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2015 Vol. 4, No. 4
Previous Issue
2015, 4(4): 375-385.
Abstract
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In recent years, there have been many studies on Synthetic Aperture Radar (SAR), and High- Resolution and Wide-Swath (HRWS) imaging has attracted increasing attention. High-resolution images of a stationary scene enabling the detection of moving targets can be provided by an SAR system combined with Ground Moving Target Indication (GMTI), which is widely used in many civilian applications. A multichannel in azimuth HRWS SAR system has been presented to effectively deal with conflicts between high resolution and low Pulse Repetition Frequency (PRF), which are encountered during HRWS SAR imaging. Because the spatial degrees of freedom in the azimuth can be employed to suppress a clutter, the multichannel configuration offers significant potential for GMTI capability. In this study, a new clutter suppression and moving target imaging method is proposed, which can be operated in an HRWS SAR system without the need for a separate high PRF operation mode.
2015, 4(4): 386-392.
In a high pulse-repetitive-frequency forward-looking radar, the clutter echo is not only severely rangedependent but also range-ambiguous. It is therefore difficult to realize clutter compensation and suppression for conventional Space-Time Adaptive Processing (STAP) radar. To alleviate this problem, in this study, a novel range-ambiguous clutter suppression method is proposed for STAP radar with Frequency Diverse Array (FDA). Because the transmit steering vector of the FDA is range-angle dependent, the range-ambiguous clutter can be separated in the spatial domain using a subspace projection approach. The separated clutter can then be compensated using the Doppler warping technique. Finally, the compensated clutter can be suppressed, and target detection is realized in both the unambiguous and ambiguous range regions. Simulation examples demonstrate the effectiveness of the proposed method.
2015, 4(4): 393-400.
A novel detection and parameter estimation method for air maneuvering target based on reconstruction time sample and Wigner-Ville Distributed (WVD) is proposed in this paper. Firstly, reconstruct time sampling method is applied to reconstruct the signal by using the space samples, which is equivalent to increasing the number of time samples within a Coherent Processing Interval (CPI). Then WVD method is utilized for reconstructing signals to achieve parameter estimation. The proposed method can achieve the detection and parameter estimation of the maneuvering target effectively without the need of precisely known of azimuth of target when the data sample is limited. The simulation result is provided to demonstrate the effectiveness of the proposed method.
2015, 4(4): 401-410.
For military or civilian activities, it is important to monitor the direction of moving targets in a wide area. Traditional regional monitoring uses the airborne scanning mode (ScanSAR) within the limits of the national airspace. The inherent characteristics of ScanSAR do not apply to spaceborne regional monitoring. In this paper, the spaceborne regional surveillance Ground Moving Target Indication (GMTI) mode based on squint-TOPSAR is proposed. The proposed method uses the TOPSAR mode that improves the low SNR of spaceborne ScanSAR. The full-aperture imaging algorithm for squint-TOPSAR is used in data focusing. The Displaced Phase Center Antenna (DPCA) and Constant False Alarm Rate (CFAR) methods are used to accomplish the moving target indication. The relation between the interferometric phase and the speed of moving target is used to estimate the speed of the moving target and mark the target location in the SAR image. The differences between real and simulation data are analyzed. The simulation results demonstrate the effectiveness of the proposed method.
2015, 4(4): 411-417.
The adaptive detection problem for mismatched useful signal is considered. A novel parametric adaptive detector is proposed, and the adaptive matched filter and double normalized adaptive matched filter are the special cases of the tunable detector. The detector parameters can be used to control the detection performance for slight mismatched signal and rejection performance for serious mismatched interference. The probability of false alarm and the probability of detection for mismatched signal are derived. The computer simulation results show that we can choose suitable parameter to control the ability to detect mismatched useful signal and the ability to reject interference.
Persymmetric Adaptive Detectors of Subspace Signals in Homogeneous and Partially Homogeneous Clutter
2015, 4(4): 418-430.
In the field of adaptive radar detection, an effective strategy to improve the detection performance is to exploit the structural information of the covariance matrix, especially in the case of insufficient reference cells. Thus, in this study, the problem of detecting multidimensional subspace signals is discussed by considering the persymmetric structure of the clutter covariance matrix, which implies that the covariance matrix is persymmetric about its cross diagonal. Persymmetric adaptive detectors are derived on the basis of the one-step principle as well as the two-step Generalized Likelihood Ratio Test (GLRT) in homogeneous and partially homogeneous clutter. The proposed detectors consider the structural information of the covariance matrix at the design stage. Simulation results suggest performance improvement compared with existing detectors when reference cells are insufficient. Moreover, the detection performance is assessed with respect to the effects of the covariance matrix, signal subspace dimension, and mismatched performance of signal subspace as well as signal fluctuations.
2015, 4(4): 431-438.
