Band Data

Multichannel in azimuth synthetic aperture radar (SAR) operating in the terrain observation by progressive scans (TOPS) acquisition mode has attracted much attention recently for its capability to achieve ultrawide-swath imaging with a high spatial resolution. In order to verify the feasibility and operability of this newly developed remote sensing concept, a C-band airborne azimuth dual-channel TOPS SAR has been designed by the Institute of Electronics, Chinese Academy of Sciences, as a test bed for future spaceborne realizations. This paper introduces the experimental SAR system and reports the data processing results of an outfield experiment conducted in late September 2014. The importance of the experiment resides in its potential to validate several important technical aspects of this novel SAR operation with real experimental data, including channel mismatch cancellation and unambiguous signal reconstruction. Besides, two kinds of processing methods are proposed to calibrate the influence of antenna phase center fluctuation occurred in the dual-channel TOPS SAR. Finally, the experimental results obtained, including the phase mismatch cancellation and the focused imageries, are presented and analyzed.

Unambiguous Imaging of Static Scenes and Moving Targets with the First Chinese Dual-Channel Spaceborne SAR Sensor.

Multichannel synthetic aperture radar (SAR) is a breakthrough given the inherent limitation between high-resolution and wide-swath (HRWS) faced with conventional SAR. This paper aims to obtain unambiguous imaging of static scenes and moving targets with the first Chinese dual-channel spaceborne SAR sensor. We propose an integrated imaging scheme with the dual-channel echoes. In the imaging scheme, the subspace-based error estimation algorithm is first applied to the spaceborne multichannel SAR system, followed by the reconstruction algorithm prior to imaging. The motion-adapted reconstruction algorithm for moving target imaging is initially achieved with the spaceborne multichannel SAR system. The results exhibit an effective suppression of azimuth ambiguities and false targets with the proposed process. This paper verifies the accuracy of the subspace-based channel error estimator and the feasibility of the motion-adapted reconstruction algorithm. The proposed imaging process has prospects for future HRWS SAR systems with more channels.

Estimating Ambiguity-Free Motion Parameters of Ground Moving Targets From Dual-Channel SAR Sensors

In this paper, we propose an approach for ambiguity-free motion parameter estimation of ground moving targets with arbitrary velocity and constant acceleration from dual-channel synthetic aperture radar (SAR) sensors. For targets with high and low signal-to-clutter-noise ratios (SCNRs), different analytic expressions for along/cross-track velocities and accelerations estimation are derived, respectively. The ambiguities induced by the motion parameters in practical applications are analyzed in detail and appropriate solutions are proposed. We use the nth-order keystone transform to mitigate the range migration induced by the target's residual motion after the ambiguity removal. The effectiveness of the proposed algorithm is verified by processing both simulated and measured stripmap SAR data.

NOVEL DATA ACQUISITION METHOD FOR INTERFERENCE SUPPRESSION IN DUAL-CHANNEL SAR

Various interference sources either intentional or unintentional can mask the synthetic aperture radar (SAR) signals and cause image degradation. With a novel data acquisition mode, a new method based on dual-channel SAR is applied to suppress the interference. Using the received dual-channel data, the two-dimensional location of the interference source can be estimated and then the interference can be removed via a Two-Channel-Cancelation method. By establishing a linear model of the interference-removed signal, the SAR image is reconstructed based on compressed sensing (CS) theory. Our method requires only a minor change to the traditional SAR system hardware while obtains a higher resolution. Simulation results are shown to demonstrate the validity of the proposed method.

Compressive sensing‐based ground moving target indication for dual‐channel synthetic aperture radar

Multi-channel synthetic aperture radar (SAR) system has excellent performance of main-lobe clutter suppression. However, the resulting enormous amount of sampling raw data increases storage and transmission load. To alleviate such payloads, the authors propose a SAR/ground moving target indication (GMTI) method using compressive sensing (CS) with a very limited number of echo samples, based on the fact that the moving targets are usually sparse although clutter scattering centres are non-sparse in most cases. In the proposed method, dual channel SAR data are sampled sparsely in the azimuth direction and jointly processed. Firstly, a transform matrix is constructed to separate the energy support areas of moving targets from that of all scattering centres. Then, the authors can roughly obtain the energy support areas of all scattering centres via CS. Finally, based on the acquired energy support areas above, GMTI is achieved by solving a weighted l 1 optimisation problem. Simulated and real data experiments demonstrate that the proposed method performs well with reduced sampled raw data, even if clutter scattering centres have a low-sparse level.

