T矩阵散射模拟双偏振测雨雷达偏振及衰减特性

通过T矩阵散射模拟,研究X、C和S波段双偏振测雨雷达在单分散雨滴谱、不同雨强和不同最大雨滴直径下的偏振和雨区衰减特性,分别建立三种不同波段双偏振雷达反射率因子和差分反射率的雨区衰减订正关系.最后,用实测雨滴谱数据和X波段双偏振雷达实测反射率数据验证散射模拟结果的准确性.     

降雨对微波传输特性的影响分析

在归一化雨滴谱分布、雨滴的介电模型和Mie散射理论的基础上计算群雨滴在微波波段的散射和衰减特性,讨论分析雨滴谱分布、降雨强度、入射波频率和温度等因素对微波传输特性的影响.数值模拟结果表明,不同谱分布的群雨滴散射能力从大到小依次为JD,MP,Gamma和JT分布,降雨强度对微波传输特性的影响最大,入射波频率次之,温度的影响最小.所得出的结论有助于准确评估降雨对微波传输的影响以及提高利用测雨雷达和毫米波雷达等定量探测降水的精度. 关键词： 微波传输 雨滴谱分布 Mie散射 散射衰减特性     

降雨对532nm和1064nm激光传输的衰减特性研究

降雨会对激光信号产生严重的衰减,从而给激光目标探测的应用带来一定影响.激光在降雨中的传输衰减已在红外波段做了大量的实验研究,而可见光波段激光在雨中的传输衰减特性还未见报道.基于夫琅禾费衍射和几何光学散射理论,建立雨滴对532 nm绿激光和1064 nm近红外激光光束的传输衰减模型,对比分析两波长激光在不同降雨量下的衰减...     

降雨对激光定距系统后向散射特性的影响

由于激光光束在降雨中传输时会产生雨滴的后向散射,引起定距系统的虚警、失效等.以几何光学散射理论为基础,建立雨滴后向散射模型并计算不同降雨条件下的后向散射系数,结合工程应用计算不同降雨条件和传输距离下的系统信噪比,分析雨滴后向散射对系统信噪比的影响.计算结果表明,传输距离一定时,系统信噪比将随着降雨率的增大而减小,暴雨中衰减率高达11%;在相同降雨条件下,作用距离从10m增大至15m,信噪比衰减率达到46%以上.降雨大小和作用距离是影响系统信噪比的重要因素,为脉冲激光定距系统在降雨环境下的目标识别以及高抗干扰性能的优化提供了依据.     

C波段机载合成孔径雷达海面风场反演新方法

针对基于散射计地球物理模型函数的机载合成孔径雷达(SAR)海面风场反演中存在的风向获取依赖于图像风条纹或数值预报、 散射计数据和浮标等背景场资料, 风向与SAR图像时空分辨率不匹配, 进而影响机载SAR海面风场反演精度等问题, 本文根据机载SAR对海探测特点, 研究一种适用于C波段机载SAR的海面风场反演新方法. 利用SAR图像距离向不同入射角的后向散射系数, 依据地球物理模型构造最小代价函数, 通过代价函数的求解直接从机载SAR数据同时反演出海面风速和风向. 利用论文提出的海面风场反演方法分别对仿真SAR数据和实测C波段机载SAR数据进行风向、 风速的反演误差分析及试验验证.研究结果表明, 该方法适用于机载SAR海面风场反演, 可不依赖背景风向直接反演出精度较高的风速和风向; 雷达后向散射系数误差是决定海面风速、风向反演精度的关键因素, 辐射定标精度越高则反演误差越小; 海面风速反演误差随着风速的提高而增大, 当海面风速大于18 m/s时, 风速反演误差显著增加, 而海面风向的反演误差与风速无明显关系. 关键词： 机载合成孔径雷达 海面风场 多入射角     

降雨粒子的无线紫外光散射特性

紫外光与降雨粒子相互作用发生散射,散射光特性改变能够反映降雨粒子的相关物理特性（如粒子尺寸参数、浓度、形态）,因此研究粒子的物理参数对散射光特性的影响对有效提高光谱法定量探测降水的精度有很大意义。由于雨滴在非球形降水粒子中具有代表性,以群雨滴粒子为例,采用T矩阵理论,利用紫外光直视和非直视单次散射模型,分析了入射光波长、群雨滴粒子形态、降雨强度、粒径大小与散射光强之间的关系。并用蒙特卡洛方法仿真分析了非球形群雨滴粒子在不同降雨强度和粒径下散射角与散射光强之间的关系,以及降雨环境中的风切变对紫外光散射特性的影响。通过理论及仿真分析,得到了不同群雨滴粒子形态下的路径损耗,不同降雨强度、风切变率和粒径下的散射光强分布。仿真结果表明：在紫外光直视与非直视通信方式下,降雨环境中的通信质量比晴天条件下的通信质量差,即路径损耗增大。当粒径分布已知时,随着降雨强度的增大,衰减系数增大,路径损耗增加,且直视通信方式的路径损耗比非直视降低7 dB左右。随着降雨强度、风切变率和粒子粒径的增大,散射光强曲线整体呈下降趋势,其中,降雨强度的变化对散射光强分布影响程度最大。相同通信距离时,不同降雨强度下的紫外光散射光强分布均随着散射角的增大而减小,当散射角继续增大到90°时,有效散射体体积逐渐减小,接收到的光子能量减小,暴雨中的散射光强衰减程度最大。相同降雨强度下考虑风切变时,相比较无风时的路径损耗增大5 dB左右。除此之外,还研究了椭球形和切比雪夫形粒子对紫外光散射光强的影响,结果表明当粒子粒径分布相同时,椭球形粒子的散射光强衰减较广义切比雪夫形粒子大。根据散射粒子的散射光强分布以及路径损耗能够区分雨滴粒子是否由相同粒径及形态组成,为粒子测量提供理论基础。分析降水中群雨滴粒子的光散射特性,为提高光谱法评估降水衰减的数值模拟方面提供理论依据,为光学技术在探测识别降水现象等气象领域的广泛应用提供了设计参考。     

