Showing papers by "Mengdao Xing published in 2017"

A 2-D Space-Variant Motion Estimation and Compensation Method for Ultrahigh-Resolution Airborne Stepped-Frequency SAR With Long Integration Time

Abstract: For the ultrahigh-resolution airborne stepped-frequency synthetic aperture radar, very large synthetic bandwidth and very long integration time may lead to a 2-D space-variant (SV) motion error when the aircraft flies off the ideally straight trajectory due to the atmospheric turbulence. This new type of error complicates the motion estimation and motion compensation (MOCO). For the motion estimation, we present a jointly 2-D SV motion error estimation method to simultaneously consider the range-variant motion error and the azimuth-variant motion error. For the MOCO, we propose a 2-D SV-MOCO method. The method is implemented through three processing steps: 1) two-step MOCO for the space-invariant motion error and the range-variant phase error; 2) range block-based chirp-z transform (CZT) for the range-variant envelope error; and 3) range block division for the range-dependent azimuth-variant phase error based on the azimuth subaperture method. Finally, processing of simulated data and real data validates the proposed methods.

Enhanced ISAR Imaging and Motion Estimation With Parametric and Dynamic Sparse Bayesian Learning

Abstract: This paper is focused on high-resolution inverse synthetic aperture radar (ISAR) imaging and motion estimation of maneuvering targets from compressively sampled echo data. Herein, a local structural sparse Bayesian learning (LS-SBL) algorithm is proposed by exploiting the joint sparsity pattern of adjacent scatterers. A structured prior by modeling the neighboring correlation or dependence is utilized to encode the joint sparsity pattern. Meanwhile, a parametric dictionary with unknown rotational parameters is constructed to represent the target maneuverability. The solution to the LS-SBL algorithm is decomposed into iterations between sparse imaging and dictionary learning. In sparse imaging, an expectation-maximization method is employed for ISAR image formation and hyperparameter estimation by using a predesigned dictionary. In dictionary learning, an efficient approach of rotational parameter estimation is presented to dynamically update the parametric dictionary. Due to the exploitation of joint sparsity pattern, enhanced performance of ISAR image reconstruction can be achieved by effectively preserving the target structure. In addition, the cross-range scaled ISAR image is obtainable by extracting the target geometry, which benefits from the rotational motion estimation. Finally, experiments on simulated and measured data demonstrate the effectiveness of the proposed algorithm.

Improved Signal Reconstruction Algorithm for Multichannel SAR Based on the Doppler Spectrum Estimation

Abstract: For high-resolution wide-swath synthetic aperture radar imaging algorithms, signal reconstruction is a key step. The steering vector plays an important role in signal reconstruction, which can be constructed by the ambiguity components. The information of the ambiguity components, e.g., number and index, is usually regarded to be constant and known. However, we find that the information of ambiguity components is always a piecewise function of the baseband frequency. This means that the steering vector cannot be preconstructed accurately and it will negatively affect the signal reconstruction. This paper presents an improved signal reconstruction method based on the Doppler spectrum estimation. The proposed method can estimate the variant ambiguity components to form the steering vector exactly by the Capon estimation. As a result, the method is able to restore the Doppler spectrum entirely and performs well on the noise reduction. Moreover, the baseband Doppler centroid and antenna pattern can be obtained in the proposed method. Simulated data and airborne raw data are processed to validate the algorithm.

A Modified Equivalent Range Model and Wavenumber-Domain Imaging Approach for High-Resolution-High-Squint SAR With Curved Trajectory

Abstract: In a synthetic aperture radar (SAR) system, the radar platform may move with a curved trajectory due to the existence of vertical velocity and acceleration, which may result in the failure of the conventional imaging methods. In order to deal with this problem, this paper proposes an improved wavenumber-domain imaging algorithm for high-resolution-high-squint SAR with a curved trajectory. It mainly includes three aspects. First, a modified equivalent range model for a curved trajectory is derived. Second, an improved wavenumber domain imaging algorithm based on the proposed range model is analyzed in detail. Finally, the imaging distortion caused by vertical velocity and acceleration is corrected via geometry and inverse projection. Simulated results and Ku-band real SAR data processing are used to validate the proposed model and imaging algorithm.

