Time-domain adaptive beamforming of HF backscatter radar signals

Experimental results relating to the use of time-domain, continuously adapting beamformers in an HF bistatic FM/CW backscatter radar system are presented. Data for the study were obtained using the Wide Aperture HF Radio Research Facility (WARF) which is located in the central valley of California and is operated by Stanford Research Institute, Menlo Park, CA. Eastward-looking transmissions were employed with an operating frequency chosen so as to provide single-hop ionospheric propagation on both the forward and backscatter paths. Digital real-time recordings were taken at eight received subarray outputs. These recordings were then processed off-line using a large general purpose computer. The beamforming methods studied, however, are computationally simple and may be readily implemented in real-time using a commercially available minicomputer. Two adaptive algorithms were studied and in both cases it was shown that signal-to-noise ratio improvements of 10 to 15 dB are readily achieved when adaptive beamforming is compared with conventional, Dolph taper beamforming methods using identical received data in an HF backscatter environment. It was also demonstrated that the time scale of coefficient variation in an adaptive processor operating in this environment is the order of 1 s. Successful tracking of the adaptive algorithm under these conditions was demonstrated. The use of moving target indication (MTI) clutter suppression filters at the subarray outputs, prior to adaptation, was investigated. No significant improvement was observed with the use of these filters on experimental data. Finally, it was shown that the presence of fading nulls can significantly affect the determination of optimal subarray location and spacing in an HF environment. In general, the adaptive beamformer performance was found to be less dependent upon array geometry than was the case for conventional processing.     

Space-time adaptive processing for manoeuvring airborne radar

Space-time adaptive processing (STAP) techniques provide simultaneous rejection of jamming and clutter in airborne radar. The greatest benefits over conventional MTI (moving target indication) approaches are in terms of a capability to detect slow-moving targets which possess the same Doppler frequency as mainlobe clutter returns. This paper examines the effects of platform manoeuvre on STAP clutter and jamming rejection performance for a forward-facing array (i.e. where the array is orientated transversally to the direction of travel). It is shown that STAP slow-target detection performance is not sensitive to the radar platform orientation. It is also demonstrated that, under conditions of manoeuvre, STAP can provide better jammer rejection performance than architectures which cascade conventional clutter filtering and spatial adaptive beamforming     

Noncausal Adaptive Spatial Clutter Mitigation in Monostatic MIMO Radar: Fundamental Limitations

The problem of a point target detection masked by clutter distributed over range and Doppler, including the range and Doppler of the target, is considered for a multimode propagation scenario commonly encountered in quasimonostatic HF over-the-horizon radars (OTHR). Here, a clutter signal spread in Doppler frequency due to propagation via a disturbed ionospheric layer competes with a target and narrowband clutter returns propagating via a stable ionospheric layer with the same group delay (radar range). Mitigation over all ranges of spread clutter propagating via a “mixed mode” path with indistinguishable direction-of-arrival (DoA) relative to the target requires (potentially adaptive) transmit beamforming to exploit the direction-of-departure (DoD) difference, which varies as a function of radar range. This range-dependent beamforming can be implemented only via the use of multiple-input multiple-output radar technology. In this paper, we explore the fundamental limitations that exist for the maximal dimension of the area in range-Doppler space occupied by spread clutter and the required properties (cardinality) of the orthogonal waveform set for efficient spread clutter mitigation.      

