三亚Ka波段卫星遥感信号降雨衰减的研究

Ka波段卫星下行遥感信号受降雨衰减影响严重,在设计信道链路和执行遥感数据接收任务时,需要考虑Ka波段雨衰问题。基于国际电信联盟无线电通信部门(ITU-R)提供的雨衰模型,结合三亚某地面站Ka波段卫星遥感数据接收设备,利用Matlab处理时间概率0.01%的降雨量数据,模拟三亚Ka波段卫星下行信号的降雨衰减率,分析降雨衰减值随天线接收仰角的变化,为设计三亚Ka波段下行信道抗雨衰余量提供了参考。同时结合自动气象站降雨观测数据,研究不同等级降雨量时的雨衰情况,为制定三亚Ka波段卫星遥感信号应对雨衰的措施提供了依据。     

星载SAR系统设计中的雨衰估算

主要讨论了微波信号大气传播中降雨衰减的估算方法。基于我国部分地区降雨强度长期累计分布 ,计算了部分地区X波段信号传输的降雨衰耗。针对星载SAR的特点 ,分析了雨衰对SAR的影响 ,提出了星载SAR设计时雨衰的指标     

卫星链路降雨衰减的测量及频率换算

利用雨量计、频谱仪、计算机等设备,对武汉地区某时段降雨率和Ku频段卫星下行链路(12GHz)的降雨衰减进行了测量.通过数据处理,得到降雨率与降雨衰减的关系.经频率换算得到了相同条件下Ka频段卫星下行链路(20GHz)的降雨率与降雨衰减的关系.针对ITU - R预测模型的不精确性,根据频率换算值对预测模型进行修正.该换算值可为该链路Ka频段卫星通信抗雨衰方案的设计提供理论依据.     

K波段卫星通信雨衰预测问题分析

对于Ku波段卫星来说，雨衰是对通信质量与通信性能产生影响的主要原因。所以，为了提升Ku波段卫星在通信过程中的质量与性能，就需要对雨衰进行有效预测。本文便从雨衰产生、降雨噪音以及去极化现象等方面入手，对雨衰机理进行概述，并分析降低雨衰影响的主要方法。     

近地空间通信中的降雨衰减影响分析

近地空间通信中链路损耗随着传输仰角的降低而增加。在低仰角传输中,大气传输损耗由于信道复杂度的增加而衰减严重。文中基于ITU-R的降雨衰减模型,结合低仰角远距离传输,分析了近地空间通信链路中冰/雨层与纬度位置的关系,并在不同频段,不同轨道高度对降雨衰减及雨衰导致天线尺寸的变化进行数值计算和仿真。仿真结果表明：在低仰角的传输环境中,降雨对近地空间通信链路的影响较大,不可忽视。其中S频段比X和Ku波段衰减小近20dB。最后总结了传输电波的工作频率、覆盖半径以及平台高度给降雨衰减带来的影响,以此来正确估计大气传输损耗对通信链路性能的影响,为近地空间在低仰角下远距离传输的链路预算提供了一个理论参考值。     

Ku波段卫星通信雨衰预测

世界许多国家都建立了很多的国内卫星通信系统和国际卫星通信系统。由于通信的容量的逐步增大,很多业务正由以前的C波段向现在的Ka、Ku波段过渡。Ku波段卫星通信比C波段卫星通信带宽大、抗干扰性强、地球站的直径小且更容易安装,但是Ku波段的雨衰相对于C波段更大。目前Ku卫星在全国大部分地区的空管单位都有装备,用作地面链路的备份。空管单位保障民用航空器正常飞行的工作特点决定了它对数据传输的可靠性有着极高的高求。对Ku卫星雨衰的研究对实际的安全保障工作具有指导意义。文章借助几种数学模型对雨衰进行预测并加以比较分析。     

降雨衰减对移动卫星系统通信的影响及补偿

针对移动卫星系统的特点，分析了降雨引起的信号衰减的原因，给出了雨衰的计算方法。Ka波段一般用于移动通信业务，从分析结果上看该波段信号受降雨影响较大。提出几种补偿雨衰的方法，给出了上行功率控制法的系统原理图。     

武警卫星通信系统的雨衰估算及分析

由于雨衰是影响卫星通信的重要因素,根据武警部队卫星通信网建设的需要,基于ITU-R制定的雨衰计算模型,选取6个典型城市进行雨衰计算,得到雨衰对Ku波段卫星通信系统的影响,分析得出地星路径仰角越小,海拔越低,降雨率越大,降雨造成的信号衰减越大。     