Thorough Understanding Property of Bistatic Forward-Looking High-Speed Maneuvering Platform SAR

Abstract: This paper investigates a new and special imaging mode, bistatic forward-looking high-speed maneuvering platform synthetic aperture radar, in which both platforms travel curvilinear trajectories with high velocities and accelerations. A thorough understanding of its new properties of motion, 2-D resolution, and echo signal coupling is given. Applicability of gradient method neglecting time dependence of gradient to this special configuration is addressed. Moreover, 2-D coupling characteristic of echo signal at different imaging distances is investigated.

Full-Aperture Focusing of Very High Resolution Spaceborne-Squinted Sliding Spotlight SAR Data

Abstract: In very high resolution spaceborne-squinted sliding spotlight synthetic aperture radar, the traditional imaging algorithms based on the equivalent squint range model (ESRM) cannot be applied, because the ESRM model is inaccurate in this case. For this problem, this paper proposes a squint equivalent acceleration range model to precisely take into account the spaceborne-squinted curved orbit. Then a full-aperture squint-imaging algorithm is proposed based on this new range model, which can handle the azimuth variation of the equivalent velocity and the range variation of the 2-D frequency spectrum. The results of the simulation validate the effectiveness of new range model and imaging algorithm.

A Two-Dimensional Beam-Steering Method to Simultaneously Consider Doppler Centroid and Ground Observation in GEOSAR

Abstract: Due to Earth’s rotation and an elliptical satellite orbit, large Doppler centroids along satellite orbit inevitably occur in geosynchronous Earth orbit synthetic aperture radar (GEOSAR). Nonzero Doppler centroid causes a large range migration, which complicates the data acquisition and design of imaging algorithms. Thus, beam steering is used to decrease the centroid. At the same time, the ground observation of interest is prerequisite for applications. Thus, a unique two-dimensional (2-D) beam-steering method to simultaneously consider the reduction of Doppler centroid and ground observation for the GEOSAR is studied. The minimum-Doppler plane is proposed to minimize the centroid and to guarantee the beams that illuminate the area of interest. Subsequently, to achieve required ground coverage, beam directions determined by the minimum-Doppler plane are slightly adjusted. The method has been validated through the simulation of two types of orbits.

A Novel Doppler Chirp Rate and Baseline Estimation Approach in the Time Domain Based on Weighted Local Maximum-Likelihood for an MC-HRWS SAR System

Abstract: In this letter, a novel estimation approach for the Doppler chirp rate and baseline in azimuth is proposed for the multichannel in azimuth high-resolution and wide-swath (HRWS) synthetic aperture radar (SAR) system. First, a range-invariant Doppler chirp rate estimation approach is developed based on a map drift algorithm and correlation function method. Then, a weighted local maximum-likelihood approach is adopted to obtain an accurate estimation of the range-variant Doppler chirp rate. With an accurate Doppler chirp rate, the baseline in azimuth can be estimated, which is developed from the correlation function between the echoes of adjective channels. The proposed approaches are successfully applied to process real five-channel HRWS SAR echo data, demonstrating the efficacy of the proposed methods.

A Novel Two-Step Approach of Error Estimation for Stepped-Frequency MIMO-SAR

Abstract: For a multiple-input and multiple-output synthetic aperture radar, stepped frequency chirps can be used to generate high-resolution range profiles (HRRPs) by using spectrum synthesis. However, the presence of channel phase errors may degrade the performance of HRRP synthesis. This letter presents a channel error estimation method to address this problem. First, to obtain a focused subband image, a range phase adjustment by contrast enhancement algorithm is proposed to estimate inner-channel high-order phase errors. Second, a sidelobe balanced model is established to estimate constant phase error from the relationship between the balanced sidelobe and constant phase; the constant phase error can be directly obtained in an efficient manner. Experimental analysis using real data demonstrates the effectiveness of the proposed method.

FM sequence optimisation of chaotic-based random stepped frequency signal in through-the-wall radar

Abstract: Chaotic-based random stepped frequency signal is applied in the multiple-input-multiple-output through-the-wall detection radar (MIMO-TWDR)recently. When the frequency modulation (FM) sequence of transmission signal is controlled by the chaotic signal, the single-frequency interference such as the power harmonics sneaking into the phase detector becomes periodical and therefore can be filtered in frequency domain. However, the target echo signal becomes random after chaotic modulation, where the matched filter usually is unable to be realised by Fourier transform and consequently the envelope of direct wave after the phase detector varies stochastically and is difficult to be eliminated by an analogue filter. The FM sequence of random disorganising cannot meet the demand of filtering out the single-frequency interference and direct wave simultaneously. Therefore, a new method of FM sequence optimisation of chaotic-based random stepped frequency signal based on genetic algorithm is proposed to solve these problems in this study. Simulations show that the optimised stepped frequency signal possesses the advantages of both chaotic-based random and linear stepped frequency signal. The proposed scheme achieves excellent performance on direct wave and single-frequency interference suppression and target detection. Moreover, it can avoid the interference between transmission antennas of MIMO radar.

