Displaced Phase Center Research Articles

Processing of Multichannel Sliding Spotlight and TOPS Synthetic Aperture Radar Data

The inherent limitation between azimuth resolution and range swath width in conventional spaceborne synthetic aperture radar (SAR) systems can be overcome by introducing the displaced phase center antenna technique. In these SAR systems, echoes from all subapertures should be reconstructed and combined together before single-channel SAR processors. However, in the multichannel sliding spotlight and Terrain Observation by Progressive Scans (TOPS) modes, azimuth beam progressive sweeping during the whole acquisition interval leads to that the total Doppler bandwidth spans over several N ·PRF intervals, where N is the number of azimuth channels and PRF is the pulse repetition frequency. As a result, conventional azimuth multichannel reconstruction algorithms for the stripmap mode are not directly suitable for sliding spotlight and TOPS modes. This paper proposes a novel imaging processor for both modes according to their azimuth echo properties. The key point of the proposed focusing processor is the first processing step of multichannel azimuth data reconstruction, which extends the two-step focusing technique to process raw data of the azimuth multichannel case. In addition to azimuth data preprocessing and accurate range cell migration correction steps, the final postprocessing step is added to correct the possible back-folded SAR images. Imaging results on simulated raw data validate the proposed imaging approach.

TWO DIMENSION DIGITAL BEAMFORMING PREPROCESSING IN MULTIBEAM SCANSAR

The novel multibeam ScanSAR takes advantage of the displaced phase center multiple azimuth beam (DPCMAB) imaging scheme and intra-pulse beam steering in elevation in ScanSAR to achieve the high-resolution ultra-wide-swath imaging capacity. This letter proposes an innovative two-dimensional (2D) digital beamforming (DBF) space-time preprocessing approach for multibeam ScanSAR. According to echo proprieties of such imaging scheme, both azimuth ambiguity and range ambiguity problems should be resolve before a conventional ScanSAR imaging processor. After range compressing in each receive channel, a 2D DBF processor is carried out in the range-Doppler domain. The azimuth DBF operation is adopted to resolve the azimuth nonuniform sampling problem in multichannel SAR systems, while the DBF preprocessing in elevation is carried out to separate echoes from difierent subswaths corresponding to difierent sub-pulses. Imaging results on simulated distributed targets validate the proposed 2D DBF preprocessing approach.

AZIMUTH MULTICHANNEL SAR IMAGING BASED ON COMPRESSED SENSING

Azimuth multichannel is a promising technique of realizing high resolution and wide swath for synthetic aperture radar (SAR) imaging, which consequently leads to extremely high data rate on satellite downlink system and confronts serious ambiguity in subsequent processing due to its strict limitation of pulse repetition frequency (PRF). Ambiguity suppression performance of conventional spectrum construction is disappointing when the samples are approximately overlapped. To overcome these weaknesses, a novel sparse sampling scheme for displaced phase center antennas based on compressed sensing (CS) is proposed in this paper. The imaging strategy sparsely sampled in both range and azimuth direction, leading to a signiflcant reduction of the system data amount beyond the Nyquist theorem, and then operated the CS technique in two dimensions to accomplish target reconstruction. Efiectiveness of the proposed approach was validated through simulation and real data experiment. Simulation results and analysis indicated that the new imaging strategy could provide several favorable capability than conventional imaging algorithm such as less sampled data, better ambiguity suppression, higher resolution, and lower integrated side- lobe ratio (ISLR).

Integrated navigation for an autonomous underwater vehicle carrying synthetic aperture sonar

As the autonomous underwater vehicle (AUV) becomes more popular in various ocean applications, its navigation technique has attracted great attention since it is often crucial that the vehicle position be known precisely. Selection of a practical navigation scheme is a tradeoff between performance, space/power budget and cost. In this study, a low-cost, small-size navigation system for an AUV carrying synthetic aperture sonar (SAS) is presented. An integrated navigation method is also developed based on the extended Kalman filter, in which two novel processes, track-based compass calibration and adaptive beamforming-based SAS micro-navigation, are incorporated. For compass calibration, the heading bias caused by hard iron interferences is estimated through a surface calibration run; for displaced phase centre antenna-based micro-navigation, sway estimation is done by applying adaptive beamforming to the overlapped phase centre array. A number of AUV field tests were implemented and the results demonstrate that a relative navigation error of as low as 0.23% of the travelling distance in circular runs can be achieved using the developed approach.

