蓝绿激光探测水下目标的脉冲传输统计模拟及回波特性初步研究

文中给出了用蓝绿激光脉冲探测水下目标时的回波特性初步研究结果,所用的方法是对激光脉冲在海水中的传输作蒙特卡洛统计模拟.研究表明,位于目标上方空中的接收机收到的回波脉冲功率随着目标物水深的增加而呈指数下降,但随目标物反射的增加而增加.水体对激光的多次散射增强了回波脉冲功率,但也使脉冲明显变宽.     

利用极化SAR数据进行土地覆盖分类研究

基于相关矩阵特征向量的目标分解将地物回波复杂的散射过程分解成相互独立的三种单一散射分量:单向散射、双向散射和交叉散射,分别对应各自的目标相关矩阵。目标分解技术降低了散射回波之间的相关性,有利于分析地物散射机理,有助于提高分类精度。对荷兰F levoland地区全极化数据进行分解,经过试验和相关性分析,选用7种数据形成多参数数据组合,对其进行最大似然监督分类,同时进行常规三种极化加相位差的分类和基于复W ishart分布的最大似然分类,逐像元计算混淆矩阵,分析对比三种分类结果的精度,试验表明:相对于常规数据组合分类,基于复W ishart分布的监督分类可以小幅度提高分类精度,而利用目标分解得到多参数组合数据进行分类则有大幅度的提高。     

略论机载激光测深系统的激光脉宽选择

依据对海面背景噪声光子的计算和激光回波幅度的涨落分析，提出机载激光测深系统的激光脉冲宽度宜选为小于或等于5ns。     

基于局部回波幅值统计的海杂波背景下小目标检测

针对传统舰载雷达和岸基雷达很难检测出海杂波背景下小目标的现状,提出一种基于局部回波幅值统计的方法来检测海杂波背景下小目标.该算法基于数学统计思想:计算不同距离单元海杂波的幅值的最大值和标准偏差,分析发现距离单元含有小目标回波的最大幅值明显小于其它距离单元只含有海杂波回波的最大幅值,距离单元含有小目标回波的幅值的标准偏差明显大于距离单元只含有海杂波回波的幅值的标准偏差,因此可以在对小目标没有任何先验知识条件下,较准确地检测出海杂波背景下的小目标.实验证明,该算法是有效的.     

基于微扰法的改进一维分形模型分层海面电磁散射研究

运用微扰法研究了平面波入射分层介质粗糙面的电磁散射,采用改进的一维分形海面模型模拟实际的分层海面,推导出了平面波入射时的散射系数计算公式。通过数值计算得到了HH极化双站散射系数随散射角的变化曲线,讨论了中间介质介电常数和厚度、摩擦风速和入射波频率对双站散射系数的影响,得到改进的一维分形分层海面散射系数的基本特征、分区特征和随频率变化的特征,结果表明散射系数近似具有"量子化"特征。这些结果在诸如海洋遥感、无线电传播与通信、粗糙面重构等方面中有着广泛的应用。     

基于文氏改进谱二维海面的电磁散射计算与数据对比

基于文氏谱的功率谱理论,结合Donelan分布函数与文氏方向谱的对比结论,提出了一种基于文氏改进谱的二维粗糙海面模型。在经典的双尺度法计算电磁散射的基础上,将二维随机粗糙海面的电磁散射计算结果与Nathanson数据进行对比,分析了不同海况、擦地角、入射频率下,两种极化方式时文氏改进谱二维随机粗糙海面的电磁散射特性。     

