基于数值方法的激光海面漫反射通信能量分布特性

 针对激光点对点通信方式的不足,提出了利用海面作为激光漫反射媒介进行组网通信的设想,采用前后向迭代的数值方法结合Green函数谱积分加速算法(FBM/SAA)对激光海面漫反射通信的能量分布特性进行了研究。通过对激光光束入射海面后产生的散射场的分析、计算及实验验证,得出了较为准确的2维激光海面双站散射系数,并对激光光束入射海面后的散射场进行了分析。研究结果表明：入射激光束经过粗糙海面散射后,能量大部分集中在前向散射区域上,而后向散射强度很弱,且在散射场的边缘处能量迅速衰减,说明了激光海面漫反射组网通信方法的可行性。通过与前后向迭代法和Kirchhoff近似方法的计算结果的比较,说明了FBM/SAA是一种高效、准确的计算方法。     

基于前后向迭代法的3维标量激光漫反射特性

 针对激光点对点通信方式的不足,提出了利用海面作为激光漫反射媒介进行组网通信的设想,并对激光入射理想导体表面后产生的散射场进行了的研究。首先利用3维标量锥形波对入射激光束进行了模拟;然后列出了积分方程,并根据矩量法进行了离散;最后通过前后向迭代法求解矩阵方程,得出了3维散射系数,准确地表示了基于理想导体界面的3维标量激光漫反射特性。     

多平台激光通信组网共焦点反射式光学天线设计

目前国际上点对点激光通信技术已较为成熟,但多平台间组网通信问题仍未能解决。提出了一种新型组网通信光学天线设计方法,可实现多点间同步双工激光通信。分析了光学天线原理,并通过仿真建模验证了设计理论,给出了光学天线的结构设计和工作模式,最后采用数学最优控制算法推导出多组光学天线多平台动态、双工通信的工作范围,当旋转抛物面开口为300 mm时,能使方位角为360°,俯仰范围为90°以上。并针对GEO(高轨道卫星)—GEO—OGS(光学地面站)间的组网通信要求进行了光学天线的初始化设计,对推动激光通信技术实现大范围宽领域应用具有重要意义。     

覆盖泡沫粗糙海面的激光散射特性研究

对潜通信及对潜探测中蓝绿激光将穿透海水表面,大气/海水界面由于风速及其他外力作用而呈现复杂状况,将影响蓝绿激光在界面的传输特性。针对泡沫-海面复合模型,采用矢量辐射传输理论、米氏理论以及粗糙面散射理论对泡沫海面的散射特性进行了分析。采用基尔霍夫近似针对JONSWAP海谱,讨论了泡沫海面在蓝绿激光波段的后向散射系数与入射角、风速、风区以及海水温度、含盐度等参量的关系。结果表明:随着海面上方风速、风区及介电常数的增大,泡沫层对海面的激光散射具有相当大的影响,尤其是在入射角度较大的状态下。     

海面冰层对声波的反射和散射特性

北极海面冰层复杂多变,其对声波的反射和散射严重影响冰下水声信道的传输特性,建立海面冰层的声波反射和散射模型对冰下水声通信研究具有重要意义.假设海面冰层为多层固体弹性介质且冰-水界面粗糙,满足微扰边界条件,导出声波从海水介质入射到海面冰层时相干反射系数满足的线性方程组.对相干反射系数随声波频率、掠射角、冰层厚度的变化进行数值分析.进一步引入根据散射声场功率谱密度计算散射系数的方法,改变掠射角,对冰层厚度、散射掠角对散射系数的影响进行研究.     

计算海面激光反射特性的几何光学方法研究

研究了计算海面激光反射的几何光学方法。首先根据JONSWAP非稳态海谱模型数值模拟出二维随机粗糙海面,再采用几何光学方法对入射激光光束在海面上的反射光进行建模,最后计算出海面激光光斑反射光强的空间分布。编制了相关的计算机程序,分析了在不同入射角度和不同风速下的激光光束反射特性。计算结果表明该方法对研究二维随机粗糙海表面激光反射特性有一定参考价值。     

基于文氏改进谱的二维粗糙海面模型及其电磁散射研究

基于文氏改进功率谱和Donelan方向分布的实验结论,结合经典的Monte Carlo法提出了一种适用于不同水深、不同风浪成长阶段的二维粗糙海面模型.在经典的双尺度法计算海面后向散射的基础上,结合小斜率近似法的仿真结果,对电磁散射计算方法进行改进.通过与典型海杂波均值散射系数GIT模型的拟合结果进行对比,验证了文氏改进谱在中国近海海面模拟的适应性与改进后电磁散射计算方法的有效性.     