Multiple Beams Spaceborne SAR Imaging

High azimuth resolution and wide swath coverage pose contradicting requirements in conventional spaceborne synthetic aperture radar (SAR) systems. In order to overcome these inherent limitations, the technique of displaced phase centers multiple azimuth beams (MAB) has been proposed in recent years. The principle of MAB SAR is discussed here and then the problems related with this mode, such as the differential path error and nonuniform sampling error, are analyzed when the Earth rotation is considered. The corresponding solutions to these problems are presented.

Detection and Estimation With RADARSAT-2 Moving-Object Detection Experiment Modes

This paper describes in detail several multichannel variants of the famous displaced phase center antenna (DPCA) method suitable for spaceborne multichannel synthetic aperture radar ground moving target indication (GMTI), which has become available through the RADARSAT-2 moving-object detection experiment (MODEX). A flexible signal processing architecture was developed at Defense Research and Development Canada to support R&D work in GMTI and to provide a tool for MODEX system validation. The presented multichannel DPCA method is one of the first algorithms that have been implemented and successfully tested using this architecture. The main objective is to provides a simple, yet effective and robust way to detect ground moving vehicles and to estimate their position and velocity. This work forms a baseline for further advancement in the GMTI area.

Analysis on the Non-uniform Sampling of Displaced Phase Center Multiple-beam SAR Systems

摘要: 该文从理论上推导了方位向的非均匀采样对偏置相位中心多波束SAR性能的影响。分析表明方位向的非均匀采样会造成脉冲压缩的主峰两侧存在虚假峰值，同时它会造成脉冲压缩主峰增益降低、主峰展宽。文中给出了峰值-假目标比及主峰增益损失的计算公式，表明了它们与不均匀性及方位向过采样率等因素之间存在定量关系。这些公式的正确性通过仿真得到了进一步验证。 关键词: 合成孔径雷达 / 方位向多波束 / 非均匀采样 / 高分辨率-宽测绘带

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.

Efficient DPCA SAR imaging with fast iterative spectrum reconstruction method

The displaced phase center antenna (DPCA) technique is an effective strategy to achieve wide-swath synthetic aperture radar (SAR) imaging with high azimuth resolution. However, traditionally, it requires strict limitation of the pulse repetition frequency (PRF) to avoid non-uniform sampling. Otherwise, any deviation could bring serious ambiguity if the data are directly processed using a matched filter. To break this limitation, a recently proposed spectrum reconstruction method is capable of recovering the true spectrum from the nonuniform samples. However, the performance is sensitive to the selection of the PRF. Sparse regularization based imaging may provide a way to overcome this sensitivity. The existing time-domain method, however, requires a large-scale observation matrix to be built, which brings a high computational cost. In this paper, we propose a frequency domain method, called the iterative spectrum reconstruction method, through integration of the sparse regularization technique with spectrum analysis of the DPCA signal. By approximately expressing the observation in the frequency domain, which is realized via a series of decoupled linear operations, the method performs SAR imaging which is then not directly based on the observation matrix, which reduces the computational cost from O(N 2) to O(N log N) (where N is the number of range cells), and is therefore more efficient than the time domain method. The sparse regularization scheme, realized via a fast thresholding iteration, has been adopted in this method, which brings the robustness of the imaging process to the PRF selection. We provide a series of simulations and ground based experiments to demonstrate the high efficiency and robustness of the method. The simulations show that the new method is almost as fast as the traditional mono-channel algorithm, and works well almost independently of the PRF selection. Consequently, the suggested method can be accepted as a practical and efficient wide-swath SAR imaging technique.

