Time-Frequency Signatures of Penetrable or Impenetrable Targets Buried in Lossy Half-Spaces

The capability of ultra-wideband (UWB) radar or sonar systems for extracting signature information useful for target-recognition purposes is well known. The signal processing techniques to be used in the microwave or ultrasonic cases are quite similar. The frequency content of the interrogating signals is usually designed to match the size and type of target and environment present. In the radar case, the complex permittivity of a soil can vary substantially with its moisture content. Both moisture content and target-depth will alter the returned signature. We investigate here the backscattered echoes of impenetrable or penetrable (i.e., dielectric, in this case) bodies of revolution (BOR) of the same size and shape, as they are illuminated by an above-ground projector pointing downwards. The targets are buried at a few representative depths, in soils of various moisture contents. The echoes are simulated by the Method-of-Moments (MoM) method, and then used to determine the target signatures in the time, the frequency, and most importantly, in the joint time-frequency domains. These signal processing methods also apply for the case of ultrasonic signals. In the time-frequency domain the signatures are generated here by a pseudo-Wigner distribution (PWD). Time-frequency distributions can be used for actual target-classification purposes using measured data.     

Suppressing GPR Clutter from Randomly Rough Ground Surfaces to Enhance Nonmetallic Mine Detection

This study attempts to quantify the ground penetrating radar rough ground surface clutter by numerical modeling of wave scattering, and establish a strategy to suppress the clutter for given test signals. The goal is to improve the GPR detection statistics for small, buried, low-contrast nonmetallic antipersonnel mines. Using a model of an experimentally measured impulse GPR signal, we simulate the ground surface and buried low-contrast mine target scattered responses. We employ a 2D finite difference time domain (FDTD) method to analyze the pulse shape, delay, and amplitude characteristics of the scattered waves—with and without buried nonmetallic mine targets—as a function of roughness parameters. Five hundred Monte Carlo simulations of various test cases of specified ground root mean square height and correlation length were run to generate statistics for the clutter and target signal variations. In addition, the effectiveness of identifying and removing the ground surface clutter signal for detecting subsurface targets is presented. Results indicate that even with moderate roughness, statistics can be generated to enhance the detection of small, shallow, low-contrast targets.     

The Huynen-Fork Polarization Parameters in the Classification of Dielectric Mine-like Objects

We present a polarimetric approach that can be used to characterize subsurface targets by means of ground-penetrating radar. Several quantities related to the Huynen-fork parameters are basic to the study. These quantities are all expressible in terms of the elements of Sinclair matrix S which in turn, can be determined as functions of frequency by an application of the method-of-moment (MoM)—which is briefly reviewed here—to a mine-like object buried at various depths in a soil of specified dielectric and moisture properties. The quantities in question are the Stokes parameters, the elevation, azimuth, and polarizability angles, as well as the polarization ratios and the scattering eigenvalues which optimize the backscattered power. Some of these quantities are studied here and displayed in various graphs. These frequency dependent graphs exhibit useful symmetry properties. The plots of the polarizability angle (β) vs. frequency seem to remain invariant with target depth, and exhibit resonance features that are later shown to agree well with other standard methods to estimate resonances such as Prony method. A simple analysis of the early-time resonances thus estimated is used to obtain acceptable mine dimensions. The agreement found implies that polarimetric techniques, such as present one, can also be used in the arsenal of techniques yielding target-ID clues.     

基于智能手机TDOA估计的被动声源定位方法与系统实现

被动目标监测广泛应用于国防、安全等领域。针对被动声目标监测,首次提出了基于手机平台的被动式声目标定位方法,并设计和开发了原型系统。针对手机间时钟同步对被动式信号到达时间差精度的影响,采用手机内置的麦克风和扬声器发送和接收同步声信号的方式,从而避免了节点间无线同步到声信号采集间的时间延迟不确定性。为了提高信号发送和接收时刻的估计精度,将时间戳信息调制到设定的线性调频声信号,并采用广义互相关方法来实现信号波形检测。进一步,针对被动目标声源的非合作特性,采用统计判决理论和语音活动性检测相结合的方法对观测信号进行联合检波,获得被动目标声源到达时间的高精度估计,即被动声源到手机间的到达时间差。最后,通过多部手机搭建了原型系统并设计实验,结果表明定位误差不超过10%的概率达到80%。     

Modeling Clutter from Bosnian and Puerto Rican Rough Ground Surfaces for GPR Subsurface Sensing Applications Using the SDFMM