基于斯托克斯参量的水下主动偏振去散射研究

受浑浊水体自身吸收与微颗粒散射的影响,水下光学成像面临成像距离短、像质差等问题。一种基于斯托克斯参量的主动偏振成像模型被应用到水下成像去散射过程中,有效提升了目标成像探测的清晰度。相比于被动偏振成像模型,引入主动光源可以实现对入射光的偏振调制,由此可以充分挖掘目标反射光与介质光的偏振特征差异进而实现分离选通。改进的主动偏振去散射模型利用了目标反射光的偏振共模抑制效应,借助偏振斯托克斯参量 S₂ 分量实现了介质光偏振角与偏振度全局分布的准确估算。同时引入图像质量评价参数对比度 contrast 作为反馈参量,通过迭代计算实现背景噪声的最大化滤除。不同散射程度、不同类型目标的水下成像实验表明：相比于传统主动偏振成像方法,改进后的成像模型可大幅提升水下目标的图像质量,特别是强散射环境下的图像 contrast 提升超 2 倍以上。该项工作为利用偏振信息进行水下图像去散射与目标重建提供了新思路。     

星载海洋高度计海面回波信号的统计特性分析

论文首先从理论上简要阐述了星载海洋高度计海面回波的散射机理和回波信号后向散射模型;其后,利用Matlab数学工具,通过对我国＂神州四号＂飞船下传的大量高度计观测数据进行IQ采样数据提取、数据处理以及统计分析,进行了星载海洋高度计海面回波信号的统计特性分析与研究.     

基于GIS的高频地波雷达回波信号的表达处理与显示

高频地波雷达的“距离-多普勒”(Range-Doppler, R-D)数据与“恒虚警率”(Constant False-Alarm-Rate, CFAR)检测结果数据存在非直观性的问题, 本文针对此问题, 分析了地波雷达回波数据特点以及结果形式, 研究了采用地理信息系统(Geographic Information System, GIS)技术对回波数据进行显示分析, 对R-D 与CFAR 检测结果进行表达, 实现高频数据在GIS 环境下的表达处理与显示。本文采用GIS的栅格表达“距离-多普勒”数据, 采用矢量数据结构表达CFAR 检测结果, 实现了地波雷达检测信息的直观显示; 研究了高频地波雷达数据中特定距离一维谱信号的提取, 实现了基于距离值的目标CFAR 检测查询; 针对雷达数据量大、处理时间长的问题, 采用多线程处理机制实现了高效实时显示和分析。     

粗糙海底波导中信号的时间反转时空聚焦

对粗糙海底波导中信号的时间反转时空聚焦进行仿真研究。利用数值计算分析不同海底粗糙度RMS(平均起伏高度)下的聚焦效果,并与平整海底时作比较。研究结果表明:在本研究计算参数范围内,粗糙海底的RMS越大散射强度越大,信号在声源处的时间反转时空聚焦效果比平整海底时稍有减弱,但随着RMS增大无明显减弱。因此,时间反转技术适用于粗糙海底波导中的目标探测。     

基于大气湍流效应的蒸发波导PJ 模型研究

本文基于海洋大气近地层相似理论和一维Kolmogorov 谱函数, 建立考虑各向同性和各向异性湍流效应的蒸发波导模型, 并基于相对误差理论, 将数值模拟的大气修正折射率与根据试验采集数据计算得到的修正折射率进行对比分析。研究结果表明： 湍流效应对蒸发波导模型的预测结果有影响, 且考虑各向异性湍流效应的蒸发波导模型预测精度更高。因此在构建蒸发波导模型时, 需要考虑各向异性湍流对大气折射率的扰动影响, 才能得到更准确的大气修正折射率廓线。通过引入各向异性大气湍流理论, 能够有效提升蒸发波导模型的适用性, 为后续反演大气波导提供较好的模型基础。     

Estimation of extreme slamming pressures using the non-uniform Fourier phase distributions of a design loads generator

Slamming pressures are predicted using a nonlinear ship motion program whose input is an ensemble of short wave trains tailored to produce a large, linear pitch response. These short wave trains are calculated via a design methodology that first creates short time series containing a specified, large ship response and then back-calculates the incident wave trains using linear systems theory. The background simulations and theory used to create these short time series are presented here. Monte Carlo simulation of moderately rare events of a random process indicate the random Fourier component phase PDFs are non-uniform, non-identically distributed, and dependent on the rarity of the target event. These PDFs are modeled using a single parameter, Modified Gaussian distribution and used to generate design time series with a given expected value at a specific time. To predict rare events without resorting to Monte Carlo simulation, the parameters of the Modified Gaussian distributions are calculated via characteristic function comparison. The characteristic functions compare a target PDF calculated from extreme value theory to a PDF based on a discrete Fourier representation of the stochastic process with non-uniform component phases. The comparison to extreme value theory helps to quantify the risk associated with rare events.     