Extraction of Vibrating Features With Dual-Channel Fixed-Receiver Bistatic SAR

Strong ground clutter may submerge the signals of vibrating targets and bring great difficulties to extract their micro-Doppler (m-D) signatures. Furthermore, if the vibrating direction is perpendicular to the radar line of sight, its m-D would be very weak in the monostatic synthetic aperture radar (SAR). In this letter, a configuration of bistatic SAR with a dual-channel fixed receiver is presented to handle the two problems. The displaced phase center antenna technique is adopted to suppress the clutter and to reserve the signals of vibrating targets in the range-compressed data domain. Then, the fractional Fourier transform is utilized to compensate for the radar Doppler shift. Finally, the m-D feature of vibration can be extracted effectively by using time-frequency transform. The validity of the proposed method is evaluated with simulated data.

Performance Comparison of Symmetric and Offset Reflector Antennas Adaptively Illuminated by Novel Triple Mode Feedhorn

Parabolic symmetric and offset reflector antennas adaptively illuminated using a novel triple-mode feedhorn (TE11+TM01+TE21) with different mode combinations and impedance and radiation performances are presented. The combination of the radiating modes in a feedhorn with proper amplitude and fixed phase values helps in electronically pointing the main beam of the radiating patterns such as that obtained in a beam-steering antenna with limited beam-scan range. This type of radiation performance virtually creates a displaced phase center location for the feedhorn, which, consequently, adaptively illuminates the reflector antenna surface. Impedance-matching bandwidths are preserved for both reflector antennas similar to the case of feedhorn alone. The copolarization gain and peak cross-polarization levels are far better with the offset reflector antenna than the symmetric reflector antenna. Such reflector antennas find applications in ground moving target indicator (GMTI) and space based radars. The investigation results are solely computed using FEKO full-wave analysis tool.

The Study of Clutter Suppression techniques for Multi-channel SAR

The paper focus on the study of several typical clutter suppression techniques for multi-channel SAR: Displaced Phase Center Antenna (DPCA), Along Track Interferometry (ATI) and Space Time Adaptive Processing (STAP) techniques. In the paper, it has been discussed that application of DPCA and ATI in the complex image domain for moving target detection in airborne single-antenna SAR real data as well as application of STAP in the space-doppler domain. In the end, some experimental results have been presented.

Study of SAR Ground Moving Target Detections Algorithms

The paper analyses the characters of ground moving targets and researches several moving targets detection algorithms such as Displaced Phase Center Antenna(DPCA), Along-track interferometry (ATI), Space-time adaptive processing(STAP) and Deconvolution used in Synthetic Aperture Radar (SAR). It describes every algorithms and provides some diagrams. In the last, the paper computes motion parameters when using ATI to detect moving targets.

Theoretical Analysis on the Mismatch Influence of Displaced Phase Center Multiple-beam SAR Systems

Theoretical Analysis on the Mismatch Influence of Displaced Phase Center Multiple-beam SAR Systems

Space–Time Coding MIMO-OFDM SAR for High-Resolution Imaging

Multiple-input and multiple-output (MIMO) radar has received renewed attention in recent years, but little work on MIMO synthetic aperture radar (SAR) remote sensing has been reported. This paper presents a scheme of space-time coding MIMO orthogonal frequency division multiplexing (OFDM) SAR for high-resolution imaging. This system employs MIMO configuration in the elevation direction and the Alamouti space-time coding scheme in the azimuth direction, along with the use of OFDM waveform diversity and displaced phase center antenna (DPCA) techniques. As an orthogonal transmission waveform is required for this novel space-time coding MIMO-OFDM SAR system, one kind of OFDM linearly frequency modulated waveforms is investigated. The matched filtering and multibeam forming in the elevation direction are detailed. Additionally, the corresponding mathematical relations and signal models for high-resolution imaging are formed. In this way, efficient spatial diversity gain and improved range resolution are obtained by the MIMO configuration, and the requirement of pulse repeated frequency for wide-swath remote sensing is reduced by the DPCA technique. The feasibility is validated by numerical simulation results.