The Steepest Descent Fast Multipole Method (SDFMM) is used to analyze the distorting effect of random rough ground surfaces on scattered and transmitted electromagnetic waves. Two well-measured loamy soils: Bosnian and Puerto Rican clay loam are investigated, each with a variety of surface roughness. This study is important in understanding the effects of different soil properties and is meant to be an a priori phase of investigating scattering from buried targets under the rough ground. In this work, we investigated the scattering from rough soil ground without buried objects. The SDFMM is an integral equation-based fast algorithm that is well suited for two-dimensional penetrable rough surfaces (3-D scattering) in the frequency domain. The scattered and transmitted near electric field of an incident Gaussian beam are calculated at different locations above and below the mean plane of the dielectric rough interface. The receiver locations above are chosen to simulate GPR measurement protocols. The obtained numerical results show that the scattered field undergoes more distortion than the transmitted field from both soil types. Moreover, the transmitted fields into the higher dielectric constant Puerto Rican soil experience more distortion than those transmitted into Bosnian soil.     

回波模拟器的研制及对卫星雷达高度计发射前的性能评估

给出了回波模拟器的原理及其对卫星高度计的测试和定标方法.通过采用chirp重建技术、全去斜坡技术、海洋回波波谱数字合成技术和高速DSP技术,研制了一台用于卫星高度计测试和定标的全信号海洋回波模拟器,实现了海洋回波信号的全程路径延时模拟和全海况模拟.延时模拟精度为0.2ns、海洋有效波高模拟精度为0.5m、后向散射系数模拟精度为1dB;并将该模拟器用于了卫星高度计全系统的测试和定标.实验结果:有效验证了回波模拟器的原理及其对卫星高度计的测试和定标方法是可行的,也有效验证了一种卫星高度计的动态工作性能.     

Detecting Nonmetallic Mines Under Rough Ground Using Semi-Analytic Mode Matching

Ground penetrating radar detection of plastic land mines buried in lossy, dielectric soils under rough ground surfaces is only possible with wide bandwidth probing signals. Using the new semi-analytic mode matching (SAMM) algorithm, we model the ultra-wide bandwidth scattering of these low-contrast buried targets, using computed time domain signatures to facilitate detection. It is shown that differences between the characteristic time peaks of the non-specular signature are primarily dependent on the size, shape and material characteristics of the target, and less so on its burial depth, background soil, or ground surface roughness. Differences between time peaks are attributed to multiple roundtrip transit times through the target which are largely independent of the ground characteristics.In the SAMM algorithm, the frequency-dependent scattered fields are constructed from moderately low-order modal superpositions of spherical waves, each satisfying the Helmholtz equation in its respective material (air, ground, or mine). By least squares fitting, mode coefficients are found which optimally match all boundary conditions at designated points along the boundary surfaces, where the boundaries may be irregularly shaped. Spherical wave expansions are chosen at multiple coordinate centers so that fewer modes are needed to give convergent results. The speed advantage of SAMM over other computational methods allows for the detailed study of ultra-wideband GPR sensing of challenging, realistic subsurface detection problems.     

Backscattered electron imaging of cultured cells: application to electron probe X-ray microanalysis using a scanning electron microscope

We report a simple method to study the elemental content in cultured human adherent cells by electron probe X-ray microanalysis with scanning electron microscopy. Cells were adapted to grow on polycarbonate tissue culture cell inserts, washed with distilled water, plunge-frozen with liquid nitrogen and freeze-dried. Unstained, freeze-dried cultured cells were visualized in the secondary and backscattered electron imaging modes of scanning electron microscopy. With backscattered electron imaging it was possible to identify unequivocally major subcellular compartments, i.e. the nucleus, nucleoli and cytoplasm. X-ray microanalysis was used simultaneously to determine the elemental content in cultured cells at the cellular level. In addition, we propose some improvements to optimize backscattered electron and X-ray signal collection. Our findings demonstrate that backscattered electron imaging offers a powerful method to examine whole, freeze-dried cultured cells for scanning electron probe X-ray microanalysis.     