基于粒子成像测速技术的雨滴微物理特性研究

为了获取自然环境条件下的雨滴形状、尺度、速度、取向等微物理特性,基于粒子成像测速的瞬态测量可视化技术,研究基于点扩散函数的图像复原算法和自相关图像的速度提取算法,设计并研制了一种基于面阵电荷耦合元件的自然降水成像测速系统.通过外场观测试验研究了雨滴的直径、速度、轴比和空间取向及其谱分布等微观物理特性;雨滴的速度随直径的增大呈指数增大,轴比随直径的增大呈线性减小,空间取向易受环境风的影响,倾斜角的平均值在2.1?左右,标准偏差为11.5?;基于实测数据拟合了雨滴速度和轴比随直径的变化关系.与国外已有模型相比,本文进一步得出了具有本地化特征的雨滴微物理特性.所得到的结论对于云降水物理学研究、天气雷达地面定标、雨致微波衰减评估等方面的应用具有重要的意义.     

非视线紫外通信大气传输特性的蒙特卡罗模拟

基于蒙特卡罗方法建立了紫外光非视线传输多次散射模型,利用单次散射近似法和实验方法验证了模型的有效性,并利用该模型完成了非视线紫外光通信大气传输特性的模拟.模拟时光波长取紫外光通信的最佳工作波段(250 nm附近),分析了不同传输距离下能见度、风、雨、雾等参量对系统能量透射比的影响.结果指出,系统能量透射比随传输距离增大而剧烈减小,在天气较差传输条件下能量衰减得更快；风力大小的变化对通信系统影响不大.较近距离通信传输时(一两百米),通信系统受天气条件的影响较小.     

雨介质中球形目标的GHz复合散射特性

将背景空间及其中存在的离散粒子作为一种连续的电介质,得出了背景空间中离散粒子存在时混合体的等效介电常量模型.利用该模型和国际电讯联盟给出的雨衰减测量数据,确定了GHz波段雨介质的等效介电常量并验证了其有效性.得出了雨环境中目标复合散射场的解析式,对雨环境中球形目标的复合微分散射进行了仿真与分析,进而研究了降雨率、电磁波频率及极化状态等因素对复合微分散射的影响.计算表明:降雨对微分散射的影响在10^-3分贝上,电磁波垂直极化时降雨的影响大于其水平极化时的影响.研究结果对精确制导和目标识别等有一定的参考意义.     

降雨对无线激光通信的影响

通过激光信号在雨介质中传输的实验发现,在大雨环境下,激光信号透射率要比在小雨或微雨的情况下大。这说明光信号在小雨时衰减较大,在大雨时衰减较小。这一现象与人们通常所认为的有所不同。针对降雨对激光信号的影响,根据Mie散射理论和Weibull雨滴尺寸分布模型,分析了不同尺寸的粒子对光的散射作用以及对衰减效率因子的影响,推导出单球粒子对光波的衰减公式,得到了衰减与降雨率的确定关系。通过数值计算发现,与小粒子相比大粒子的前向散射光强更大且更加集中;激光信号在雨介质中传输时,衰减系数在小雨时较大,中雨、大雨时较小,暴雨时不断增大。这一结果与实际情况较吻合,为激光在通信系统中的应用提供了一定的理论依据。     

基于微波链路的降雨场反演方法研究

本文基于微波雨衰的幂律关系, 研究了使用微波链路反演降雨场的方法, 采用层析技术建立了降雨场反演模型. 并利用SIRT算法与正则化算法实现对降雨场层析反演模型的求解. 数值模拟结果表明, 该模型与反演算法能够较为准确地重建降雨场强度与空间分布特征, 能够提供高时空分辨率的二维降雨强度分布. 因此, 利用微波衰减数据进行降雨探测可以作为常规的雨量计与天气雷达观测手段的有效补充. 关键词： 微波雨衰 微波链路 降雨场重建 层析反演     

Polarization ratio characteristics of electromagnetic scattering from sea ice in polar areas