Vibration target detection and vibration parameters estimation based on the DPCA technique in dual-channel SAR

This paper proposes a new method based on the displaced phase center antenna (DPCA) technique for the detection and parameters estimation of vibration target in dual-channel synthetic aperture radar (SAR). The echo model of vibration target is established in dual-channel SAR according to the equivalent phase center principle. By accumulating the DPCA signal amplitude in the azimuth direction, we realize the vibration target detection in clutter and noise background. Through the analysis of DPCA signal amplitude and phase characteristics of vibration target, we then convert the vibration parameters estimation to estimation of a signal with the form of absolute value of a sine function, and the converting conditions is also given. Accordingly, an algorithm combining the Fourier transform with least squares is proposed for estimation. The simulation results show that the algorithm can estimate each vibration parameter precisely at a low signal-to-noise ratio.

NOVEL PRE-PROCESSING TECHNIQUES FOR COHERENCE IMPROVING IN ALONG-TRACK DUAL-CHANNEL LOW FREQUENCY SAR

The coherence between the complex image pair from two channels is important for improving the capability of along- track interferometry (ATI) processing in synthetic aperture radar (SAR) ground moving target indication (GMTI). The along-track dual-channel low frequency SAR can be easily in∞uenced by not only mismatch errors of the image pair but also the radio frequency interference (RFI). RFI has great impacts on the joint probability density function (PDF) of magnitude and phase in the interferometric image. However, little work has been done to investigate the coherence improvement under RFI. This study develops an algorithm to improve the coherence of the image pair for along-track dual-channel low frequency SAR, which can be used by ATI. After analyzing RFI imaging in detail, it is proposed that the along-track interferometric image in the range frequency and cross-range slow time domain can be used to detect RFI. Median fllters are proposed to further suppress RFI. This suppression has the same implementations to the image pair without heavy computation load. Considering RFI suppression and mismatch errors compensation, a pre-processing ∞ow is proposed to achieve high coherence of the interferometric image in low frequency SAR. It is shown that the coherence of the complex image pair can be improved greatly by using this pre-processing ∞ow. The efiectiveness of this algorithm is demonstrated with real data acquired by an airborne along-track dual-channel P-band SAR GMTI system.

Performance analysis of the TerraSAR-X Traffic monitoring concept

With TerraSAR-X, Germany will launch its first dual-channel SAR satellite mission in the year 2006. Besides other commercial applications TerraSAR-X should also be used as a demonstrator for traffic monitoring from space. This paper revises the theoretical background of traffic monitoring with space-based SARs and applies the conclusions to a performance analysis of the TerraSAR-X Traffic monitoring system. As it is well known, an object moving with a velocity deviating from the assumptions incorporated in the focusing process of the synthetic aperture radar (SAR) principle will generally appear both displaced and blurred in the image. To study the impact of these (and related) distortions in focused SAR images, the analytic relations between an arbitrarily moving point scatterer and its conjugate in the SAR image have been reviewed and adapted to the dual-channel satellite specifications of TerraSAR-X. To be able to monitor traffic under these boundary conditions in real-life situations, a specific detection scheme is proposed. This scheme integrates complementary detection and velocity estimation algorithms with knowledge derived from external sources as, e.g., road databases. The main focus of this article lies on an extensive analytical and empirical accuracy analysis for both vehicle detection and velocity estimation. The accuracy analysis includes a theoretical accuracy evaluation and a validation with real data.

Compensating the effects of target acceleration in dual-channel SAR–GMTI

The authors examine the influence of uncompensated target acceleration on the focusing of moving targets in airborne synthetic aperture radar (SAR) imagery and present one method of detecting and compensating for its effects. Although vehicles travelling on roads and highways routinely experience acceleration, the majority of ground moving target indication algorithms assume a constant velocity scenario, which may result in a defocused target response. Both along-track and across-track accelerations are examined through simulations and experimental data from Environment Canada's airborne CV 580 dual-channel SAR system. Acceleration can have severe effects on focusing and may result in azimuthal shift, azimuthal smearing and a significant loss in peak power in the SAR image. Having determined the effects of acceleration, time-frequency (TF) analysis implementing the pseudo-Wigner-Ville distribution is used to improve target focusing and to detect the presence of significant acceleration. Accelerating targets in experimental airborne data are presented and are identified as such by their non-linear TF histories. Estimations of the instantaneous frequency of the signals yield reconstructed target phase histories, which may be used to identify the presence of certain acceleration components and to obtain a focused image of each target. However, estimates of a target's acceleration and velocity vector may not be uniquely determined using only two receive channels.