Multichannel Synthetic Aperture Radar Systems Research Articles

Velocity Estimation of Moving Targets in Multi-channel SAR images by Optimally-weighted Velocity Averaging of Multiple Pixels

ABSTRACT A scheme is proposed for the radial-velocity estimation of moving targets in multi-channel SAR systems. A moving target consists of multiple pixels in SAR images, generally. The radial velocity of each pixel can be estimated from its interferometric phase. The radial velocity of the moving target is then obtained by averaging the radial-velocity estimates of its multiple pixels. In the averaging, the weight of each radial-velocity estimate is designed to minimize the mean-square-error (MSE) between the estimate and the truth. This scheme has a higher estimation accuracy than those conventional schemes, which take the radial-velocity estimate of one pixel as the final radial-velocity estimate of the moving target. The experimental results of both simulated and real data show the effectiveness of this scheme.

Quadratically Constrained Ambiguity Suppression Algorithm for APC/Multichannel SAR Systems With Nonuniform Spatial Sampling

The azimuth phase coding (APC) technique is known for its very low implementation complexity and its effectiveness for point and distributed ambiguities in conventional synthetic aperture radar (SAR) systems. In recent years, as an extension, the APC technique has been briefly discussed for multichannel SAR systems. However, the properties of the APC technique are no longer guaranteed in the multichannel SAR systems based on the digital beamforming (DBF) on-receive, and only a slight APC gain in the suppression of the range ambiguity can be obtained. In this article, we first provide a more thorough analysis for an APC-multichannel SAR system with respect to a uniform pulse-repetition frequency (PRF). Then, the APC/multichannel SAR system with nonuniform spatial sampling is briefly discussed, and an improved reconstruction approach based on a quadratically constrained optimization model is proposed to increase greatly the APC gain with respect to existing multichannel reconstruction algorithms. This proposed approach allows the minimization of the range ambiguity with a given azimuth-ambiguity constraint. In particular, for some specific PRFs, the proposed method permits a cancellation of the odd-order range ambiguity. Finally, simulation experiments are performed to verify the advantages and effectiveness of the proposed approach.

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.

Deceptive Targets Generation Simulation Against Multichannel SAR

Traditional synthetic aperture radar (SAR) deceptive jamming can effectively generate deceptive scenes or false targets in SAR images. However, these false targets or scenes can be easily distinguished or eliminated by the multichannel SAR system. To interfere with the multichannel SAR, we first analyzed the results of SAR deceptive jamming generated by one transponder and two transponders against three-channel SAR- ground moving target indication (GMTI). Then, we propose a new deceptive jamming method against three-channel SAR-GMTI by using three synergetic transponders. By modulating each transponder with a complex coefficient, three synergetic transponders can generate false moving targets with the controllable radial velocity and located azimuth position in three-channel SAR-GMTI. Besides, in this paper, we also introduce an algorithm to deploy three transponders reasonably by utilizing the minimum condition number. In the end, a general architecture of multiple transponders deceiving multichannel SAR is given. The proposed method can not only generate deceptive false targets against multichannel SAR-GMTI, but also guide the production of a deceptive digital elevation model (DEM) against multichannel interferometric SAR (InSAR). Simulations verify the effectiveness of the proposed method.

Multi-Channel Synthetic Aperture Radar Based Classification of Maritime Scenes

We present novel experimental evidence that demonstrates the effectiveness of exploiting scene motion information for the analysis of scene structure in maritime imaging applications. We analyze data captured by our novel airborne Multi-channel SAR (MSAR) system that is particularly suited to sampling the velocity profile of scatterers in the maritime environment. While previous works have shown the utility MSAR systems for correcting scene motion induced blurring artifacts, our work shows, for the first time, how the information furnished by an MSAR system can systematically render accurate classification of maritime scenes into different perceptual categories. We offer a methodology that is superior to traditional classification techniques that are based purely on the spatial structure of an image. Furthermore, the simplicity of the feature space involved together with the demonstrated classification performance on imagery captured by our airborne MSAR system underscore the strength of the methodology.

Multichannel W-Band SAR System on a Multirotor UAV Platform With Real-Time Data Transmission Capabilities

In recent years, the demand for small-scale remote sensing, which is used in disaster monitoring, agriculture, and ground subsidence has increased. A multichannel synthetic aperture radar (SAR) system can provide image and topographic information of the illuminated scene, regardless of adverse weather conditions. As a cost-effective solution to radar imaging, a multichannel W-band SAR system mounted on a multirotor unmanned aerial vehicle (UAV) is presented. The radar module was designed to operate at W-band to achieve small size and weight allowing the module to be mounted on multirotor UAVs with small payload. A detailed description of the design and measurement of the system is provided in this paper. The radar imaging capability of the developed system was verified by performing outdoor experiments using isolated buildings as targets. The multichannel functionality of the system was verified by measuring height of a point target placed above the ground. The measurements and experiments verified the feasibility of a multichannel radar mounted on a multirotor UAV for imaging and topographic applications.