Paired echo suppression algorithm in helicopter-borne SAR imaging

Abstract: A helicopter may vibrate at high frequencies during its flight, which can give rise to paired echoes in the helicopter-borne synthetic aperture radar (SAR) imagery. This phenomenon substantially degrades the quality of SAR images since it may generate unwanted ghost targets in the images. In this study, the authors propose a method consisting of three steps to suppress paired echoes in helicopter-borne SAR imagery based on the characteristics of the helicopter vibration. The proposed method can suppress paired echoes with high efficiency and accuracy in the case of vibrations with varying amplitude. Furthermore, this method does not require the presence of isolated or strong points in a range cell. Both simulated and acquired data are processed to demonstrate the effectiveness of the proposed algorithm.

Performance Improvement and System Design of Geo-SAR Using the Yaw Steering

Abstract: The yaw steering may be ineffective to compensate for the large Doppler centroid in geosynchronous SAR (Geo-SAR) in many cases. Therefore, the available studies are concentrated on the 2-D beam-steering, rather than the yaw steering. However, the range and azimuth may be severely coupled without any beam-steering or with the 2-D beam-steering. Fortunately, we found that the yaw steering could overcome this serious problem, and can bring several benefits in terms of the performance improvement of the system. In this paper, we first illustrate the benefits of the yaw steering, and then provide two aspects of the system design with the incorporation of demonstrating the benefits of the yaw steering. The demonstrated three main benefits of the yaw steering are: 1) makes the 2-D side-lobes nearly orthogonal, and improves the imagery quality; 2) largely reduces the synthetic aperture time, and decreases the computation burden of imaging processing; and 3) substantially mitigates the azimuth-variation of raw data, and simplifies the imaging processing.

A Novel 3-D Imaging Configuration Exploiting Synthetic Aperture Ladar

Abstract: Traditional three-dimensional (3-D) laser imaging systems are based on real aperture imaging technology, whose resolution decreases as the range increases. In this paper, we develop a novel 3-D imaging technique based on the synthetic aperture technology in which the imaging resolution is significantly improved and does not degrade with the increase of the range. We consider an imaging laser radar (ladar) system using the floodlight transmitting mode and multi-beam receiving mode. High 3-D imaging resolutions are achieved by matched filtering the linear frequency modulated (LFM) signals respectively in range, synthetic aperture along-track, and the real aperture across-track. In this paper, a novel 3-D imaging signal model is given first. Because of the motion during the transmission of a sweep, the Doppler shift induced by the continuous motion is taken into account. And then, a proper algorithm for the 3-D imaging geometry is given. Finally, simulation results validate the effectiveness of the proposed technique.

Two dimensional phase unwrapping method based on minimum infinite norm

Abstract: The invention belongs to the radar signal processing field, and discloses a two dimensional phase unwrapping method based on minimum infinite norm. The two dimensional phase unwrapping method based on minimum infinite norm includes the steps: acquiring an interference phase diagram so as to obtain a wrapping phase matrix; obtaining a horizontal wrapping phase gradient matrix of the interference phase diagram and a vertical wrapping phase gradient matrix of the interference phase diagram through calculation; calculating a horizontal absolute phase gradient estimate matrix and a vertical absolute phase gradient estimate matrix of the interference phase diagram; and obtaining an absolute phase matrix of the interference phase diagram through calculation, wherein the absolute phase matrix of the interference phase diagram is the unwrapping result of the interference phase diagram, so that the two dimensional phase unwrapping method based on minimum infinite norm can effectively improve the solution precision of a traditional two dimensional phase unwrapping method.

Scan-ISAR imaging for translational motion targets

Abstract: The cross-range resolution of Inverse Synthetic Aperture Radar (ISAR) image is highly dependent on the relative motion between radar and observed targets, in which the rotational motion can induce different Doppler modulation for different scatterers However, in practice there exists only translational motion of target In this paper, a scan-ISAR imaging algorithm for translational motion targets is proposed, where the thought that synthetic aperture is accomplished by the rotation of antenna Experimental results have demonstrated the effectiveness of the proposed method