Implementing Wide-swath SAR Based on Displaced Phase Center Azimuth Multi-beam

Implementing Wide-swath SAR Based on Displaced Phase Center Azimuth Multi-beam

Shifting MIMO SAR System for High-Resolution Wide-Swath Imaging

A multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) system is presented for high-resolution wide-swath imaging. For a system with N transmit and N receive antennas, the proposed method using slope-varying chirp signals, azimuth beamwidth extension, and transmit antenna shift can give an increased azimuth resolution or a decreased pulse repetition frequency (PRF), which results in wider swath width, by a factor of N compared to a SAR system based on a displaced phase center antenna. To verify the proposed shifting MIMO SAR system, we carried out the point target simulations and the timing analysis which showed enhanced resolution and widened swath width performances.

Analysis of Displaced Phase Center Error in MIMO Radar

为便于MIMO (Multiple-Input Multiple-Output) 雷达在接收端相干处理多通道回波数据，该文首先基于相位中心近似来简化MIMO雷达信号模型。以产生等效相位中心误差为代价，将MIMO雷达的多收发阵列结构转化成为收发同置的单站形式。通过分析等效相位中心与收发阵元之间的几何关系，解析描述了等效相位中心误差。而后对等效相位中心误差进行了理论分析，推导出一些与误差校正有关的基本性质。针对收发阵元间距的不同取值范围，设计了相应的等效相位中心误差校正表示式。最后利用数值仿真实验完成了等效相位中心误差分析有效性的定量评估。

Fore and Aft Channel Reconstruction in the TerraSAR-X Dual Receive Antenna Mode

The TerraSAR-X satellite is a high-resolution synthetic aperture radar (SAR) system launched in June 2007 which provides the option to split the antenna in along-track direction and sample two physical channels separately. Modern SARs are equipped with active phased array antennas and multiple channels. In order to keep costs low, TerraSAR-X uses the redundant receiver unit for the second channel such that fore and aft channel signals are combined by a hybrid coupler to form sum and difference channel data. The dual receive antenna (DRA) mode can either be used to acquire along-track interferometric data or to acquire signals with different polarizations at the same time (Quad-Pol). Fore and aft channel reconstruction is necessary if ground moving target indication (GMTI) algorithms such as the displaced phase center antenna technique or along-track interferometry shall be applied, and in order to separate the horizontally and vertically polarized received signal components. The proposed approach uses internal calibration pulses from different calibration beams in order to estimate and compensate the hardware impact. The theoretical framework together with the results from the experimental data evaluation for the fore and aft channel reconstruction of the TerraSAR-X DRA mode are presented. The impact of the receive hardware transformation matrix estimation accuracy on errors in the reconstructed fore and aft channel image data is studied, and first examples on the GMTI capability of the TerraSAR-X DRA mode are given.

Automatic Extraction of Traffic Flows Using TerraSAR-X Along-Track Interferometry

Spaceborne synthetic aperture radar (SAR) offers great potential for the measurement of ground traffic flows. A SAR with multiple receiving apertures aligned in flight direction repeatedly images the same ground area with a short time lag. This allows for an effective detection of moving ground objects, whose range variation translates into an interferometric phase signal between the receiving channels. The high-resolution German SAR satellite TerraSAR-X offers several ways to create multiple along-track apertures. We exploit this to demonstrate satellite-based traffic-flow measurements using along-track interferometry (ATI) and Displaced Phase Center Array techniques. In this paper, we address the usage of different TerraSAR-X ATI modes for data acquisition and describe an automatic near-real-time processing chain for the extraction of traffic information. The performance of this TerraSAR-X traffic processor is significantly driven by incorporating <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a priori</i> knowledge of road networks. We present examples of automatic traffic detection as well as empirical evaluations thereof using different kind of reference data.