宽带扩展性目标时反算子分解成像

在时反算子分解技术的基础上,提出了宽带全信号子空间加权成像方法。虽然扩展性目标与时反算子的特征向量不再是一一对应关系,传统的时反选择性聚焦仍然选择某一特定的信号子空间成像,而时反MUSIC只利用某一频点成像,导致定位结果背景起伏较高。为克服这些缺陷,利用时反算子的特征值判断信号子空间的个数,以广义散射系数为加权系数,实现全宽带和全信号子空间成像。结果表明,此方法的旁瓣水平明显下降。     

前后混响零点约束下基于时反算子分解的信混比增强方法研究

结合时反算子分解和时反混响置零方法,提出了一种前后混响零点约束下基于时反算子分解的信混比增强新方法。利用目标回波出现时刻及其出现前后时刻的回波信号,通过时反算子分解获得目标距离对应的一个二维信号子空间和目标前后距离附近的海底聚焦权向量,根据该信号子空间和海底聚焦权向量,以目标前后距离附近的海底声场置零、目标处最大为约束条件,导出了时反阵的一个约束最优激励权向量。该方法解决了现有直接时反处理、混响置零方法以及时反选择性聚焦方法存在的局限性,可以在没有探测声源、没有目标和海底散射强度谁大谁小的先验知识的条件下,实现声场在目标处聚焦,同时在目标距离附近海底处设置混响凹槽,进一步增强回波信混比。针对典型浅海波导环境,通过计算机仿真验证了所提方法的有效性及其改进的信混比增强能力。     

传感器网络基于特征值分解的信号被动定位技术

基于传感器网络的信号被动定位技术在电磁学、声学、声呐系统以及传热学等领域具有广泛的应用前景,当传感器网络节点所接收噪声强度不同或传输信道存在阴影衰落效应时,给出了目标信号到达距离比定位关联度量的估计方法与基于信号到达距离比的被动定位算法.将特征值分解技术引入到信号到达距离比定位关联度量估计中,通过接收信号协方差矩阵特征值分解技术估计各节点所接收噪声强度,并通过网络参考节点轮换与特征值分解方法消除阴影衰落效应所引入的定位误差,最后给出该算法的最小二乘定位解.该方法可较好的消除由于节点接收噪声强度不同以及阴影衰落效应等因素所带来的定位性能恶化.     

基于声传播算子的声源定位实验模拟方法研究

声传播算子是一种高效的时域声场计算方法,它能够很方便地计算出给定系统参数下任意时刻任意位置的声场变化情况,本文采用这种方法计算所得的二维房间声场信息进行传声器阵列的声源定位仿真实验。计算结果表明,用该方法获取的阵列数据能有效地应用于阵列信号处理算法中,准确地估计出初始高斯脉冲声源的方向。声传播算子声场计算方法能为传声器阵列声源定位的实验提供方便,使得传声器阵列声源定位算法在不同混响时间的鲁棒性实验研究变得更加简捷。     

多输入多输出与时反联合的波导声呐有源目标定位

为了提高对浅海静止小目标的探测能力,提出了将多输入多输出和时反处理相结合的波导声呐处理框架。利用收发合置垂直阵,假设目标为点目标,每一个阵元轮流时反发射正交信号照射目标,整个阵接收对应的目标回波。当所有的阵元发射正交信号结束以后,对所有接收的目标回波数据进行分集处理,然后与时反发射驾驶向量做匹配滤波完成对目标的定位。分时发射策略克服了水声信道时延扩展和多普勒扩展导致常规多输入多输出处理接收端不同回波信号分离的困难。采用物理时反发射和数值时反接收聚焦,有助于抑制混响、提高回混比。数值仿真和波导水池实验结果表明多输入多输出时反处理较常规的有源匹配场处理以更高的精度对波导中的目标进行定位。     

一种新颖的基于截集分解的形态学算子

数学形态学算子在机器视觉应用领域中 ,如目标识别、图像分割及缺陷探测中越来越引起人们的重视。二值形态学基本操作膨胀与腐蚀也已经扩展到灰值图像处理中。目前已存在有几种不同的灰值形态学算子的定义 ,但对于结构元为灰值图的灰值图像处理 ,还未有一种具有实时处理结构的形态学算子。文中给出了这样的一种灰值形态学算子的定义。为了进一步增强这种算子的处理能力 ,在模糊集构成的完备格框架下完成了这种算子的构造过程。     