含泡沫面元模型的海面电磁散射研究

传统计算海面电磁散射的方法都是通过求集平均的方法来统计得到不同输入参数下海面总回波系数的均值曲线,并不需要具体的几何样本.随着合成孔径雷达以及雷达成像的发展,为了充分描述海面各点的分布特征,需要得到海面具体面元的散射结果,同时随着海面上方风速的增大,海面泡沫层的出现会对散射结果产生相当大的影响.本文采用海面模型面元化的思想,将海面散射的贡献面元化,同时考虑泡沫层对大入射角下散射结果的影响,计算了不同风速下海面的后向散射系数,并与实测数据做对比分析,验证了方法的准确性.     

基于海面散射模型的全极化合成孔径雷达海洋环境探测关键技术参数设计仿真研究

技术参数设计研究是提升全极化SAR海洋环境探测能力的有效途径. 本文通过分析噪声等效后向散射系数和辐射分辨率与海洋环境探测的地球物理联系,提出一种全极化SAR关键技术参数设计方法. 采用全极化海面散射模型计算不同海洋环境条件下的雷达后向散射系数,据此确定全极化SAR对海探测的噪声等效后向散射系数,再将其作为合成孔径雷达方程输入参数,以辐射分辨率和信噪比的函数关系为约束条件,开展信噪比、辐射分辨率和系统功率孔径积等技术参数设计研究. 论文通过全极化海面散射模型的仿真计算,发现对海探测的噪声等效后向散射系数设计为-35.0 dB,可满足全极化SAR不同海洋环境条件的探测需求. 通过研究辐射分辨率与信噪比的函数关系,发现对海探测的最优信噪比为8.0 dB. C波段机载全极化SAR设计结果表明,由于兼顾了海洋环境探测需求,本文设计方法使得全极化SAR技术参数同时满足了海洋环境应用需求和系统设计要求. 关键词： 海面散射模型 全极化合成孔径雷达 海洋环境探测 技术参数设计     

海面与其上方双矩形截面柱复合散射的混合算法研究

采用Pierson-Moscowitz(PM)海谱和Monte Carlo方法模拟实际的粗糙海面, 基于矩量法和基尔霍夫近似的混合算法研究了海面与其上方双矩形截面导体柱的复合电磁散射特性, 得出了复合散射系数的角分布曲线, 计算了复合散射系数随海面参数、矩形截面柱参数以及入射波参数的变化情况, 并做了详细分析与讨论, 得到了PM谱海面与其上方双矩形截面柱复合散射特性. 结果表明, 与单纯的矩量法相比较, 采用基于矩量法和基尔霍夫近似的混合算法, 既可获得较高的准确性, 同时又可减少计算时间和内存占用量, 而且粗糙面尺度越大该优势越明显.     

Bistatic scattering coefficient from one- and two-dimensional random surfaces using the stationary phase and scalar approximation with shadowing effect: comparisons with experiments and application to the sea surface

In this paper, the bistatic scattering coefficient from one- and two-dimensional random surfaces using the stationary phase method and scalar approximation with shadowing effect is investigated. Both of these approaches use the Kirchhoff integral. With the stationary phase, the bistatic cross section is formulated in terms of the surface height joint characteristic function where the shadowing effect is investigated. In the case of the scalar approximation, the scattering function is computed from the previous characteristic function and in terms of expected values for the integrations over the slopes, where the shadowing effect is analysed analytically. Both of these formulations are compared with experimental data obtained from a Gaussian one-dimensional randomly rough perfectly-conducting surface. With the stationary-phase method, the results are applied to a two-dimensional sea surface.     

Bistatic scattering coefficient from one- and two-dimensional random surfaces using the stationary phase and scalar approximation with shadowing effect: comparisons with experiments and application to the sea surface

Abstract

In this paper, the bistatic scattering coefficient from one- and two-dimensional random surfaces using the stationary phase method and scalar approximation with shadowing effect is investigated. Both of these approaches use the Kirchhoff integral. With the stationary phase, the bistatic cross section is formulated in terms of the surface height joint characteristic function where the shadowing effect is investigated. In the case of the scalar approximation, the scattering function is computed from the previous characteristic function and in terms of expected values for the integrations over the slopes, where the shadowing effect is analysed analytically. Both of these formulations are compared with experimental data obtained from a Gaussian one-dimensional randomly rough perfectly-conducting surface. With the stationary-phase method, the results are applied to a two-dimensional sea surface.     

