Azimuth Multichannel Synthetic Aperture Radar Research Articles

A Minimum-Energy-Based Algorithm for Multichannel Reconstruction

For the azimuth multichannel synthetic aperture radar (SAR), nonuniform sampling is common since the pulse-repetition frequency cannot be selected arbitrarily, which leads to the contradiction between ambiguity and noise suppression in the azimuth signal reconstruction. This letter constructs a novel multichannel signal model by taking the coherence among different Doppler bins into account and proposes a minimum-energy-based multichannel reconstruction algorithm, which can obtain the unambiguous signal spectrum without increasing noise in the nonuniform sampling cases. By comparing the simulation results formed by digital beamforming (DBF), improved DBF (IDBF), and space-time adaptive processing (STAP) algorithms, the proposed algorithm shows better performance in balancing ambiguity and noise.

Signal Reconstruction Algorithm for Azimuth Multichannel SAR System Based on a Multiobjective Optimization Model

This article establishes a multiobjective optimization model to suppress the azimuth ambiguity power and noise simultaneously in signal reconstruction for a multichannel synthetic aperture radar (SAR) system. This multiobjective optimization model extends the theory of multichannel signal processing for reconstructing the SAR signal from the aliased signals. Linear scalarization and a quadratically constrained method for the multiobjective optimization model are applied to obtain $l_{1}$ norm optimization, $l_{2}$ norm optimization, and quadratically constrained optimization, respectively, in signal reconstruction. Azimuth ghosts can intuitively reflect the effects of azimuth ambiguity on SAR images. The $l_{1}$ norm optimization solution leads to a minimum upper bound of azimuth ghosts. A lowest azimuth ambiguity-to-signal ratio (AASR) can be derived by $l_{2}$ norm optimization. By relaxing the constraint of total ambiguity power suppression, one can obtain a minimum noise level in the case of quadratically constrained optimization. The reconstruction performances of the multiobjective optimization model in terms of AASR, signal-to-noise ratio (SNR), and signal-to-ambiguity-plus-noise ratio (SANR) are investigated with respect to the pulse repetition frequency (PRF) and compared with other methods for a multichannel SAR system.

Multiaperture Antenna Architecture Design for Azimuth Uniform Sampling in High-Resolution Wide-Swath SAR

Azimuth multichannel synthetic aperture radar (SAR) is an essential remote sensing device to obtain images with high resolution and wide swath (HRWS). However, using a conventional multichannel phased array antenna, azimuth uniform sampling cannot be ensured, if the operated pulse repetition frequency of the system deviates from the optimum value. A novel multichannel antenna architecture design approach in HRWS SAR is proposed for uniform sampling. Phase center positions of both transmitting aperture and receiving subapertures can be adjusted flexibly by invalidating the elements of the antenna. First, a limited number of elements of receiving subapertures can be closed to adjust receive phase center spacing. The transmit phase center could be further adjusted, if the azimuth nonuniform sampling still cannot be fully compensated for. With this multiaperture antenna architecture, azimuth uniform sampling could be ensured and system performances are obviously improved. Advantages of the proposed azimuth multiaperture antenna architecture adjust method are verified by a simulation experiment.

A Novel Approach to Doppler Centroid and Channel Errors Estimation in Azimuth Multi-Channel SAR

Multi-channel synthetic aperture radar (SAR) in azimuth can overcome the minimum-antenna-area constraint of the conventional SAR in high-resolution and wide-swath (HRWS) imaging. However, the SAR system suffers from amplitude and phase mismatch among channels and nonideal antenna pattern, which will result in azimuth ambiguity and ghost targets in the final image. Therefore, taking the nonbandlimited signal and channel errors into account, a practical azimuth ambiguity-to-signal ratio (AASR) model of multi-channel SAR system is established. Meanwhile, the baseband Doppler centroid (DC) frequency related to channel errors also has an influence on image quality. Then, an effective method is proposed to calculate the baseband DC frequency according to the jumping points of the channel phase errors estimate. Subsequently, considering the effect of azimuth antenna pattern (AAP), a corresponding relationship between the ideal steering vectors and the signal subspace from the decomposing covariance matrix is established. After that, based on the uniqueness of the signal subspace and the correct corresponding relationship, an accurate method is proposed to estimate the channel phase errors by minimizing the minimum mean square error (MMSE) of the signal subspace. Finally, an accurate multi-channel SAR imaging diagram is shown to effectively mitigate the azimuth ambiguous energy caused by channel errors. Simulation and real data experiments, including four channel airborne SAR data with a bandwidth of 210 MHz and the Chinese Gaofen-3 dual receiving channel (DRC) spaceborne SAR data, validate the effectiveness of the proposed calibration method, particularly in low signal-to-noise ratio (SNR).