Velocity estimation of rock-fall in multi-channel SAR system with low PRF sampling

Rockfalls influence the safety of the surrounding infrastructure and the normal operation of transportation lines. The application of synthetic aperture radar (SAR) technology is a new method of rock-fall monitoring and disaster prediction. This paper introduces a novel method to velocity estimation of rock-fall target utilizing SAR with low pulse repetition frequency (PRF) sampling. As is known to us, when PRF is less than the Doppler spectrum bandwidth, rock-fall target suffer both Doppler centroid frequency ambiguity and Doppler spectrum ambiguity. Under this condition, the traditional velocity estimation method in the Doppler domain is out of action. To solve above problem, this paper designs an improved reconstruction filter bank to reconstruct the Doppler spectrum of rock-fall target, and then Doppler in-band energy and Doppler out-of-band energy ratio of rock-fall target is utilized to estimate the velocity of rock-fall target. Simulation experiment results show the effectiveness of proposed method.

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.

Clutter Suppression via Subspace Projection for Spaceborne HRWS Multichannel SAR System

Traditional clutter suppression methods are mainly studied under the condition that the pulse repetition frequency (PRF) of the system is not less than the Nyquist frequency. Whereas in the high-resolution and wide-swath (HRWS) multichannel synthetic aperture radar (SAR) system, a low PRF is used to break through the minimum antenna area constraint. The low PRF case brings new challenges to the traditional clutter suppression methods. In this letter, a subspace projection clutter suppression method is proposed based on the fact that moving targets and the clutter consist in different signal subspaces. This method can be directly applied to the HRWS multichannel SAR system, and it shows better performance compared to the space-time adaptive processing (STAP) when the moving target components cannot be ignored in the clutter covariance matrix calculation. Simulated data and airborne measured data are processed to verify its effectiveness.

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.

Study of synthetic aperture radar and automatic identification system for ship target detection

In the past, the main goals of Synthetic Aperture Radar (SAR) systems were the study of the interaction of electromagnetic waves with the earth surface. Recently, the development of multi-channel SAR systems has enabled the development of more sophisticated techniques for the surveillance activities. SAR is the most efficient instrument, which provides high-resolution data for wide ocean area surveillance under all weather conditions. The intrinsic capability of this instrument is to provide a quick view of the oceanic surface features such as vessels, waves and currents, oil spills, laver facilities and wind fields. The ship detection or recognition is achieved in two steps: the first step is to identify the target in SAR images of a busy traffic, which corresponds to Automatic Identification System (AIS) signals by the “dead-reckoning (DR) position”, and the second step is to estimate the position, size and speed of the ship from SAR images and compare these results with the AIS “true” data. This paper presents the fundamentals of SAR and its integration with the AIS data for the ship target detection.

The maiden flight of Hinotori-C: The first C band full polarimetric circularly polarized synthetic aperture radar in the world

Synthetic Aperture Radar (SAR) is a very powerful tool in microwave remote sensing due to its capability of all-weather and day-to-night time operation [1]. Carl Wiley invented SAR in 1951 to overcome the poor azimuth resolution in conventional Side-Looking Airborne Radar (SLAR), followed by a patent filed in 1954 [2]. Unlike SLAR, using the forward motion of the platform and the principle of Doppler beam sharpening, the azimuth resolution of a SAR is equal to half the antenna length and is independent of the range distance [3]. Since the invention of SAR, steered by the breakthrough in science and technology, many advanced SAR techniques have been proposed and realized. These techniques include, but are not limited to, spotlight SAR for a finer image resolution [4], scan SAR for a wider swath coverage [5], and the remarkable polarimetric [6] and interferometric [7] SAR techniques using multichannel SAR system [8], [9], [10] for advanced remote sensing applications. Over the years, SAR has been widely used in different types of application, particularly in Earth observation such as disaster damage assessment [11], land deformation observation [12], oceanography [13], terrain classification [14], target detection [15], and so on. The diversified applications of SAR have encouraged the rapid development of airborne and spaceborne SAR sensors.

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.