风驱粗糙海面背景下海雾对可见和红外光多次散射特性研究

通过测量空间辐射反演海雾气溶胶的微观特性是一种重要的遥感方法,但是海雾对辐射的反射会受到海面背景的影响。该工作研究了海雾与海面的耦合多次散射。利用Mie理论研究了海雾气溶胶的单次散射特性,利用辐射传输理论研究太阳光在无下垫面海雾中传输多次散射特性;在基尔霍夫近似下研究了风驱粗糙海面的散射特性,并考虑了海面的遮蔽效应,得到了风驱海面随风速的反射函数。根据累加法研究了海雾和海面的耦合散射,计算结果表明海雾在有粗糙海面作为背景的情况下,其整体反射有较大增强。     

Anomalous sea surface reverberation scale model experiments

Low frequency sea surface sound backscattering from approximately 100 Hz to a few kHz observed from the 1960s broadband measurements using explosive charges to the Critical Sea Test measurements conducted in the 1990 s is substantially higher than explained by rough sea surface scattering theory. Alternative theories for explaining this difference range from scattering by bubble plumes/clouds formed by breaking waves to stochastic scattering from fluctuating bubble layers near the sea surface. In each case, theories focus on reverberation in the absence of the large-scale surface wave height fluctuations that are characteristic of a sea that produces bubble clouds and plumes. At shallow grazing angles, shadowing of bubble plumes and clouds caused by surface wave height fluctuations may induce first order changes in the backscattered signal strength. To understand the magnitude of shadowing effects under controlled and repeatable conditions, scale model experiments were performed in a 3 m x 1.5 m x 1.5 m tank at the Technical University of Denmark. The experiments used a 1 MHz transducer as the source and receiver, a computer controlled data acquisition system, a scale model target, and a surface wave generator. The scattered signal strength fluctuations observed at shallow angles are characteristic of the predicted ocean environment. These experiments demonstrate that shadowing has a first order impact on bubble plume and cloud scattering strength and emphasize the usefulness of model scale experiments for studying underwater acoustic events under controlled conditions.     

Radar backscattering from Gerstner's sea surface wave

In the framework of a two-scale scattering model, radar backscattering from the rough sea surface was considered. The sea surface was modelled as a superposition of a nonlinear, large-scale Gerstner's wave and small-scale resonant Bragg scattering ripples. The zero-order diffracted field was found by a geometrical optics approach, with shadowing taken into account, and by an 'exact' solution of the diffraction problem obtained numerically. For vertical and horizontal polarizations, the spatial distribution of specific scattering cross sections along the large-scale wave was obtained. The spatially averaged specific backscattering cross sections, as well as the mean Doppler frequency shifts at both polarizations, obtained by the geometrical optics approach are compared with those obtained by using the 'exact' solution of the large-scale diffraction problem. The roles of shadowing and multiple wave scattering processes are discussed, and qualitative explanations of the difference between these two approaches are given.     

Computation of Near Sea Target Scattering from Complex Background Radiation

The background radiation contribution is an important component of target scattering characteristics. A study is made on target scattering characteristics from complex background radiation, with contributions of the sun and the sea and sky background and that among each components of target. In this paper, the shadowing function of sea surface is numerically calculated by using the Z-BUFFER shadowing elimination algorithm, and the scattering of each facet of sea surface and target from the sun and the sky background infrared radiation is computed based on the rough surface scattering model as well as the infrared self-radiation of the sea surface. Finally, a numerical calculation of the forward and backward radiance of a near-sea circular cylinder scattering from the sun, the sea and sky background infrared radiation of near sea target and the infrared self-radiation of the target skin have been made. The results show the important influences of each contribution on target scattering from the sea and sky background radiation in 3-5 m and 8-12 m bands.     

一维粗糙面散射的射线追踪法研究

在考虑遮蔽效应的情况下,应用射线追踪法对一维导体和介质的高斯粗糙面的散射系数进行了研究,并分别计算了考虑一次反射和二次反射的散射系数。同时,利用蒙特卡罗法生成一维高斯粗糙面,计算了考虑一次、二次反射和遮蔽时不同均方根斜率粗糙面的平均散射系数。