The classical Frequency-Wavenumber (F-K) imaging algorithm can efficiently reconstruct the image for homogeneous medium; however, it cannot generate focused and properly-located images for layered medium. Considering the electrical properties of individual layer and the discontinuity between layers, the phase shift migration for layered medium is derived in this paper. The analysis on the backscattered transfer function shows that some assumptions and mathematical approximations are applied in the proposed method. The numerical and experimental results are presented to show the feasibility of the proposed method for real-time imaging of layered medium.
2015, 4(4): 439-444.
For Impulse Radio Ultra-Wide Band (IR-UWB) radar, the time jitter transforms the static clutters to dynamic clutters. Thus, strong residue exists and false alarms form after traditional direct current suppression. The effect of the time-domain jitter on the life detection capacity is analyzed, and then the phenomenon that the relative time delays between the coupling echoes can reflect the time jitter is pointed out. Based on the coupling echo, a method of time jitter suppression is proposed. Experimental data demonstrate that the proposed method can effectively suppress the residue of strong static clutter, and further improve the life-detection capacity.
2015, 4(4): 445-451.
Because of the limitations of traditional Principal Component Analysis (PCA) in clutter reduction, an improved PCA subspace method is proposed based on the 2D wavelet transform. Moreover, the combination of the improved subspace method and adaptive filtering ensures the signal fidelity and learning adaptability of adaptive filtering. Then, an adaptive clutter reduction algorithm based on wavelet transform and PCA, as well as adaptive filtering, is proposed. The experimental results suggest that the proposed method improves the signal to clutter ratio and target image definition.
2015, 4(4): 452-459.
It is difficult to accurately and rapidly partition measurement sets of multiple extended targets in cluttered environment. Hence the affinity propagation method is introduced and a novel measurement partition algorithm is proposed. First, the measurement set is preprocessed by using density analysis to remove clutters from the measurements. Second, the number and location of the extended targets is determined via competition among the measurements. Finally, state estimates are obtained by using the probability hypothesis density filter. Simulations show that the proposed algorithm offers good performance in measurement partitioning of extended target tracking with clutter disturbance. Compared with the distance partition and K-means++ methods, the proposed method effectively minimizes the computation time and retrieves the number of targets iteratively.
2015, 4(4): 460-466.
In the modern radar signal environment, hybrid modulation signals are increasingly used in many systems. This study focuses on the Time Difference Of Arrival (TDOA) estimation problem of FSK-BPSK hybrid modulation signals. The method combines the characteristic of modulated signal and utilizes the sum of simple correlation functions of sub-pulses to elicit the complex correlation function of the entire pulse signal. Finally, a correlation function fitting algorithm is used to estimate the exact TDOA. Experimental results indicate an obvious improvement in the accuracy and noise immunity of the method, and the method is appropriate for the TDOA estimation of low-bandwidth hybrid modulation signals.
2015, 4(4): 467-473.
Airborne array antenna SAR is used to obtain three-dimensional imaging; however it is impaired by motion errors. In particular, rotation error changes the relative position among the different antenna units and strongly affects the image quality. Unfortunately, the presently available algorithm can not compensate for the rotation error. In this study, an airborne array antenna SAR three-dimensional imaging model is discussed along with the effect of rotation errors, and more specifically, the yaw angle error. The analysis reveals that along- and cross-track wavenumbers can be obtained from the echo phase, and when used to calculate the range error, these wavenumbers lead to a target position irrelevant result that eliminates the error's spatial variance. Therefore, a wavenumber-domain subblock compensation method is proposed by computing the range error in the subblock of the along- and cross-track 2-D wavenumber domain and precisely compensating for the error in the space domain. Simulations show that the algorithm can compensate for the effect of yaw angle error.
2015, 4(4): 474-480.
In this study, a digital dechirp approach for Synthetic Aperture Radar (SAR) is discussed. It can be applied to lightweight MiniSar systems with longer pulse width than swath. With this method, and after data acquisition with lower sampling rate than the bandwidth of the transmitted LFM signal and digital dechirp, pulse compression can be obtained by spectral analysis in the range direction without aliasing. The approach is described in detail and the sampling frequency selection criteria are derived. Theoretical formulations and numerical simulations verify the validity of the proposed approach.
2015, 4(4): 481-489.
In this paper, the definition and the key features of Software Radar, which is a new concept, are proposed and discussed. We consider the development of modern radar system technology to be divided into three stages: Digital Radar, Software radar and Intelligent Radar, and the second stage is just commencing now. A Software Radar system should be a combination of various modern digital modular components conformed to certain software and hardware standards. Moreover, a software radar system with an open system architecture supporting to decouple application software and low level hardware would be easy to adopt user requirements-oriented developing methodology instead of traditional specific function-oriented developing methodology. Compared with traditional Digital Radar, Software Radar system can be easily reconfigured and scaled up or down to adapt to the changes of requirements and technologies. A demonstration Software Radar signal processing system, RadarLab 2.0, which has been developed by Tsinghua University, is introduced in this paper and the suggestions for the future development of Software Radar in China are also given in the conclusion.
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