基于时反算子分解的时反高分辨率定位技术研究

基于时反算子分解技术,提出了主动时间反转选择性定位方法。为提高此定位方法的分辨率,根据时反算子和协方差矩阵的相似性,提出了时反MUSIC和对角线加载时反MVDR定位技术。传统的获取时反算子的方法是通过单个阵元依次发射,该方法的缺点是每次只有单个元发射,导致输入信噪比较小,本文提出通过多个波束正交发射获取时反算子,能得到较大的输入信噪比,从而改善三种定位方法的定位性能。波导实验分别对三种定位方法和正交发射技术进行了验证,结果显示三种定位算法中,子空间基于的时反MUSIC方法具有较好的定位精度,时反MVDR技术具有较高的分辨率。      

多基地空时码探测信号设计及时反相关检测技术

为实现浅海复杂环境下的多基地声纳多源目标回波分辨,本文设计了一种适用于多入多出垂直阵信道环境下的空时码探测信号,并针对倾斜垂直阵的多途子信道差异问题,提出了信号的时反相关检测技术.空时码探测信号采用伪随机信号调制,具有良好的正交性,能在抗子信道严重衰落的同时,分辨多源目标回波.垂直阵受水流冲击,呈倾斜状态时,其多途子信道不一致性会导致各子信道传递信号无法在接收端聚焦,使阵列增益受损,同时导致时延测量能力下降和信号判决错误率上升,为此本文设计了信道训练信号用以估计多途子信道环境,通过虚拟时间反转镜获得子信道不一致条件下的最佳匹配检测信号,实现对接收信号的时反相关检测.仿真结果表明,本文所设计的探测信号和检测方法,能够克服复杂的信道条件和多途子信道不一致性引起的检测问题,满足多基地声纳探测需求,实现多源目标回波分辨.     

时间反转处理用于掩埋目标检测

研究了有PS(Probe Source)源发-收时间反转在强混响背景下对掩埋目标的检验问题.所提出的发-收互动时反处理是:在目标处放一个PS声源产生引导信号, SRA(Source Receiver Array)接收引导信号,并对接收稿:的信号做物理时反(即多路径补偿反波束形成),将声能聚焦在目标位置处,对SRA接收稿:的目标散射回波做与发射聚焦互动的接收聚焦(即多路径补偿波束形成),得到聚焦后的目标回波,实现发-收互动的聚焦.并对常规BS(Broadside)发射检测(没有时间反转)和时间反转检测两种方法各自的性能进行了比较.实验室波导掩埋目标实验数据处理验证了理论分析的结论,并证明时间反转检测比常规BS发射检测可得到更大的信混比,从而提高了掩埋目标的检测概率.     

浅海中水平阵时反聚焦性能研究

在浅海波导中,垂直时反阵可以跨越整个波导,能对大多数简正波模态进行精确采样。相比垂直阵,水平阵因便于安装在舰艇或永久布在海底放而更具有实际意义。本文在给定声源及海洋环境信息的条件下,通过仿真计算在声源处取得了较好的聚焦效果。对比垂直阵,计算了同种环境下水平阵取得较好时反聚焦效果所需孔径的大小,并分析了水平时反阵孔径一定,基元数减小;发射信号频宽的变化对时反聚焦性能造成的影响。得出大孔径的水平时反阵具有较好的时反聚焦性能,窄带信号较单频信号能改善水平阵时反聚焦性能的结论。最后处理了2002年海试数据,根据反演出的海底参数选择其中两组信号数据做时反处理,水平距离的定位误差为1.9%和2.2%。     

Costas编码跳频信号多普勒容限及其在多声源宽带正交检测中的应用

针对Costas信号的传统宽带匹配滤波因拷贝信号生成过程复杂而带来的计算复杂度高的问题,给出了一种通过计算宽带回波与窄带近似回波的互相关度进而得到Costas信号多普勒容限的数值计算方法,并提出利用容限值对预测目标多普勒范围进行分段窄带近似处理,从而避免了大量的时域伸缩变换,简化了拷贝信号的生成复杂度,提高了宽带匹配滤波的处理效率。随后,将这种方法扩展应用到多声源多目标的回波检测中,利用Costas信号的正交性及多普勒容限值,设计了一种宽带正交匹配检测器,大大提高了多目标声探测的效率。实验仿真和分析证明了多普勒容限求取方法的正确性及其应用于宽带正交匹配检测的有效性。      