Introduction to space-time adaptive processing

The detection of slowly moving targets by air- and spaceborne MTI radar is heavily degraded by the motion-induced Doppler spread of clutter returns. Space-time adaptive processing (STAP) can achieve optimum clutter rejection via implicit platform motion compensation. The fundamentals and properties of STAP applied to air- and spaceborne MTI radar are summarised     

地基相控阵雷达STAP应用研究

针对现代地基雷达传统方法抑制杂波不彻底的情况,根据空时二维信号处理技术（STAP）的特点和发展趋势,提出了将空时二维信号处理技术应用于地基二维相控阵雷达中的设想。根据地基雷达运动杂波的特点,在考虑误差情况下,对运动杂波二维耦合谱进行分析和仿真;初步构想了应用STAP的地基雷达无源干扰抑制结构,并对全空时自适应处理器抑制运动杂波进行仿真分析。结果表明,空时二维信号处理抑制运动杂波可以取得很好效果。     

平台运动引起Bragg海浪谱的展宽及其压缩方法

基于运动叠加原理，推导了以Ｄｏｐｐｌｅｒ频率表征的雷达平台运动条件下高频段海浪杂波的运动方程及由平台运动引起的Ｂｒａｎｎ海浪谱的展宽式，给出了受平台运动调制的Ｂｒａｇｇ浪回波与目标信号在Ｄｏｐｐｌｅｒ域里的二阶模糊关系和动用ＳＴ－ＷＤ（Ｓｈｏｒｔ Ｔｉｍｅ Ａｖｅｒａｇｅ Ｗｉｇｎｅｒ－Ｖｉｌｌｅ Ｄｉｓｔｒｉｂｕｔｉｏｎ；短时间平均Ｗｉｇｎｅｒ－Ｗｉｌｌｅ分布）技术对一阶海浪回波信号的高频相位进     

基于空时导向约束的天基雷达离散旁瓣杂波判别方法

由于天基雷达覆盖范围广,大量强离散杂波(小型岛礁、陆地铁塔等)会从天线旁瓣进入雷达系统,其多普勒特征与目标相同,极易造成虚警。针对以上问题,该文提出基于空时导向约束的天基雷达离散旁瓣杂波判别方法,该方法首先选取空时自适应处理(STAP)杂波抑制后检测到的潜在“目标”(包含真实目标与离散旁瓣杂波)距离多普勒单元及其附近单元;然后根据杂波多普勒频率与空间角度的耦合关系获得各杂波单元对应的空时导向矢量;最后利用获得新的导向矢量构成的滤波器再次对“目标”距离多普勒单元及其附近单元进行滤波处理,此时真实目标信杂噪比会大幅度降低,而离散旁瓣杂波信杂噪比变化不大,从而实现离散旁瓣杂波的判别。理论分析及机载实测数据处理证明该方法具有良好的稳健性和可靠性。     

基于载波域自适应迭代滤波器的无源雷达多径杂波抑制方法

在无源雷达系统中,监测通道信号中存在零频和非零频多径杂波,影响目标的检测。时域自适应迭代滤波器(如LMS, NLMS, RLS等)常被用于无源雷达杂波抑制,但这些方法只适用于零频多径杂波。该文针对零频和非零频多径杂波的问题,结合数字广播电视信号的正交频分复用波形特征,提出一种基于载波域自适应迭代滤波器的杂波抑制算法。该算法利用同一载频下含有相同多普勒频移的多径杂波的相关性原理,进行杂波抑制。仿真和实测数据处理结果证明了算法的有效性。     

一种MTD的优化设计及实际应用

根据某型雷达的具体情况，MTD的滤波器组采用了切比雪夫加权设计方法，对相关的信号处理方法如恒虚警检测等关键问题作了讨论。测试结果表明，该MTD系统对地杂波的抑制比大于55dB，对动杂波的抑制比大于45dB，能够达到较好的动目标检测效果。     

自适应杂波抑制技术在雷达中的应用

动目标显示(MTI)技术是雷达在杂波环境中发现运动目标的有效手段。首先讨论了雷达信号处理中常用的自适应动目标显示技术,即在多杂波环境下的自适应杂波抑制技术。然后针对雷达杂波抑制中常用的级联MTI滤波器,提出了一种新的设计方法,即采用时变加权原理,通过对动杂波速度估值,实现对运动杂波抑制滤波器权值的优化。最后给出了这2种杂波抑制级联型MTI的工程实现。     