南海北部海洋生态模型的参数分析及遗传算法优化

海洋生态系统动力学模型是研究海洋生态环境的重要手段。随着模型的发展, 生态参数取值不确定性增加, 对模型结果的影响逐渐增大, 因此模型参数优化显得尤为重要。本研究在南海北部应用一维物理-生态耦合模型, 通过对模型生态参数进行敏感性分析, 获取关键生态参数, 利用遗传算法对参数进行优化。结果表明, 模型中的敏感参数主要集中于浮游植物生长和浮游动物生长、摄食和死亡以及碎屑沉降等过程。针对以上参数利用遗传算法优化, 发现仅加入表层卫星数据, 模型表层和垂向模拟误差分别降低27.80%和21.40%; 加入垂向观测数据, 表层和垂向模拟误差分别降低14.90%和32.70%。遗传算法应用于海洋生态模型的关键参数优化研究, 所获取的参数对模型有明显的改善效果, 提高了耦合模型对生态系统的模拟精度, 为参数优化在三维模型中的应用提供了依据。     

蒸发波导环境下风浪对电磁波传播影响的数值模拟研究

蒸发波导是一种特殊的大气波导,在其中传播的电磁波信号会被陷获在近海大气层中,实现超视距传播。受海表面温度、湿度、风速、微波频率等因素的影响,海洋蒸发波导环境中的微波传播特性起伏变化很大,规律十分复杂。以往的工作主要通过计算这些气象因素对蒸发波导条件下大气折射率剖面的影响来分析它们对路径损失的作用,其结果与实验数据仍有较大差异。本文在一定的蒸发波导条件下,利用一维分形海面模型产生海面“地形”,将其作为抛物方程电磁波传播模型的边界条件进行计算,得到相应的路径损失,并与传统计算方法进行对比,分析了不同蒸发波导高度、不同频率及不同接收天线高度时的数值模拟情况,可为舰艇通信系统或者雷达系统的设计提供相应的依据。     

Ionisation of Rydberg hydrogen atom near a metal surface by short pulse laser

In the ionisation of Rydberg hydrogen atoms near a metal surface, the electron will escape from the nucleus and arrive at the detector in a time sequence. This probability flux train relies on the initial electron wave packet irradiated by the laser pulse. For simplicity, the laser pulse is usually simplified to a delta function in energy domain, resulting in a sharp initial arrival time with an exponentially decaying tail at the detector. Actually and semiclassically, the initial outgoing wave should be modeled as an ensemble of trajectories propagating away from the atomic core in all directions with a range of launch times and a range of energies. In this case, each pulse in the pulse train is averaged out rather than a sharp profile. We examine how energy and time averaging of the electron wave packet affects the resolution of escaping electron pulses and study the energy dependence of the arrival time for each pulse in the ionisation train. An optimization condition for the laser pulse shape to generate narrow ionisation electron pulse in the train is obtained. The ionisation rates with various excitation energy are calculated also, which show the excitation to higher N Rydberg states will narrow the electron pulse as well.     