基于正交传播算子的闪电宽带甚高频辐射源定位方法研究

闪电甚高频辐射源定位技术为闪电放电特征及其物理机制的研究提供了重要手段.基于空间谱估计理论,可将正交传播算子方法应用于闪电放电通道时空演变过程的成像.该方法将阵列数据协方差矩阵进行线性分解形成正交传播算子,然后以子空间的正交性构造空间谱,通过空间谱搜索实现辐射源定位.针对闪电宽带甚高频信号,采用非相干子空间处理方法将带宽内的有效频点进行平均,减小噪声干扰.利用数值仿真分析了该方法的定位性能,验证该方法定位弱辐射源的有效性,并与时间反转技术进行了对比.针对人工触发闪电过程的定位结果表明该方法可以较高的时空分辨率清晰地描绘出闪电通道的基本结构及放电通道的发展过程,并且其对双源同窗事件的定位能力优于时间反转方法.该方法对提高宽带甚高频阵列在闪电弱辐射源定位、闪电起始机制的研究中的应用价值具有重要意义.     

基于本征正交分解函数的波前动态高分辨率重构

 应用Hartmann波前传感器测量了准直平行光通过低速热射流后的畸变波前时间序列,对该时间序列进行本征正交分解得到系列本征函数和时间系数,对本征函数进行双线性插值处理得到未布置探测器处的本征函数值,新本征函数与时间系数相乘从而获得未布置探测器处的波前数据。在时域和频域比较了波前预测值与实际值,结果表明了该插值方法的有效性,从而实现对波前的高时间、高空间分辨率测量。     

基于改进Canny算子的SOP贴片元件的定位识别

SOP贴片元件的定位识别主要采用的是Canny算子进行图像边缘检测,传统的Canny算子在滤波和阀值分割方面不能很好地满足SOP元件的识别精度。针对此问题,对Canny算子进行改进,采用中值滤波及形态学闭运算滤波替代Canny算子的高斯滤波器,能更好达到去噪效果;提出一种基于改进的最大类间方差法自适应的搜索出Canny算子的双阀值,解决了SOP元件对外界环境变化敏感问题;利用Canny算子提取出边缘轮廓结合基于邻接矩阵的搜索方法和最小二乘法实现元件的定位识别。仿真结果表明,改进后的算法能更好的提高SOP元件的识别精度,具有较好的自适应能力。     

基于DEM的战场目标侦察定位技术研究

针对现有侦察手段侦察定位精度低、速度慢的问题,运用数字摄影测量技术的相关理论与方法,提出了在DEM的基础上,以摄像机为"视点",通过透视模型的变换与相关算法计算获取所摄对像三维坐标的单摄像机侦察定位方法,给出了具体的实现步骤。目标定位实验表明,该方法具有定位精度高、实时性强的特点。     

Experimental subwavelength localization of scatterers by decomposition of the time reversal operator interpreted as a covariance matrix

The D.O.R.T. method (French acronym for Decomposition of the Time Reversal Operator) is an active remote sensing technique using arrays of antennas for the detection and localization of scatterers [Prada et at., J. Acoust. Soc. Am. 99, 2067-2076 (1996)]. The analogy between the time reversal operator and the covariance matrix used for classical sources separation in passive remote sensing [Bienvenu et al., IEEE Trans. ASSP 31, 1235-1247 (1983)] is established. Then, an experiment of subwavelength detection and localization of point-like scatterers with a linear array of transducers is presented. Using classical estimators in reception like Maximum-Likelihood and Multiple Signal Characterization (MUSIC), two point-like scatterers separated by lambda/3 and placed at 100lambda from the array of transducers are resolved. In these experiments, the role of multiple scattering and the existence of additional eigenvectors associated with dipolar and monopolar radiation of each scatterer is discussed.     

Experimental detection and focusing in shallow water by decomposition of the time reversal operator

A rigid 24-element source-receiver array in the 10-15 kHz frequency band, connected to a programmable electronic system, was deployed in the Bay of Brest during spring 2005. In this 10- to 18-m-deep environment, backscattered data from submerged targets were recorded. Successful detection and focusing experiments in very shallow water using the decomposition of the time reversal operator (DORT method) are shown. The ability of the DORT method to separate the echo of a target from reverberation as well as the echo from two different targets at 250 m is shown. An example of active focusing within the waveguide using the first invariant of the time reversal operator is presented, showing the enhanced focusing capability. Furthermore, the localization of the scatterers in the water column is obtained using a range-dependent acoustic model.     