In the global climate system, the polar regions are sensitive indicators of climate change, in which sea ice plays an important role. Satellite remote sensing is a significant tool for monitoring sea ice. The use of synthetic aperture radar(SAR) images to distinguish sea ice from sea water is one of the current research hotspots in this topic. To distinguish sea ice from the open sea, the polarization ratio characteristics of sea ice and sea water are studied for L-band and C-band radars, based on an electromagnetic scattering model of sea ice derived from the integral equation method(IEM) and the radiative transfer(RT) model. Numerical experiments are carried out based on the model and the results are given as follows. For L-band, the polarization ratio for sea water depends only on the incident angle, while the polarization ratio for sea ice is related to the incident angle and the ice thickness. For C-band, the sea water polarization ratio is influenced by the incident angle and the root mean square(RMS) height of the sea surface. For C-band, for small to medium incident angles,the polarization ratio for bare sea ice is mainly determined by the incident angle and ice thickness. When the incident angle increases, the RMS height will also affect the polarization ratio for bare sea ice. If snow covers the sea ice, then the polarization ratio for sea ice decreases and is affected by the RMS height of snow surface, snow thickness, volume fraction and the radius of scatterers. The results show that the sea ice and the open sea can be distinguished by using either L-band or C-band radar according to their polarization ratio difference. However, the ability of L-band to make this differentiation is higher than that of C-band.     

Statistical Prediction Modeling of Rain Induced Attenuation and Depolarization

Millimeter wave rain co-polarization attenuation, CPA, and cross polarization discrimination, XPD, measurements have been made at 35GHz and 94GHz over a line-of-sight link. On the basis of these experimental results, a study of this rain medium has been made with the method of link measurements. In this paper, we presented a statistical prediction modeling of rain-induced attenuation and depolarization from the statistical distribution of the rain intensity, and the size, and canting angle of raindrops. Our computational results are in good agreement with data of measurements.     

A three-dimensional radiative transfer model to interpret ranging scatterometer measurements from a pine forest

This paper addresses the propagation of microwaves in forested media. It interprets the vertical distribution of backscatter acquired by a ranging scatterometer in a pine forest at X band (9.5 GHz) and vertical incidence by means of modelling. A plane parallel radiative transfer model for the forest canopy exhibits a large discrepancy between the measured and estimated distribution. This is attributed to failure to represent the horizontal heterogeneity in the forest. Taking into account the height variability of the trees significantly improves the estimated distribution at the top of the canopy but the backscatter from the lower layers and the ground is still underestimated. To deal with this, a fully three-dimensional (3D) radiative transfer model is developed. By modelling precisely the propagation of the wave through the medium, the 3D approach allows accurate estimation of the ground backscatter and slightly improves the estimated backscatter from the lower layers. It is demonstrated that 3D modelling is required to estimate accurately the vertical distribution of backscatter, total backscatter and the height of the centre of backscatter in the experimental data. These results have implications for the interpretation of more general measurements scenarios, especially as regards forest height measurement by radar interferometry.     

基于支持向量机的微波链路雨强反演方法

为提高微波链路雨致衰减反演雨强精度, 在Mie散射理论、气体吸收衰减模型以及Gamma雨滴谱分布的基础上, 将支持向量机引入到微波链路测量降水中, 提出了基于支持向量机的微波链路雨强反演方法, 并开展了15–20 GHz频段的视距微波链路与地面雨滴谱仪的同步观测降雨实验. 实验结果表明, 基于支持向量机的微波链路雨强反演模型的反演雨强与实测雨强的相关系数全部高于0.6, 最高达到0.9674; 雨强的均方根误差最小值为0.5780 mm/h, 累积降雨量的绝对最小误差仅为0.0080 mm; 相对偏差大部分在10%以内, 最小偏差为0.7425%. 实验结果验证了基于支持向量机的微波链路雨强反演方法的有效性、准确性和适用性, 对于进一步提高微波链路反演降雨精度、改善降水监测效果具有重要意义.     

A three-dimensional radiative transfer model to interpret ranging scatterometer measurements from a pine forest

Abstract

This paper addresses the propagation of microwaves in forested media. It interprets the vertical distribution of backscatter acquired by a ranging scatterometer in a pine forest at X band (9.5 GHz) and vertical incidence by means of modelling. A plane parallel radiative transfer model for the forest canopy exhibits a large discrepancy between the measured and estimated distribution. This is attributed to failure to represent the horizontal heterogeneity in the forest. Taking into account the height variability of the trees significantly improves the estimated distribution at the top of the canopy but the backscatter from the lower layers and the ground is still underestimated. To deal with this, a fully three-dimensional (3D) radiative transfer model is developed. By modelling precisely the propagation of the wave through the medium, the 3D approach allows accurate estimation of the ground backscatter and slightly improves the estimated backscatter from the lower layers. It is demonstrated that 3D modelling is required to estimate accurately the vertical distribution of backscatter, total backscatter and the height of the centre of backscatter in the experimental data. These results have implications for the interpretation of more general measurements scenarios, especially as regards forest height measurement by radar interferometry.