浅海波导中水平接收阵被动时反混响抑制方法研究

使用基于射线简正波异地混响理论建立的水平接收阵接收海底混响模型,提出了水平阵被动时反混响抑制方法。以水平接收阵接收回波信号的某一时刻作为开始,分别对其前一时刻和后一时刻的回波信号做时反算子分解获得两个混响子空间,将这两个混响子空间的平均作为该时刻的混响子空间的一个估计,然后利用投影处理来抑制该时刻接收信号中的混响成分。该方法可以在一定程度上克服由于海底粗糙引起的局部时间(距离)段内接收回波信号的不平稳性,使得估计得到的混响子空间更加接近真实混响子空间。因此,利用其进行被动时反混响抑制可以增强回波信混比,提高对目标的检测和方位估计能力。针对典型浅海波导环境开展的仿真验证了本文方法的有效性。      

基于垂直阵模态分解的低频水声信号盲解卷处理研究

研究了基于人工时反处理的水声信号盲解卷方法,并在此基础上提出了一种基于简正波模态分解的低频水声信号的盲解卷处理方法。该方法适用于浅海波导中垂直阵接收的远程低频水声信号的盲解卷处理。该方法首先从浅海中垂直阵接收的信号中提取(估计)出波导中传播的简正波模态函数信息,然后,根据估计的模态函数信息通过模态滤波来实现水声信道盲解卷处理。针对典型的浅海波导环境,进行了计算机仿真试验,结果表明:(1)远程低频条件下,模态分解方法可以从垂直阵接收的信号中提取出波导中有效传播的模态函数信息,因此这种方法解决了目前人工时反处理方法需要准确的模态函数先验信息的问题;(2)在一定带宽条件下,接收信号信噪比较低时,本文给出的这种基于模态滤波的盲解卷方法比人工时反处理具有更好的解卷性能。     

结合主动时间反转算法的短基线定位研究

短基线声学定位技术是水下运动目标定位的重要研究内容。为了解决定位系统工作于浅水或近岸时多途扩展严重造成的时延估计误差和系统工作不稳定的问题,本文提出一种结合时间反转算法的短基线宽带应答定位技术,该方法利用短基线阵元发射宽带信号,通过应答器接收该信号时间反转处理后再返回给短基线阵元位置,实现信道多途的自适应聚焦,进而提高时延估计精度及信号检测的鲁棒性。仿真研究和湖上试验表明,该方法能够充分利用信道多途信息实现聚焦,减少信道多途对定位信号时延估计的影响,具有较强的抗噪声和多途干扰的能力。相对传统定位方法,该方法可以抑制短基线定位过程中误差及野点的产生,改善了物体定位导航的精度。      

用于浅海有源声呐目标深度估计的匹配相位处理EI北大核心CSCD

针对浅海波导中有源声呐目标深度估计问题,提出了通过单个声源发射低频宽带信号,垂直双接收水听器接收目标回波,利用宽带目标回波比值的相位特征进行匹配相位处理的方法,不仅消除了目标散射特性对有源声呐目标深度估计的影响,而且仅需计算单程的信道传递函数,运算量小,有利于实时处理的实现需求。首先通过理论推导,定义了一个隐含目标深度、且与目标散射特性无关的声场特征——单程传播向量宽带干涉结构,在Pekeris波导条件下仿真分析了简正波阶数与单程传播向量宽带干涉结构的幅度和相位的关系,发现选取较多的简正波阶数贡献的单程传播向量宽带干涉结构进行匹配相位处理可以提高目标深度估计的性能。进一步仿真分析表明,在发射信号选取一定的时间长度和带宽的条件下,利用全部阶简正波且信噪比大于-10 dB时,方法的深度估计误差在5 m之内。最后分析了信号时间长度和处理带宽对有源声呐目标深度估计性能的影响,以及海底声速、海深和声速剖面失配时方法的鲁棒性。     

