复杂大气条件对微波传播衰减的影响研究

为了进一步提高大气对微波传播衰减影响的描述精度,为微波链路遥感反演大气新应用提供理论基础,该文系统性研究了大气主要吸收气体和各种大气粒子对微波传播的衰减情况。利用ITU-R模型计算大气主要气体成分对微波的吸收衰减,然后在降水粒子、云雾粒子和沙尘粒子的介电模型、形状、相态和谱分布的基础上,计算得到群粒子对微波的衰减特性,系统讨论降水强度、相态、含水量、谱分布、气压和温度等因素对微波传播衰减的影响。数值模拟结果表明,大气主要气体成分在60 GHz, 180 GHz和320 GHz附近存在强烈的吸收带,其衰减系数与水蒸气含量和气压呈正相关,与温度呈负相关;降水强度、谱分布、相态以及冰水比例对降水的微波衰减存在不同程度的影响,云雾的含水量和相态,沙尘的数密度、谱分布和含水量是影响微波衰减的主要因素,而温度的影响较小;大气各因素的衰减系数从大到小依次为爆炸沙尘、降水、气体吸收、水雾、冰雾和自然沙尘。     

降水对Ku/Ka频段星地链路衰减特性的影响研究

近年来,Ku和Ka卫星在广播、通信和军事等领域应用数量越来越多,传统的Ku和Ka波段星地链路降水衰减预报模型基于经验关系和理想假设,考虑降水微物理特性不足,针对此问题,在实测降水粒子微物理特征的基础上定量分析了降水垂直分布、粒子形状、粒子取向、粒子相态等对Ku和Ka频段信号衰减的影响特性.结果表明,与考虑降水非均匀垂直分布的计算结果相比,ITU和SAM模型是基于降水分布均匀的假设,无法代表降水垂直分布不均匀时的衰减情况;降水粒子形状和取向对衰减的影响较小,在13GHz和32GHz频段,球形和非球形粒子衰减值的平均绝对偏差均在0.01dB以下,不同粒子取向时衰减系数的平均绝对偏差最大值为0.00098dB/km和0.0207dB/km;不同相态的降水引起的衰减差异较大,衰减值从大到小依次是湿雪、水和冰.研究结果可以为Ku和Ka波段星地链路传播特性评估及降雨反演新方法提供基本的理论支撑和数据参考.     

地基35 GHz测云雷达回波强度的衰减订正研究

为了订正大气和云对毫米波雷达信号的衰减,选取了层状云、降水性层状云及对流云个例,进行了衰减订正试验.根据雷达反射率因子Z与衰减系数k的经验关系,采用分级逐库订正法进行订正.结果表明,对于回波强度比较弱的层状云来说,订正后数据可靠性较高; 对于降水回波来说,雨水及传播路径中的丰富水汽都会造成毫米波信号的严重衰减,因此降水云的衰减订正难度大.经过订正后,虽然还存在误差,但对毫米波雷达数据的质量有明显改善.     

Ka及毫米波段大气传输衰减特性研究

由于Ka及毫米波频段电磁波频率极高,大气传播特性不同于低频,受大气气体、降雨等大气环境的影响非常大。本文重点考虑大气中的氧气和水蒸气对电磁波的吸收衰减作用,分别从电磁波频率、大气含水量和温度三个方面具体分析大气对Ka及毫米波频段电磁波的衰减特性,并得出相关结论。     

基于盲源分离的毫米波雷达抗雨杂波干扰研究北大核心CSCD

毫米波雷达信号在降雨环境下传播时，会产生雨杂波，对雷达目标测距精度产生较大影响。本文研究了毫米波雷达信号在降雨环境下的衰减计算方法，给出了单次散射情况下毫米波雷达的体反射率，明确了降雨环境下的雷达方程。并根据雨滴尺寸分布特性建立降雨环境下的雨滴谱三维分布模型与雷达探测范围模型，仿真分析了降雨环境下的雷达测距结果。提出了一种基于四阶累积量与正交联合近似对角化的盲源信号分离算法，对降雨环境下雷达混合接收信号进行分离，实验结果证明了该方法的有效性。     

云雾对THz波传播的衰减研究

基于云雾的粒子尺寸分布,利用Rayleigh近似和Mie理论计算了不同能见度和含水量的云雾对THz波的传播衰减.结果表明:THz波段的云雾衰减较微波、毫米波段的衰减严重得多;THz波段的云雾衰减随能见度的减小和含水量的增加而增大,对于相同能见度和含水量的云雾衰减并非随频率的提高而单调增加;同时温度对THz波段的云雾衰减影响不大,在整个THz波段并不存在明显的规律.     