Diffraction effects and inelastic electron transport in angle‐resolved microscopic imaging applications

We analyse the signal formation process for scanning electron microscopic imaging applications on crystalline specimens. In accordance with previous investigations, we find nontrivial effects of incident beam diffraction on the backscattered electron distribution in energy and momentum. Specifically, incident beam diffraction causes angular changes of the backscattered electron distribution which we identify as the dominant mechanism underlying pseudocolour orientation imaging using multiple, angle‐resolving detectors. Consequently, diffraction effects of the incident beam and their impact on the subsequent coherent and incoherent electron transport need to be taken into account for an in‐depth theoretical modelling of the energy‐ and momentum distribution of electrons backscattered from crystalline sample regions. Our findings have implications for the level of theoretical detail that can be necessary for the interpretation of complex imaging modalities such as electron channelling contrast imaging (ECCI) of defects in crystals. If the solid angle of detection is limited to specific regions of the backscattered electron momentum distribution, the image contrast that is observed in ECCI and similar applications can be strongly affected by incident beam diffraction and topographic effects from the sample surface. As an application, we demonstrate characteristic changes in the resulting images if different properties of the backscattered electron distribution are used for the analysis of a GaN thin film sample containing dislocations.     

一种信号合成电路的设计与实现

主要设计一种能够合成指定波形的信号波形发生电路,电路基于傅里叶合成,通过产生不同频率的正弦信号,将这些信号处理后送入加法电路可合成所需信号。主要由方波产生模块产生方波,分频与滤波模块对所得方波分频并滤成正弦波,放大模块对所得正弦波幅值进行调节,移相模块调整各频率正弦波相位,最终通过信号合成模块合成所需波形。     

Effects of windowing filters in leak locating for buried water-filled cast iron pipes

Arrival time difference or time delay estimation is critically important for detection of leak location in buried water supply pipes. Because the exact leak locating depends upon the precision of the arrival time difference estimation between leak signals measured by sensors and the propagation speed of the leak-related elastic wave, the research on the estimation of time delay has been one of the key issues in leak locating. The arrival time difference was estimated with the peak time of cross correlation functions of the measured signals. In this study six different window functions, including the basic rectangular, Roth, Wiener, SCOT, PHAT and maximum likelihood windows were applied. Experimental results against an actual buried pipe made of cast iron showed that the introduction of the window functions improved the precision of time delay estimation. In this paper, a new statistical approach, that combines all results of each window function, is suggested for better leak locating. Apart from the experiment, an intensive theoretical analysis in terms of signal processing is described. This paper was recommended for publication in revised form by Associate Editor Eung-Soo Shin Young-Sup Lee received his BS degree in Naval Architecture from Pusan National University, Korea, in 1987. He then received his MSc and PhD degrees from the University of Southampton, United Kingdom, in 1997 and 2000, respectively. Dr. Lee is currently an assistant professor at the Department of Multimedia Systems Engineering in the University of Incheon, Korea. His research interests include active control of sound and vibration, real-time signal processing, and smart sensors and actuators.     

基于小波分析的声发射源定位技术

声发射全波形采集技术为基于波形分析的声源定位方法提供了可能。在板状构件中声波的传播模式较为复杂,且不同模式的波到达时间和波速均不相同,从而带来了声源定位误差。将时频幅度分析方法引入声发射源定位技术：从接收到的信号中提取出某一频率的柔性波随时间变化的规律,在理论分析的基础上证明,这个分离信号的最大幅值点所对应的时间就是该频率柔性波群速度的到达时间。根据这个到达时间,以及实际测量出的群速度就可以实现声发射源更精确的定位,试验结果也证明了这种方法的可靠性。     

多层印制板的高速过孔仿真分析

采用CST软件对高速多层PCB板上的过孔进行了建模仿真,分析了过孔尺寸对信号完整性的影响规律,给出了一些相应的S参数以及时域反射波形和时延波形。     

谐振型软开关逆变器控制信号补偿研究

针对谐振型逆变器输出电压波形畸变及开关管发烫的问题,分析了逆变器的拓扑结构及软开关控制原理,推导了并联谐振的频率响应特性,分析了波形过零点畸变的原因是由于芯片响应时间对控制信号产生了时延.针对信号时延问题,设计了基于CD4046锁相环频率跟踪技术的补偿电路,给出了补偿电路的电路图.最后,研究结果表明所设计的补偿电路有效...     