Azimuth Phase Center Adaptive Adjustment upon Reception for High-Resolution Wide-Swath Imaging.

A spaceborne azimuth multichannel synthetic aperture radar (SAR) system can effectively realize high resolution wide swath (HRWS) imaging. However, the performance of this system is restricted by its two inherent defects. Firstly, non-uniform sampling is generated if the pulse repetition frequency (PRF) deviates from the optimum value. Secondly, multichannel systems are very sensitive to channel errors, which are difficult to completely eliminate. In this paper, we propose a novel receive antenna architecture with an azimuth phase center adaptive adjustment which adjusts the phase center position of each sub-aperture to improve multichannel SAR system performance. On one hand, the optimum value of the PRF can be adaptively adjusted within a certain range by adjusting receiving phase centers to obtain uniform azimuth sampling. On the other hand, false targets introduced by residual channel errors after azimuth multichannel error compensation can be further suppressed. The effectiveness of the proposed method to compensate for non-uniform sampling and suppress false targets is verified by simulation experiments.

A Simultaneous Imaging Scheme of Stationary Clutter and Moving Targets for Maritime Scenarios with the First Chinese Dual-Channel Spaceborne SAR Sensor

The technique of azimuth multichannel synthetic aperture radar (SAR) system has become a potential solution to the irreconcilable conflict between high-resolution and wide-swath (HRWS) confronted with in a traditional SAR system. Unambiguous imaging, especially for a scene with moving targets, is one of the crucial research topics in the HRWS SAR system. This paper proposes a simultaneous imaging scheme of moving targets and stationary clutter for maritime scenarios. First, the moving target echoes are extracted from the stationary clutter. After that, two methods working in completely different principles are used to estimate the radial velocity of each moving target, and the estimated result is used for phase compensation. After that, the moving target echoes are added back to the stationary scene echo and sent to the reconstruction filter. Lastly, the reconstructed echo can be processed by the classical Chirp Scaling (CS) algorithm. Experiments are carried out using the Chinese GaoFen-3 dual-channel data. The estimated velocities of the moving targets are verified by automatic identification service (AIS) information, and the imaging results show that the false targets are effectively suppressed and the moving targets also return to their correct positions along the azimuth.

A Multifeature Autosegmentation-Based Approach for Inshore Ambiguity Identification and Suppression With Azimuth Multichannel SAR Systems

To address the problems of location and suppression of inshore azimuth ambiguous clutter for the azimuth multichannel synthetic aperture radar (SAR) systems, a multifeature autosegmentation-based approach is developed in this paper. This proposed method can segment a SAR image automatically according to the distinctions among main land clutter, ambiguous land clutter, and sea clutter in the features of interferogram's phase and magnitude. First, the finite mixture clutter model for a multilook covariance matrix (MLCM) is built, where the off-diagonal elements of the MLCM contain the information of magnitude and interferogram's phase between azimuth channels. Then, SAR image autosegmentation is carried out by using the expectation maximum algorithm with combination of the aforementioned mixture model, and the isolated points that are segmented incorrectly can be eliminated via exploiting the Markov random field smoothing technique. Finally, azimuth ambiguous clutter can be suppressed by means of the clutter covariance matrix, which is constructed by the training samples of segmented ambiguities. The experiments on simulated data and real data measured by TerraSAR-X demonstrate that the proposed approach can obtain the more accurate position information and good cancellation performance for the azimuth ambiguous clutter, without the accurate system parameters and the information of the sources account for azimuth ambiguities.