不确实环境下目标检测与定位的宽容性最小方差时反波束形成

揭示了时间反转技术的本质,指出时反不仅是一个物理过程,更是一种信号处理方法,进而提出"时反波束形成"的概念.针对时反空时聚焦特性受到环境不确实性限制的问题,研究了用模型探查源(MS)代替实探查源(PS)的发射时反波束形成实现发射聚焦,以及用对角线加载的宽容性MVDR接收时反波束形成实现接收聚焦,并最后用于目标检测.水池实验的结果表明了该发射聚焦和接收聚焦在不确实环境下用于目标检测及其距离估计的有效性.     

TR-MIMO detection of a small target in a shallow water waveguide environment

A kind of multiple-input multiple-output (MIMO) sonar model is presented for enhancing detection of small targets in an underwater acoustic waveguide. A co-located vertical linear array is partitioned into several sub-arrays illuminating orthogonal waveforms at a target of interest. When the target is modeled as an extended target consisting of multiple scatterers, time reversal is utilized in the MIMO transmission scheme for capturing target diversity. The feasibility of time-reversal beams simultaneously esonifying the target’s scatterers is verified by the analysis of spatial–temporal focusing in simulations. The tank experimental results have further demonstrated the effectiveness of the TR-MIMO sonar model.     

Robust time reversal focusing based on Maximin criterion in a waveguide with uncertain water depth

Time reversal processing(TRP) might be regarded as matched field processing with known environmental knowledge.However,the performance of TRP is degraded in an uncertain environment.A technique based on the Maximin criterion is proposed for enhancing the robustness of TRP in a waveguide with uncertain water depth.The relationship between the water depth and the focal spot translation is examined based on the waveguide-invariant theory.Then the time reversal transmission scheme with the Maximin criterion is performed to maximize the minimum transmission power on a target of interest.At the receiving end,coherent summation operation is carried out over the received data by a reception focusing bank.If it is necessary to enhance the target echo further,the iterative time reversal can be considered where the target echo corresponding to the first time reversal transmission is regarded as a secondary source.Numerical simulations and experimental results of the target localization in a waveguide tank have verified the effectiveness of robust TRP.     

Combination of time reversal and synthetic aperture beamforming for active detection of small bottom objects in waveguide environments

To enhance detection of small targets, the combination of time reversal processing (TRP) and synthetic aperture beamforming (SABF) is investigated. With the spatial–temporal focusing, the potential application of TRP for active detection has been demonstrated [Kim S, Kuperman WA, Hodgkiss WS. Echo-to reverberation enhancement using a time reversal mirror. J Acoust Soc Am 2004;115(4):1525–31]. When a physical probe source (PS) replaced by a modeled source (MS), the “potential” is turned into being more practical. Similar to matched field processing, the robustness of TRP with MS needs to be considered. Meanwhile by the improvement of the extended towed array measurement (ETAM) algorithm of passive SABF, a segmented ETAM algorithm is discussed for its use in active sonar. With the echo-signal enhancement by time reversal transmission, the echo-to-reverberation ratio is further improved by SABF. Finally a matched filter is used to detect the target and the range of the target is estimated by the time delay referenced to the transmission time. The results of the waveguide tank experiment demonstrate that the TRP–SABF method can effectively detect and locate a bottom cylinder shell of 0.51 m long and 0.21 m in diameter.     

Time reversed reverberation focusing in a waveguide

Time reversal mirrors have been applied to focus energy at probe source locations and point scatterers in inhomogeneous media. In this paper, we investigate the application of a time reversal mirror to rough interface reverberation processing in a waveguide. The method is based on the decomposition of the time reversal operator which is computed from the transfer matrix measured on a source-receiver array [Prada et al., J. Acoust. Soc. Am. 99, 2067-2076 (1996)]. In a similar manner, reverberation data collected on a source-receiver array can be filtered through an appropriate temporal window to form a time reversal operator. The most energetic eigenvector of the time reversal operator focuses along the interface at the range corresponding to the filter delay. It is also shown that improved signal-to-noise ratio measurement of the time reversal operator can be obtained by ensonifying the water column with a set of orthogonal array beams. Since these methods do not depend upon a priori environmental information, they are applicable to complex shallow water environments. Numerical simulations with a Pekeris waveguide demonstrate this method.     