大气气溶胶对激光传输衰减的影响研究

在大气气溶胶类型、谱分布的基础上,利用Mie散射理论计算气溶胶在可见光和近红外波段的散射和衰减特性,讨论分析了大气气溶胶对可见光和近红外波段激光传输衰减的影响,计算结果表明,激光传输在很大程度上受粒子的折射率和谱分布的影响;不同类型气溶胶粒子具有不同的折射率和谱分布参数,从而决定了其具有不同的散射和吸收特性;气溶胶粒子的数密度越大,其衰减和散射能力就越强,对激光传输的衰减也就越大;但前后向散射的不对称性只与气溶胶谱分布有关,与数密度无关.所得出的结论有助于准确评估大气气溶胶对激光传输的影响以及提高激光传输、通信和激光测风等应用.     

降雨环境下电磁波短距离传播衰减研究

随着无线电技术的不断革新，在降雨等复杂环境下由于电磁波传播特性变化造成对武器系统的影响逐步被重视。与以往降雨环境下的远距离电磁波传播研究不同，着重阐述了近距离传播过程中电磁波的衰减问题。通过电磁仿真软件的帮助，根据层状云降水情况建立仿真模型，模拟多个频段下电磁波短距离传播过程，并通过计算路径前后电磁波能量差值以反映出电磁波衰减情况。结果表明，在降雨环境中，短距离的电磁波传播由于雨滴的影响，存在较大的衰减，随着频率和传播距离的增加，电磁波的衰减也在增加。     

太赫兹波段信号在雾中的传输特性研究

太赫兹（THz）波提供的通信带宽和容量远大于毫米波。与可见光和红外光相比,THz脉冲的波长较长,在随机介质中传播时,不但会发生时域和空域的形变,介质中的粒子还会对入射波发生散射,这些都会使得脉冲信号发生衰减。根据Mie理论与随机离散分布粒子的波传播与散射理论,计算了THz波信号入射下雾滴粒子的消光系数,分析了不同THz波波长下,雾滴粒子消光系数随粒子尺寸的变化。结合雾滴粒子谱分布,考虑粒子群的平均体系散射特性,得到了不同波长下的平均反照率与相函数。最后分析了THz波段信号在不同能见度雾中的传输特性。结果表明:大气环境中,雾对THz波产生的吸收和衰减不容忽视,不同THz信号的水的折射率虚部的变化严重影响了THz信号在雾中的传输。     

基于非球形雨滴的降雨对激光传输衰减的影响

为了进一步确定降雨对激光在大气中传输衰减的影响,将雨滴的非球形效应考虑在内,在雨滴的近似椭球模型、归一化谱分布的基础上,利用射线追踪法计算群雨滴在可见光和近红外波段的散射和衰减特性,讨论分析雨滴谱分布和降雨强度对可见光和近红外波段激光传输衰减的影响。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;激光衰减不仅受降雨强度的影响,也与雨滴的数密度有关,数密度越大,激光衰减越大;而形状对群雨滴的影响较小。所得出的结论有助于准确评估降雨对激光传输的影响以及改善激光传输、通信和激光测风等应用。     

Scattering of radiowaves by a melting layer of precipitation inbackward and forward directions

A melting layer of precipitation is composed of melting snowflakes (snow particles); the assumption of spherical particles along with mass conservation is used. The melting layer is studied by deriving the size distribution of the melting snow particles, the thickness of a melting layer, the density of a dry snow particle, and the average dielectric constant of a melting snow particle. Vertical profiles of radar reflectivity and specific attenuation are computed at 1-100 GHz by using the Mie theory for five raindrop size distributions at rain rates below 12.5 mm/h. The radar bright band is explained with computed radar reflectivities at 3-10 GHz. It is shown that the radar bright band can be absent in the melting layer at frequencies above 20 GHz. This agrees with radar observations at 35 and 94 GHz. The specific attenuation, as well as the average specific attenuation of the melting layer, is divided into absorption part and scattering part. The latter is increasingly significant with the increase of frequency. The total zenith attenuation due to stratiform rain is divided into the rain zenith attenuation and the additional zenith attenuation, which is the difference between zenith attenuation, due to the melting layer, and attenuation, due to the same path length of the resulting rain. The additional zenith attenuation increases with the increase of rain rate even at frequencies above 20 GHz. This should be taken into account in radar remote sensing and satellite-Earth communications     

Monte Carlo simulation for millimeter wave propagation and scattering in rain medium

As the operating frequencies of communications systems more higher into the millimeter wave range, the effects of multiple scattering in precipitation media become more significant. This paper treats the problems of electromagnetic multiple scattering in rain medium by the Monte Carlo method. The em wave is regarded as a Markov chain of photon collisions in a medium in which it is scattered and absorbed. For the sake of simplicity, the polarization is not taken into account, the above mentioned problems are described by the scale integro-diffierential equation of transfer. When the plane wave through a random medium with particle size distribution, the technique of weighted average is used to characterize the radiation intensity, including average scattering, absorption coefficients and phase function. The Monte Carlo simulation algorithms are done for the rain attenuation and reflectance at millimeter wavelength region. Our computational results are in good agreement with experimental data of rain attenuation.     