Comparison of linear and nonlinear extreme wave statistics

An extremely large (“freak”) wave is a typical though rare phenomenon observed in the sea. Special theories (for example, the modulation instability theory) were developed to explain mechanics and appearance of freak waves as a result of nonlinear wave-wave interactions. In this paper, it is demonstrated that the freak wave appearance can be also explained by superposition of linear modes with the realistic spectrum. The integral probability of trough-to-crest waves is calculated by two methods: the first one is based on the results of the numerical simulation of a wave field evolution performed with one-dimensional and two-dimensional nonlinear models. The second method is based on calculation of the same probability over the ensembles of wave fields constructed as a superposition of linear waves with random phases and the spectrum similar to that used in the nonlinear simulations. It is shown that the integral probabilities for nonlinear and linear cases are of the same order of values     

无人机及三维激光扫描仪技术在沙滩剖面动态监测中的应用及测量精度分析

沙滩剖面动态监测是沙滩整治修复工程效果评价的基础性工作,同时也是工程实践中及时发现问题并提出解决方案的关键步骤。由于常规测量手段存在工作效率低、周期长、难以全面覆盖测区等局限性,新型技术在沙滩剖面动态监测中的应用研究显得尤为重要。本研究以厦门天泉湾沙滩养护工程为案例,通过无人机航拍技术和三维激光扫描仪获取沙滩数字正射影像和高程数据,与人工测量结果进行比对和统计分析,研究两种技术的应用价值。研究结果表明,无人机平面坐标精度较高,但高程精度有所偏离;三维激光扫描仪测量数据满足沙滩剖面地形测量精度要求,但易受视场角限制,结果存在扫描噪点和盲区。因此,采用无人机及三维激光扫描仪技术,结合传统测量手段进行沙滩剖面动态监测,在我国广泛分布的沙滩养护工程实践中,具有良好的应用价值。     

利用单飞秒激光脉冲驱动类氖钛X射线激光的研究

提出了一种利用单飞秒激光脉冲驱动类氖钛X射线激光的物理方案.利用自相似方法研究了不同脉冲前沿的单飞秒激光辐照钛平板靶产生的类氖钛X射线激光等离子体的特性,得到了电子温度、电子密度和定标长度三者的定标律,讨论了给定输入参数下各定标律曲线的特性.研究表明,利用单个飞秒激光能够实现X射线激光的产生,而且脉冲前沿强度随时间增长平缓的飞秒激光有利于驱动X射线激光.本研究为实验上实现单飞秒激光脉冲驱动X射线激光提供了一种新的方案.     

Dynamic Behavior of Beam–Pile Structure Under Vertical Transient Excitation

The dynamic response of beam–pile–soil system under vertical transient excitation is investigated. Both piles and beam are assumed to be one-dimensional rods and subjected to vertical exciting forces. The uniformly distributed Voigt models are introduced to simulate the pile tip resistances, and the dynamic interactions between piles and beam are simplified as a set of concentrated point loads. Then, the plane strain model, the theory of longitudinal vibration of one-dimensional rod, and the Timoshenko beam theory are used to establish the mathematical models for the motion of soil, piles, and beam, respectively. On this basis, the matrix equation for solving the governing equations is constructed in the Laplace domain and the time-domain response is then obtained by the discrete inverse Fourier transform. Comparisons with numerical simulations and model tests are conducted to evaluate the rationality of the present solution. The results show that the dynamic responses calculated by the proposed solution are generally consistent with simulated curves and experimental data.     

Brillouin scattering study on elastic and piezoelectric properties of laser irradiated ZnO single crystals

Brillouin light scattering technique can be successfully used to determine the whole set of elastic and piezoelectric constants of a ZnO single crystal irradiated by different laser energy densities, into a micron range (radiation layer thickness). It is found that the scattering intensity, the linewidth and the Brillouin scattering shift of acoustic phonons are all strongly dependent on laser energy density. Based on the sound propagation equations and these results, the directional dependences of the compressional and shear moduli of the irradiated ZnO sample in the (001) plane are investigated. It is found that under an appropriate laser condition, 248 nm KrF excimer laser irradiation can significantly improve the surface quality and increase the elastic properties of ZnO single crystal. This procedure has potential applications in the fabrication of ZnO-based surface acoustic wave and optic-electronic devices.