Ku波段卫星通信雨衰分析及对抗措施

雨衰是影响Ku波段卫星通信系统传输质量及系统性能的主要因素之一。本文介绍了Ku波段卫星通信中雨衰产生的机理及其对信道的影响,并提出了有效减少雨衰对Ku波段卫星通信不利影响的措施。     

基于Turbo码的自适应Ka频段抗雨衰控制

为了减小雨衰对Ka频段的影响,通过分析Ka频段雨衰和信号频率的关系以及Turbo码在不同天气下的性能,提出了一种自适应的抗雨衰控制方法,并建立了相应的系统模型。根据不同的天气状况自适应地提高码重和控制信号频率,提高了系统在中高信噪比时的抗雨衰能力。仿真结果表明,当误码率为10-4～10-5时,其性能可提高3 dB左右。     

Rain-induced attenuation for Ku-band satellite communications in the west coast of Peninsular Malaysia, Penang

Accurate rainfall rate is important for predicting rain attenuation over a satellite link. Raindrops are able to take in, spread out, and change the polarization of satellite signals that pass through the atmosphere of the earth, especially in equatorial regions like Malaysia where heavy rainfall normally occurs. Satellite communications system network operating at Ku-Band will experience rain fade due to absorption and scattering of signal. This paper aims to discuss the range of effects of rain on the transmission of electromagnetic signal and to determine the significance of rainfall rate in the prediction of rain attenuation based on measurements carried out in the northern region of Peninsular Malaysia (Penang). At 0.01% of time, 140?mm/h and 22?dB of rain rate and attenuation were observed, respectively.     

基于雨滴谱分布数据库的近红外辐射衰减研究

为了研究降雨对近红外辐射的衰减,定量评估降雨对近红外设备的影响,在雨滴谱分布二元函数模型建立的基础上,提出了一种雨滴谱分布数据库的建立方法,对LAWS和PARSONS在美国华盛顿地区测量的雨滴谱分布数据进行拟合,建立雨滴谱分布数据库。基于雨滴谱分布数据库,结合米散射理论和衰减系数公式,得到了1μm,2μm和3μm波长近红外辐射的消光效率因子与雨滴直径的关系曲线,并计算了在降雨量分别为0.25mm/h,1.25mm/h,2.5mm/h,12.5mm/h,25mm/h和50mm/h条件下的近红外辐射衰减系数。结果表明,不同降雨量条件下,对于雨滴谱分布的函数关系式,正态分布具有更好的拟合效果;消光效率曲线的包络线随波长的增大而增大;拟合衰减系数和降雨强度的函数关系发现,指数函数具有很好的拟合效果。该计算结果对研究降雨对近红外辐射的衰减具有重要意义。     

非致命激光武器雨中传输衰减系统设计研究

非致命激光武器实际应用中会受到大气环境影响,造成激光能量衰减,严重影响其作战效果。为精确了解激光在雨中传输的衰减,本文建立了激光在雨中传输的衰减模型,分析研究了激光在雨中的透过率,搭建了激光在雨中传输的衰减实验平台,实验研究了激光透过率与降雨强度和传输距离的关系。结果表明,激光在雨中传输衰减很大程度上取决于降雨强度和传输距离,雨量越大,传输距离越远,衰减越严重,当雨量为60 mm/h,激光传输200 m距离时,透过率已不到40％。仿真和实验结果吻合较好,说明所建立的衰减模型基本符合实际情况,同时也表明搭建的激光雨中传输衰减系统可以用于研究非致命激光武器雨中传输衰减,具有结构简单、易于操作、灵活性较好等优点。     

雨滴前向散射对光传输影响的研究

利用Mie理论和M-P雨滴谱分布，分析了粒子尺度及不同波长对消光效率因子的影响，详细推导了光波在雨中衰减的计算公式，得到了衰减与降雨率之间的确定关系，介绍了前向散射修正系数，分析了雨滴的前向散射对532nm波长光信号传播的影响，得到了经过前向散射修正后的衰减计算公式。     

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.     