A New Prediction Model of Rain Attenuation That Separately Accounts for Stratiform and Convective Rain

An improved version of the exponential cell (EXCELL) rain attenuation model is presented here. Analogously to the original one, it predicts attenuation through a cellular representation of precipitation, but, in addition, is able to discriminate between stratiform and convective rain by means of an embedded algorithm. Accordingly, two separate physical rain heights, derived from the ERA-15 database, are used to calculate stratiform and convective rain attenuation and, when considering stratiform precipitation, the melting layer contribution to attenuation is added. Eventually, the predicted cumulative distribution function (CDF) of excess attenuation is the combination of the contributions due to stratiform and convective precipitation types. Some input parameters of the prediction model, such as those defining the melting layer process or the rain plateau embedding rain cells, can be modified in order to account for the local meteorological characteristics.      

Inference of raindrop size distribution from attenuation and rain rate measurements

A method to infer the raindrop size distribution from the measurements of point rain rate and rain attenuation at a frequency in centimeter or millimeter wave bands is proposed. This method is applied to the results of field propagation experiments at 81.8, 34.5, and 11.5 GHz through natural rain. It is shown that, if an appropriate frequency is employed, this method is effective as a frequency-scaling method for short terrestrial propagation paths.     

Measurements of cloud contribution to rain millimeter wave attenuation by using a radar and radiometers

The measurement technique of cloud contribution to rain attenuation and the equipment consisted of the coherent pulse Doppler radar at wavelength λ=3.2 cm, the radiometers at λ=0.4; 0.8 and 1.35 cm and apparatus for signal recording and processing is described. The results of such measurements are given. The Doppler spectrum of the rain backscattered radar signal was used for determination of rain drop size distribution height profile then rain attenuation was calculated and cloud attenuation was determined as the difference between the total attenuation measured by using the radiometers and rain attenuation. The results of this work gave possibility to improve the known rain model of P.Misme for prediction of rain attenuation statistics for Earth-satellite links at millimeter wave.     

A dual-polarization rain profiling algorithm

A new attenuation correction algorithm based on profiles of reflectivity, differential reflectivity, and differential propagation phase shift is presented. A solution for specific attenuation retrieval in rain medium is proposed, which solves the integral equations for reflectivity and differential reflectivity with cumulative differential propagation phase shift constraint. The conventional rain profiling algorithms that connect reflectivity and specific attenuation can retrieve specific attenuation values along the radar path assuming a constant intercept parameter of the normalized drop size distribution. However, in convective storms, the drop size distribution parameters can have significant variation along the path. This paper presents a dual-polarization rain profiling algorithm for horizontal looking radars incorporating reflectivity as well as differential reflectivity profiles. The dual-polarization rain profiling algorithm has been evaluated with X-band radar observations simulated from drop size distribution derived from high-resolution S-band measurements collected by the Colorado Statue University CHILL radar. The analysis shows that the retrieved specific attenuation, differential attenuation, reflectivity, and differential reflectivity from the dual-polarization rain profiling algorithm provide significant improvement over the current algorithms.     

非球形冰晶在94/220 GHz毫米波的散射特性模拟计算

针对94/220 GHz双频雷达的数据处理,分析了不同形状冰晶对这两个波段的单散射特性及衰减特性,探讨了单形状冰云及具体冰云模型的回波特性,结果表明:1)当冰晶较大时,冰晶的后向散射及衰减对冰晶形状较敏感,相同最大尺度下,六角形冰晶后向散射及衰减最大、子弹花次之、雪花最小;2)单形状冰晶云的雷达反射率因子对冰晶形状、冰水含量、滴谱的中值尺度较敏感,同样滴谱条件下,220 GHz的衰减系数约是94 GHz的5~25倍;3)具体冰云模型的雷达反射率因子随粒子浓度、冰水含量、中值尺度增加而增加,对粒子谱的形状参数敏感性较低.     

A simplified approach to the evaluation of EMW propagationcharacteristics in rain and melting snow

Electromagnetic propagation characteristics in rain and melting snow are calculated by treating these media as artificial dielectrics. Computed values of attenuation and phase shift in rain, obtained by this approach, are compared with those derived by using Mie scattering theory over a frequency range of 1-1000 GHz and for rain rates up to 100 mm/h. Very close agreement is generally obtained over these entire ranges. Melting snow is treated in the same manner, where comparison is possible these results tend to agree well with the available, but rather limited, published data. Attenuation and phase shift are calculated as a function of the degree of melting of the snow particles. Subject to assumptions relating the degree of melting to depth in the melting layer, average values of attenuation and phase shift are computed as functions of frequency. The attenuation values compare well with those derived from an empirical formula over the range of its validity