Adaptive Detection and Interference Rejection of Multiple Point-Like Radar Targets

We present an adaptive algorithm aimed at detecting multiple point-like radar targets embedded in correlated Gaussian noise. The proposed detector modifies and improves the adaptive beamformer orthogonal rejection test (ABORT) idea to address detection of multiple targets. More precisely, it relies on the so-called two-step generalized likelihood ratio test (GLRT) design procedure implemented without assignment of a distinct set of secondary data. The newly proposed detector can guarantee the constant false alarm rate (CFAR) property and the performance assessment, conducted resorting to simulated data, has shown that it exhibits better rejection capabilities of mismatched signals than previously proposed detectors, at the price of an acceptable performance loss for matched signals     

雷达仙波的抑制方法研究

仙波在特殊的气候条件下出现,会导致雷达终端显示器出现成片的慢速运动“目标冶,从而严重影响雷达的性能。文中结合某中重频雷达的实测数据,分析了雷达实测数据中仙波的特征,根据仙波的多普勒频率特征提出了分别基于自适应动目标显示、自适应动目标检测(MTD)处理的仙波抑制方法。由于仙波的多普勒频率未知,提出一种在MTD处理基础上对检测结果所在多普勒通道进行简单判断的仙波抑制方法。实测数据处理结果表明,三种方法都对仙波具有良好的抑制作用。     

Time Reversal Imaging by Adaptive Interference Canceling

We develop the time reversal adaptive interference canceler (TRAIC) time reversal beamformer (TRBF), a new algorithm to detect and locate targets in rich scattering environments. It utilizes time reversal in two stages: (1) Anti-focusing: TRAIC time reverses and then reshapes the clutter backscatter to mitigate the clutter response; (2) Focusing: TRBF time reverses the residual backscatter to focus the radar image on the target. Laboratory experiments with electromagnetic radar data in a highly cluttered environment confirm the superiority of TRAIC-TRBF over conventional direct subtraction (DS) beamform imaging.     

机载雷达极化空时联合域杂波抑制性能分析

针对当前机载雷达面临强杂波抑制和微弱目标检测等难点问题,提出了一种极化空时联合处理新方法.该方法充分利用了目标与杂波在极化域、空域和时域(Doppler域)的特征差异,提高了抑制杂波、增强目标的能力.本文首先建立了机载极化阵列雷达接收信号模型和滤波算法模型.其次,利用分辨格思想及特征值、特征向量和功率谱的关系,构建了完全极化情况下杂波协方差矩阵新的等价关系.在此基础上,详细推导了新方法的杂波抑制性能,得到了输出信干噪比(SINR)与空间匹配系数,Doppler匹配系数,极化匹配系数,杂波脊斜率,信噪比(SNR),杂噪比(CNR)等的定量解析表达式.并与空时自适应处理(STAP)进行比较,从理论上分析得到了极化空时处理性能优于空时处理.特别在检测慢速运动目标时,新方法具有较强的稳健性.最后,仿真算例验证了模型的正确性.     

一种有效的机载火控雷达地面动目标检测方法

研究机载火控雷达检测地面慢速目标的方法。由于机载火控雷达采用前向阵,脉冲重复频率低,使得杂波多普勒频率多重模糊。分析了机载火控雷达检测地面慢速目标的特点,提出了一种非自适应的干涉对消法与自适应的AMTI方法相结合的检测方法。分析结果表明,这种方法对雷达平台运动参数的估计精度要求较低,工程上易于实现。     

基于冗余小波变换的高频地波雷达目标检测算法

为了从高频地波雷达(High Frequency Surface Wave Radar, HFSWR)信号生成的复杂距离多普勒(Range Doppler, RD)图像中准确提取运动点目标, 提出了一种基于冗余小波变换(Redundant Discrete Wavelet Transformation, RDWT)的RD图像点目标检测算法.该算法根据点目标与海杂波、电离层杂波等特征的差异, 首先在距离方向进行自适应RDWT以去除海杂波和地杂波, 并在多普勒方向进行自适应RDWT以去除电离层杂波; 然后利用图像形态学运算对背景噪声进行了抑制; 最后进行阈值自适应分割以得到点目标.实验结果表明:该算法能有效抑制RD图像中的海杂波、电离层杂波和背景噪声, 能从复杂的RD图像中实现点目标的有效检测, 其检测性能优于改进的恒虚警率(Constant False Alarm Rate, CFAR)算法.     