Azimuth Multichannel GMTI Based on Ka-Band DBF-SCORE SAR System

The azimuth multichannel synthetic aperture radar (SAR) system is used in ground moving target indication (GMTI). But for Ka-band SAR, the conflicts between high gain and wide swath coverage are very strict. The digital beamforming (DBF) scan-on-receive (SCORE) algorithm can solve this problem very well. This letter introduces a multichannel SAR system with three receiving antennas in the azimuth direction. For each azimuth antenna, eight subapertures are arranged in the elevation direction in order to obtain higher gain and a lower data rate through the DBF-SCORE algorithm. Airborne flight experiments with this system are accomplished successfully and quantities of experimental data are acquired. In this letter, we first derive the signal forms and weighting coefficients in the DBF-SCORE procedures. Then, different GMTI methods including along-track interferometry and displaced phase center antenna are introduced for azimuth multichannel SAR–GMTI based on the DBF-SCORE technology. Finally, airborne raw data with cooperating moving targets are processed successfully, and the estimation errors of the radial velocities meet the expected accuracy requirements.

Fast channel phase error calibration algorithm for azimuth multichannel high-resolution and wide-swath synthetic aperture radar imaging system

The configuration of multiple uniformly spaced channels in azimuth can achieve high-resolution and wide-swath images for synthetic aperture radar (SAR) systems. The unambiguous Doppler spectrum can be reconstructed via digital beamforming (DBF) techniques in the azimuth multichannel SAR system. However, the performance of DBF deteriorates significantly because of the inevitable channel errors in practical applications. Since the SAR antennas are generally unweighted in azimuth, the power of the SAR signal is symmetrically distributed around the Doppler centroid in the azimuth frequency domain. Based on this observation, the covariance matrix of multichannel output in the range-Doppler domain can be regarded as a real matrix when there is no existence of channel errors and the correction of the baseband Doppler centroid is applied. Consequently, we propose a fast calibration algorithm to calculate channel phase errors. The proposed method can acquire the estimation of phase errors directly from the covariance matrix. Thus, the proposed method has the properties of high robustness and low computation load. Theoretical analysis and experiments validate the performance and the efficiency of the proposed algorithm.

Robust Channel Phase Error Calibration Algorithm for Multichannel High-Resolution and Wide-Swath SAR Imaging

High-resolution and wide-swath synthetic aperture radar (SAR) imaging can be achieved by the azimuth multichannel system. The minimum variance distortionless response (MVDR) beamformer can be utilized to suppress azimuth ambiguities. However, the presence of channel phase errors significantly deteriorates the performance of the azimuth multichannel SAR system. Instead of employing subspace techniques, this letter proposes a robust channel phase error calibration algorithm via maximizing the MVDR beamformer output power. Compared with the conventional subspace-based calibration methods, there is no redundancy of channels required to estimate the subspaces in the proposed algorithm. Also, the proposed algorithm is relatively robust, because it avoids the subspace swap phenomenon, which probably takes place at low signal-to-noise ratios for the subspace techniques. Moreover, the proposed method has the advantage of estimating the channel phase errors without covariance matrix decomposition, which reduces the computation load. The simulation experiments and the real data processing validate the effectiveness of the proposed calibration method.

A Weighted Backprojection Algorithm for Azimuth Multichannel SAR Imaging

A backprojection algorithm (BPA), a time domain synthetic aperture radar (SAR) imaging algorithm, can be widely used without modification in various imaging modes, like stripmap, spotlight, sliding spotlight, etc. However, BPA also suffers when used in azimuth multichannel SAR due to the nonuniform sampling problem caused by a nonideal pulse repetition frequency. In this letter, we propose a weighted BPA (WBPA) for azimuth multichannel SAR imaging. Derived from the filter-bank-based reconstruction method, WBPA adds a weighting procedure to BPA. Simulations and an airborne SAR data experiment demonstrate that WBPA can perfectly suppress the azimuth ambiguities under band-limited circumstances. WBPA extends the applicable scope of BPA.