Wideband imaging of extended targets with the decomposition of the time reversal operator

The weighted wideband imaging approach of full signal subspaces is proposed based on the decomposition of the time reversal operator(DORT). Although each singular vector of nonzero singular values does not correspond to one of the extended targets any more, the conventional approach of selective time reversal focusing still chooses one of the signal subspaces for imaging. Simultaneously, the time-reversal MUSIC imaging is carried out at a single frequency for wideband signal. The imaging of both...     

Adaptive instant record signals applied to detection with time reversal operator decomposition

Time reversal arrays are becoming common tools whether for detection or tomography. These applications require the measurement of the response from the array to one or several receivers. The most natural way to record the impulse responses for several sources is to generate pulses successively from each emitting point and record simultaneously the signals from the receivers. However, this method is very time consuming or inefficient in terms of signal-to-noise ratio. To overcome this limitation quasi-orthogonal pseudonoise signals like Kasami sequences can be used. For guided wave propagation, a very high degree of orthogonality between the signal is necessary to allow an accurate measure of the whole multipath structure of the transfer function. Hence, in this work, we propose a new family of pseudo-orthogonal signals that is adapted to the environment and more specifically, to highly dispersive media. These adaptive instant records signals are used experimentally to detect targets using the time reversal operator decomposition method. The accuracy of the 15 x 15 transfer functions acquired simultaneously, and therefore the detection capability, are demonstrated in an experimental ultrasonic waveguide as a small-scale model of shallow water propagation including bottom absorption and reverberation.     

Imaging in the presence of grain noise using the decomposition of the time reversal operator

In this paper, we are interested in detecting and imaging defects in samples of cylindrical geometry with large speckle noise due to the microstructure. The time reversal process is an appropriate technique for detecting flaws in such heterogeneous media as titanium billets. Furthermore, time reversal can be iterated to select the defect with the strongest reflectivity and to reduce the contribution of speckle noise. The DORT (the French acronym for Decomposition of the Time Reversal Operator) method derives from the mathematical analysis of the time reversal process. This detection technique allows the determination of a set of signals to be applied to the transducers in order to focus on each defect separately. In this paper, we compare three immersion techniques on a titanium sample, standard transmit/receive focusing, the time reversal mirror (TRM), and the DORT method. We compare the sensitivity of these three techniques, especially the sensitivity to a poor alignment of the array with the front face of the sample. Then we show how images of the sample can be obtained with the TRM and the DORT method using backpropagation algorithm.     

Spatio-temporal invariants of the time reversal operator

The decomposition of the time reversal operator, known by the French acronym DORT, is a technique to extract point scatterers' monochromatic Green's functions from a medium. It is used to detect, locate, and focus on scatterers in various domains such as underwater acoustics, medical ultrasound, and nondestructive evaluation. A limitation of the method arises from its single-frequency nature, when the signals used in acoustics are often broadband. Reconstruction of the broadband Green's functions from the single-frequency Green's functions can be very difficult when numerous scatterers are present in the medium. Moreover, the method does not take advantage of the axial resolution associated with broadband signals. Time domain methods are investigated here as an answer to these problems. It is shown that the time reversal operator in the time domain takes the form of a tensor. The properties of the invariants are discussed. It is shown they do not have all the expected properties. Another method is proposed that requires a priori information on the medium.     

Resolution enhancement and separation of reverberation from target echo with the time reversal operator decomposition

Time reversal operator (TRO) decompositions are performed in a model of an ocean wave guide containing a target and having different kinds of bottom. The objective is to study the effects of bottom reverberation and absorption by means of ultrasonic experiments. It is shown experimentally that the echo from a target can be separated from the bottom reverberation. Reverberation eigenvectors are back propagated in the wave guide leading to focus on the bottom. An amplitude correction is applied to both reverberation and signal eigenvectors to compensate for bottom absorption and thus to improve target resolution.     

The time reversal operator for semigroup evolutions

A quantum theory combining an irreversible time evolution semigroup with a time reversal operator is presented.     

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.