Time accurate finite difference method for performance prediction of a silencer with mean flow and nonlinear incident wave

Transmission losses of various reactive silencers are predicted, using a time accurate finite difference method. The numerical scheme is the 3rd order upwind scheme for axisymmetric Euler equations. Main advantage of the present method is that it can simulate linear and nonlinear wave propagation phenomena in a flow field directly with minimum numerical oscillation errors. The special treatments of incident wave condition, i.e. multiple harmonics of the transparent acoustic condition are applied to the transmission loss prediction for calculation efficiency. For the validation of the present approach, circular expansion chamber silencers without mean flow and an exponential pipe with mean flow are simulated in case of linear incident wave. The computed transmission losses have quite good agreements with those of the others. The nonlinear incident wave case is also investigated to check the usefulness of this method. The periodicN wave is clearly captured without numerical oscillation errors, and the insertion losses of two different incident frequencies are compared.     

考虑重力影响的索结构损伤行波识别法

因索中某几股断裂而导致索等效截面减小是索结构常见的损伤模式。文中分析此损伤模式下索受扰动后的行波响应，提出重力场中索结构损伤的行波识别法。该方法由索上一点的实测响应数据，根据入射波和反射波到达该点的时间确定损伤位置，根据入射波和反射波的信号能量比及能量的时域分布确定损伤程度和损伤段宽度。为适应现场有噪声测试数据的影响，应用多分辨率分析理论，以Daubechies小波分解行波响应信号，对信号去噪并作出白噪声情况下损伤识别方法的误差估计。仿真结果表明，该方法可采用噪声污染响应信号对重力场中索结构的损伤进行准确的估计，适合小损伤情况下索结构的损伤识别。     

Measuring the angular dependent energy distribution of backscattered electrons at variable geometry

An aluminium semisphere system with 120 points of entry and eight detection areas, assembled on a meridian covering 0.0026 steradian each, was put over a solid bulk sample (e.g., aluminium), which was mounted in the eucentric point so that the incident electron beam could be varied by a polar rotation of the sphere in steps of 11.25 degrees. The complete angular distribution of the backscattered electrons became available by a rotation in steps of 11.25 degrees azimuthally. For this particular setup, the signals from the detection areas as well as the signal from the rest of the semisphere were amplified by operational amplifiers (Burr-Brown OPA128LM). However the signal of the semisphere was not available at that time. Specimen current measurements made the total amount of electrons accessible, providing a possibility for normalization of the results and comparison with total backscattering coefficients. By use of counter voltage variable up to 10 kV inside the detection assembly, it was possible to measure an energy resolution of the backscattered electrons for each detection area at the same time. Details of the construction and calibration procedures, possible errors, and sources of systematic deviations as well as first test results are discussed.     

Laser-Induced Acoustic Imaging of Buried Objects

A 100 mJ, 100 ns pulsed CO₂ laser incident on the surface of soil is used as a localized acoustic source for the detection and imaging of underground objects. The acoustic pulse produced by the impulsive heating of the soil surface due to the absorbed 10.6 m radiation is detected with an acoustic transducer suspended over the surface. Application of a Fourier domain filter enables the separation of the direct acoustic return from the faint echo from an object buried 3–25 mm below the surface. Scanning of the laser pulses across the position of a buried object allows the resolution of the shape and the depth of the buried object. The application of this technique to image buried landmines is demonstrated in trials at an outdoor test track.     

Fast scanning of a pulsed terahertz signal using an oscillating optical delay line

We describe a fast measurement of a pulsed terahertz signal generated by a femtosecond laser and a photoconductive antenna using an oscillating optical delay line. The method to measure the amplitude of the retroreflector in the oscillating optical delay line is proposed and the displacement of the retroreflector is exactly calculated to acquire the optical delay time in the fast scan mode. With the different oscillation frequency and amplitude of the retroreflector, the pulsed terahertz signals are measured and analyzed. The comparison of the temporal waveform and frequency spectrum between the fast scan mode and the slow scan mode shows a good agreement with the decrease in the scanning time from 60 to 1 s at a signal to noise ratio of 430.     

Monte Carlo simulation of scanning electron microscope signals for lithographic metrology

Two computer codes for simulating the backscattered, transmitted, and secondary-electron signals from targets in a scanning electron microscope are described. The first code, MONSEL-II, has a model target consisting of three parallel lines on a three-layer substrate, while the second, MONSEL-III, has a model target consisting of a two-by-two array of finite lines on a three-layer substrate. Elastic electron scattering is determined by published fits to the Mott cross section. Both plasmon-generated electrons and ionized valence electrons are included in the secondary production. An adjustable quantity, called the residual energy loss rate, is added to the formula of Joy and Luo to obtain the measured secondary yield. The codes show the effects of signal enhancement due to edge transmission, known as blooming, as well as signal reduction due to neighboring lines, known as the “black-hole” effect.