Effect of Atmospheric Parameters on Satellite Link

The rainfall path attenuation measured at Universiti Sains Malaysia (USM) for 4 years (January 02 to January 06) is presented. The data obtained are useful to investigate the impairment due to rainfall attenuation in satellite links operating in tropical and equatorial climates. It shows that the logarithmic function with ground rain rate deviates at very high rain rate. A rainfall rate of 130 mm/h causes the rainfall attenuation threshold (> 20 dB) to be exceeded. Maximum exceedences for rain rate and attenuation were observed during the wet months. The cumulative distributions of attenuation derived from the measured data are presented and compared with those obtained with existing prediction methods.     

Numerical investigation of intense rainfall effects on coherent and incoherent slant-path propagation at K-band and above

A model investigation is carried out to analyze the impact of intense rainfall on slant-path microwave propagation, using a rainfall microphysical model. The effects are evaluated both for path attenuation, undergone by coherent radiation, and for multiple scattering phenomena, originating incoherent radiation along the path. Atmospheric spatial inhomogeneity is taken into account. The EM propagation model is formulated by means of the radiative transfer theory. The propagation model is applied both to simplified rain slabs and to vertically and horizontally inhomogeneous raining cloud structures in order to compare the impact of atmospheric models on coherent and incoherent propagation. Beacon frequencies between 20 and 50 GHz, elevation angles between 20/spl deg/ and 40/spl deg/ and surface rain rates from 1 to 100 mm/h are considered. Appropriate sensitivity analysis parameters are defined to present and discuss the numerical results. Our main conclusion is that the impact of the convective rainfall structure can be significant both in determining total attenuation and quantifying the contribution of multiple scattering to the received power. For intense rainfall, the use of a rain slab model can both overestimate coherent attenuation and underestimate incoherent intensity. The analysis of realistic raining clouds structures reveals the significance of modeling the volumetric albedo of precipitating ice, particularly at V-band. Total path attenuation can strongly depend on the pointing direction of the receiving antenna due to the intrinsic variability of the precipitating cloud composition along the slant path. Coupling cloud-resolving models with radiative transfer schemes may be foreseen as a new approach to develop statistical prediction methods at Ka-band and above in a way analogous to that pursued by using weather-radar volume data.     

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     

A new theoretical formulation for calculation of the specific attenuation due to precipitation particles on terrestrial and satellite radio links

The propagation of high frequency electromagnetic waves through rain is affected by absorption and scattering, which both lead to signal attenuation on satellite or terrestrial super high frequency (SHF) and extremely high frequency (EHF) links. Many factors govern rain effects on waves propagating in the atmosphere, namely raindrop size distribution, rain cumulative distribution, water temperature, refractive index and operating frequency. A high degree of accuracy in the calculation of specific attenuation caused by rain (attenuation per km) is of importance when planning high frequency radio communication systems. Until now, the kR^α expression has been widely used for such calculations. This relationship in which R (mm/h) represents the rain rate, α and k parameters determined for each frequency by interpolation, can be considered to be a compromise between the complexity and a simplified approach of the Van de Hulst relationship. In this paper, we present a new theoretical model based on physical laws and allowing the calculation of specific attenuation caused by all kinds of hydrometeor particles on high frequency electromagnetic waves. This method, which we derived from the forward scattering amplitude and the Van de Hulst relationship, is governed by wave polarization, the wave incidence angle, the radio link operating frequency, and precipitation particle size distribution in the climate of interest. Its application gives theoretical results which are in very good agreement with data gathered on terrestrial or earth–satellite links in several localities around the world. This new proposed model can be easily used and will constitute a very useful tool for evaluating satellite and terrestrial radio link performances, mainly in the future which will bring an expansion of high frequency satellite systems. © 1997 John Wiley & Sons, Ltd.     

The Prediction of Rain-Induced Attenuation in MM Wave Band by Rain Medium System Model

While the millimeter radio wave propagates through rainfall, it will be attenuated heavily due to assimilation and scattering of rain. It is imperative to establish a simple and effective model to predict the rain-induced attenuation. In this paper, the rainfall is taken as a random system that can attenuate the radio wave. The transfer function matrix model is selected to be the random system model. Using experiment rain attenuation data at different rain rate, the correlation entropy and residue error of the system is obtained by system identification method. On the basis of correlation entropy and residue error, we can determine the order of the predication system. At last, the predication model that can forecast heavy rain attenuation by small rain attenuation is gotten by applying the least square method. The comparison shows that the discrepancy between the predication result of the obtained model and the experiment rain attenuation data is relatively minor.