Improved Azimuth Multichannel SAR Imaging for Configurations With Redundant Measurements

The traditional spectrum reconstruction algorithm for nonuniform sampling of a displaced phase center antenna (DPCA) synthetic aperture radar (SAR) system can provide an unambiguous recovery of an original signal through the inverse computation of a joint filter matrix. However, when the sampling positions of different receivers coincide spatially, the joint filter matrix will be non-invertible, thus causing the ineffectiveness of the algorithm. This letter proposes a regularized spectrum reconstruction scheme when the pulse repetition frequency of a multichannel system operates on these critical frequencies, as well as in their neighborhoods. First, a low-order regularization of the joint filter matrix is suggested so that the original spectrum of the multichannel system can be recovered more accurately with a decreased number of nonredundant channels' measurements. Furthermore, in order to improve the output signal-to-noise ratio (SNR) of the reconstruction network, a combinational low-order spectrum recovery scheme is additionally presented, which takes all redundant channels' samples into account. The optimizations bring significant benefits to both the suppression of aliased ambiguities and the advance of the output SNR of the reconstruction network.

Design Considerations of PRF for Optimizing GMTI Performance in Azimuth Multichannel SAR Systems With HRWS Imaging Capability

Using multichannel in azimuth to suppress the Doppler ambiguities allows for high-resolution wide-swath (HRWS) synthetic aperture radar (SAR) imaging, thus overcoming the minimum antenna area constraint of the conventional space-borne SAR. If the degrees of freedom in azimuth are used for the clutter suppression, the ground moving target indication (GMTI) can be achieved. Therefore, a space-borne multichannel SAR system for GMTI has the potential to offer HRWS imaging capability to some extent. However, here, the GMTI performance may suffer from the Doppler ambiguity caused by the undersampling. This paper focuses on the analysis of the influence of pulse repetition frequency (PRF) on GMTI performance in Doppler ambiguity. The multichannel signal models of the clutter and the moving target with Doppler ambiguity are derived in complex image domain. Considering the Doppler ambiguity and the multichannel spatial ambiguity, the influence of PRF on the output signal-to-clutter-plus-noise ratio after adaptive clutter suppression and the signal-to-noise ratio loss after SAR imaging is discussed. Accordingly, the design considerations of PRF for optimizing GMTI performance in multichannel SAR systems with HRWS imaging capability are given, along with a simulation example. Finally, the real-data experiments verify the theoretical investigations.

A NOVEL ARRAY ERROR ESTIMATION METHOD FOR AZIMUTH MULTICHANNEL SAR

Minimum side-zone power to center-zone power ratio (MSCR) method is presented to estimate array errors of azimuth multichannel synthetic aperture radar (SAR). Spaceborne azimuth multichannel SAR is one of the most promising candidates for achieving high-resolution wide-swath imaging. However, array errors brought in by instrument in∞uences and aperture position errors need to be compensated. MSCR method is designed to obtain phase error estimates by minimizing side-zone power to center-zone power ratio, where the side-zone and the center-zone indicate the intervals far from and around Doppler centroid respectively. The proposed method achieves signiflcantly improved performance on phase error estimation especially when signal to noise ratio is low. Experiment results conflrm the validity and solidity of the method.

COMPLETE COMPLEMENTARY SEQUENCE CODING WAVEFORM BASED AZIMUTH MULTI-CHANNEL SPACE-BORNE SAR WITH ULTRA-LOW RANGE SIDELOBE RATIO PERFORMANCE

Sidelobes of strong targets substantially impact image quality of synthetic aperture radar (SAR) using linear frequency modulation (LFM) waveform, especially in urban areas. A novel space-borne azimuth multi-channel SAR scheme with ultra-low range sidelobe-ratio (RSLR) performance was proposed, employing complete complementary sequence (CC-S) coding waveform. The CC-S waveform was utilized to acquire ultra-low RSLR performance in range direction. Azimuth multi-channel scheme was introduced, to compensate reduction of efiective PRF due to employing CC-S, and to mitigate azimuth resolution lost resulted from strong azimuth weighting, in order to implement low side-lobe performance in both range and azimuth direction. The method for pre-processing the CC- S based multi-channel SAR data was proposed, which would both compensate receiving time difierence of sub-sequences and reconstruct azimuth spectrum of multi-channel SAR data. Furthermore, the corresponding image formation algorithm for accurately focusing raw data of the SAR system was also proposed. Computer simulation results were presented, which demonstrated the validity of the proposed SAR scheme and image formation algorithm.