Sparse LCMV beamformer design for suppression of ground clutter inairborne radar

Cancellation of the ground clutter received at an airborne phased array radar is an inherently two dimensional problem. Clutter returns are Doppler shifted due to platform motion forcing the use of processors that can resolve targets in both velocity (Doppler) and azimuth. Fully adaptive processors that operate in both dimensions require prohibitively large computation so that reduced adaptive dimension, or partially adaptive processors must be considered. In conventional partially adaptive linearly constrained minimum variance (LCMV) beamformer design the approach taken has been to represent the interference subspace with some reduced set of vectors, typically the eigenvectors associated with the largest eigenvalues of the interference covariance matrix. This technique does yield good performance but will not give the optimum performance for a given partially adaptive dimension. In this paper, an off-line method for selecting the “best” degrees of freedom to be retained in a partially adaptive design is presented. The sequential algorithm described selects those degrees of freedom that best minimize the beamformer output mean square error. This approach leads to a sparse structure for the transformation matrix, which when implemented in a generalized sidelobe canceller (GSC) structure results in a reduction in the computational load. This approach also allows a reduction in the required adaptive dimension as compared to the eigenvector based approach. Illustrative examples demonstrate the effectiveness of this method     

机载火控雷达慢速目标检测

该文研究机载火控雷达对慢速目标的检测方法以及发射信号带宽对慢速目标的检测性能的影响。通常情况下,机载火控雷达地杂波多普勒频率多重模糊,主瓣杂波带宽大,常规处理性能很差。该文结合实际情况。重点分析了机载火控雷达检测地面慢速目标的特点,分析了主瓣杂波这一制约系统检测性能的主要因素,比较了两类空时二维处理方案对慢速目标的检测性能。采用较大的发射带宽是火控雷达的发展方向,雷达发射信号的带宽是影响系统对慢速目标检测性能的重要因素,该文通过理论分析和仿真试验比较了不同带宽情况下系统对慢速目标的检测能力。     

陆基雷达接收机采用PD体制的设计需求研究

针对采用脉冲多普勒(PD)体制的陆基雷达接收机在强重叠杂波条件下进行弱目标检测的应用背景,首先介绍了PD处理的原理;然后,重点分析了杂波重叠对接收回波模型的影响;最后,通过工程实例分析给出了提高陆基PD雷达近距离强杂波背景下弱动目标检测能力的有效措施:1)频率源的设计不建议锁相源体制,相位噪声关注范围扩展到MHz量级;2)瞬时无杂散动态范围指标需提高到75 dB;3)保证系统噪声系数恶化有限的前提下,尽量降低系统接收增益。     

频控阵雷达空距频聚焦信号处理方法

针对复杂环境下对低可观测运动目标探测的迫切需求,该文在系统回顾近几年频控阵雷达国内外发展现状、频控阵雷达阵列结构设计及波束形成、距离和角度的联合估计等技术的基础上,提出了基于空距频聚焦的频控阵雷达信号新方法。充分利用频控阵雷达提供的发射波形自由度、阵元位置自由度、波束方位与距离相关性以及凝视观测的特点,即在空间(角度)、距离和频率(多普勒)的灵活自由度和能量集性,实现空-距-频聚焦和联合参数估计。仿真分析表明该方法具有提高复杂环境下雷达微弱动目标检测和参数估计的潜力,在杂波和干扰抑制、动目标精细化处理等方面有广阔的应用前景。