Modal time reversal of waves for shallow-water sea

A method is proposed for calculating the time-reversed wave field generated by a point source in a waveguide by using signals received by a vertical antenna array. The procedure of time reversal is based on representing the wave field in the form of the decomposition into modes of the ideal waveguide. In contrast to the earlier proposed simplified numerical method of time reversal of waves, the method presented here allows one to obtain the reversed field for the entire thickness of the waveguide. The method is successfully applied to a shallow-water sea with a depth of 120 m, at distances of 7, 10.5, and 12 km. It is shown that an opportunity arises to increase the gain of the array; to determine the parameters of the medium, including its stability in the presence of currents; and to match the point of transmission of an arbitrary received signal and the point of transmission of the reversed signal.     

Time reversal multiple-input/multiple-output acoustic communication enhanced by parallel interference cancellation

Multiple-input/multiple-output (MIMO) techniques can lead to significant improvements of underwater acoustic communication capabilities. In this paper, receivers based on time reversal processing are developed for high frequency underwater MIMO channels. Time reversal followed by a single channel decision feedback equalizer, aided by frequent channel updates, is used to compensate for the time-varying inter-symbol interference. A parallel interference cancellation method is incorporated to suppress the co-channel interference in the MIMO system. The receiver performance is demonstrated by a 2008 shallow water experiment in Kauai, Hawaii. In the experiment, high frequency MIMO signals centered at 16 kHz were transmitted every hour during a 35 h period from an 8-element source array to a wide aperture 16-element vertical receiving array at 4 km range. The interference cancellation method is shown to generate significant performance enhancement, on average 2-4 dB in the output signal-to-noise ratio per data stream, throughout the 35 h MIMO transmissions. Further, communication performance and achieved data rates exhibit significant changes over the 35 h period as a result of stratification of the water column.     

正交多相码连续波主动声呐回波检测算法设计

与常规脉冲式主动声呐相比,连续波主动声呐能够提高目标回波的时间处理增益和目标信息更新速率。该文提出一种由正交多相码合成的连续发射波形,分析了发射信号和目标回波模型,设计了多通道匹配滤波器组完成回波检测。为了进一步提高接收机检测性能,提出了多通道非相干积累的处理方法。通过数值仿真,分析提出的连续波形的多普勒分辨性能和目标信息更新率。通过蒙特卡罗法获取接收机工作特性曲线,比较了脉冲式主动声呐和连续波主动声呐在均匀混响背景下对单目标检测性能的差异。仿真结果表明,该文设计的连续波波形具有较好的多普勒分辨性能,在均匀混响背景下,回波检测算法能够明显提高单个目标的探测能力。     

Echo-to-reverberation enhancement based on decomposition of time reversal operator with forward and backward reverberation nulling constrains

Combined the decomposition of time reversal operator and the time reversal reverberation nulling, a new time reversal processing approach for echo-to-reverberation ratio enhancement is proposed. In this method, a 2-dimensional signal subspace for the range of the target and two bottom focusing weight vectors for the ranges near the target are obtained by the decomposition of time reversal operator. From the signal subspace and focusing weight vectors, a constrained optimal excitation weight vector of source receiver array can be deduced to null the acoustic energy on the corresponding bottom and maximize the energy at the tar- get. This method remedies the shortages of conventional time reversal processing, time reversal reverberation nulling and time reversal selective focusing method. It focuses sound energy at the target and nulls the energy at the bottom near the target range simultaneously, therefore enhancing the echo-to-reverberation ratio without probe source and prior-knowledge of the relative scattering intensity of target and bottom. Numerical simulations in typical shallow water environments showed the effectiveness of the proposed method and its improved performance for echo-reverberation enhancement than conventional time reversal processing.