High-Resolution Imaging of Multi-Channel Forward-Looking Synthetic Aperture Radar Under Curve Trajectory

Forward-looking synthetic aperture radar is an essential tool in modern remote sensing applications, but its imaging is a challenging task. Few existing methods consider the curve trajectory. Aiming at the difficulty of resolution and the Doppler ambiguity in forward-looking SAR under the curve trajectory, this paper establishes a forward-looking multi-channel SAR system model and proposes a novel approach for the forward-looking SAR imaging under the curve trajectory. The approach is implemented by a two-step process: 1) The Doppler ambiguity under forward-looking mode is solved by adaptive beam-forming processing; and 2) A sparsity-driven optimization imaging algorithm is developed to enhance the azimuth angle resolution in the forward-looking direction, where the resolution gain aroused by the curve trajectory is exploited. The extensive simulation confirms the validity of the proposed approach.

GMTI processing for multi-channel high-resolution wide-swath SAR systems

ABSTRACTGround moving target indication (GMTI) is an important application of future multi-channel high-resolution wide-swath (MC-HRWS) synthetic aperture radar (SAR) system. This paper improves the extended displaced-phase-center antenna (EDPCA) method for efficient GMTI with a MC-HRWS SAR System. This processing method called improved EDPCA is derived in this paper and is applied to simulated data and real data in order to validate the theory. Compared with EDPCA, the improved EDPCA can decrease the degrees of freedom (DOFs) required by GMTI processing and the false alarm rate induced by the ambiguous image of moving target. In addition, signal-to-noise ratio (SNR) loss of moving target caused by linear range cell migration (RCM) can be avoided without a priori knowledge of the radial velocity.

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.

Velocity estimation of moving targets for spaceborne multichannel synthetic aperture radar systems based on equivalent along‐track interferometry technique

Velocity estimation of a moving target is an important yet challenging issue facing the special mode of high-resolution and wide-swath (HRWS) synthetic aperture radar. HRWS imaging is achieved by splitting the antenna into multiple receive channels in azimuth with each channel working at low pulse repetition frequency sampling. In this study, the authors propose a novel approach to estimate the velocity of a moving target in HRWS mode, along with a complete moving target indication, estimation, and imaging scheme. The proposed radial velocity estimation algorithm is called the equivalent along-track interferometry method, which is implemented by twice imaging after reconstructing the data of several leftmost and rightmost channels. Moreover, the along-track velocity is also estimated for focused imaging of the moving target. The motion-adapted reconstruction and imaging algorithm is applied to achieve unambiguous imaging of the moving target. Experimental results verify the performance of the algorithm and its superiority for a moving target with interference from clutter, noise, and channel mismatches. The proposed algorithm is free of search and iteration, and featured by high accuracy and low computational load.

A Novel Weighted Doppler Centroid Estimation Approach Based on Electromagnetic Scattering Model for Multichannel in Azimuth HRWS SAR System

Similar to the conventional squint mode synthetic aperture radar (SAR) imaging processing, the estimation of Doppler centroid is one key problem for the low-squint-mode (within the range of [−5°, 5°]) multichannel in an azimuth high-resolution and wide-swath (HRWS) SAR system. In this paper, based on the electromagnetic scattering model, a novel weighted Doppler centroid estimation approach is proposed for the multichannel in the azimuth HRWS (MC-HRWS) SAR system. First, Maxwell’s equations are employed to derive the echo model for the multichannel SAR system. Then, an improved backscattering model is presented based on the small perturbation method and the available echo model, which is adopted to produce the weights for Doppler centroid estimation. More importantly, in order to improve the precision of Doppler centroid estimation, a weighted ambiguity-free Doppler centroid estimation approach is proposed, where the range-variant characteristic of Doppler centroid is employed to resolve the ambiguity number of Doppler centroid, and the echoes from different range bins with different signal-to-noise ratios are considered. In addition, the Cramer–Rao low bound of the estimated Doppler centroid is also discussed in this paper. The effectiveness of the proposed Doppler centroid estimation approach is verified via simulated and real measured low-squint-mode MC-HRWS SAR data.

Application of an improved DPCA algorithm in rock-fall monitoring based on synthetic aperture radar

Rockfalls influence the safety of the surrounding infrastructure and the normal operation of transportation lines. The application of synthetic aperture radar (SAR) technology is a new method of rock-fall monitoring and disaster prediction. This paper proposes a new method of dynamic monitoring and parameter estimation for rockfalls based on a multichannel airborne SAR system. First, the signal model of the multichannel airborne SAR system is derived. Then, to improve performance, the traditional displaced phase centre antenna (DPCA) method is extended to three or more receiver channels. This method can be effectively applied for rock-fall target detection, as well as to precisely estimate rock-fall target parameters and resolve the ambiguity issues associated with the estimation of rock-fall target parameters. Finally, simulation results are used to validate the proposed approach.