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.     

基于非球形雨衰模型的微波链路雨强反演方法

以T矩阵理论、Gamma谱分布为理论基础,基于Pruppacher-Beard降雨粒子模型,对OTT雨滴谱仪的Gamma谱参数历史资料与降雨强度值进行非线性拟合得到具有实地谱分布的幂律系数,建立适合于本地区的雨衰模型,提出了基于非球形雨衰模型的微波链路雨强反演方法,分析了温度对模型幂律系数的影响,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:反演雨强的相关系数全部高于0.6,最高达到了0.96,RMSE最小值为0.79,累积降雨量的绝对偏差在2.47 mm以内,最小偏差仅为0.28 mm,相对误差低于1.84%.实验结果验证了基于非球形雨衰模型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.     

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     

基于1–10GHz空地链路信号的雨强监测方法可行性研究

空地链路上的微波信号受降雨影响, 会产生功率衰减和去极化效应. 基于这些物理特性, 本文提出利用1–10 GHz空地链路信号的降雨干扰项获取雨强的方法, 并开展了相关理论研究. 根据空地链路信号与雨滴复杂的相互作用, 研究了空地链路信号频率为1–10 GHz时, 雨强 (rain rate, R) 对衰减 (attenuation, A) 和交叉极化分辨率 (cross-polarization discrimination, XPD)的影响, 分别建立了A-R和XPD-R关系模型. 通过数值模拟, 分别分析了利用上述两个关系模型估测雨强的可行性, 并系统研究了不同频率、极化方式和仰角条件下的适用性. 研究结果表明, 对于水平极化或圆极化, 且频率较高的空地链路信号, 利用A-R关系反演强降雨具有理论上的可行性; 对于不同频率和极化方式的信号, XPD-R关系模型都可以用于反演雨强, 并且对于1–50 mm·h^-1范围内的雨强, XPD较为敏感; 不同仰角条件下, A-R和XPD-R 模型都适用. 在4–10 GHz时, 本文的XPD-R模型和国际电信联盟ITU-R中XPD预测模型的结果非常接近. 所得出的结论对于下一步开展相关的验证实验, 拓展卫星系统的气象应用, 实时估测降雨强度, 实现全球降雨观测具有重要的参考价值. 关键词： 空地链路信号 雨强 衰减特性 交叉极化分辨率(XPD)     

基于星地链路的垂直降雨场反演方法

在分析星地链路几何结构及传播模型的基础上,研究基于联合代数重建技术的星地链路反演二维垂直降雨场的方法.利用实测降雨资料构建3类降雨场,并搭建3条17 GHz垂直极化星地链路进行数值仿真.实验结果表明:单星地链路无法实现二维垂直降雨场的重构,反演场与真实场的相关系数分别为0.556,0.504和0.364;基于双星地链路的反演结果和真实场的相关系数均高于0.98,平均偏差分别为0.122, 0.159和0.537 mm/h,欧式距离均低于0.9 mm/h,熵的相对误差均小于1.6%,基本实现了二维垂直降雨场的反演;三星地链路的应用进一步提升了反演精度,相关系数接近于1,能够精准重构降雨场.实验结果验证了基于星地链路的垂直降雨场反演方法的可行性、准确性和有效性.星地链路的架设和维护简单,探测降雨范围广,易于在高原、山区、海岛等传统地面观测资料缺失的地区使用,可以作为已有降水测量手段的补充.     

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     

雨滴碰击光缆后光纤应变相位调制分析

基于云动力学理论,分析了雨滴碰击光缆后径向应变导致光纤纤芯折射率及形状发生改变,使光纤内传输光相位受到调制的过程.建立了雨滴碰击光缆引起光纤内传输光相位调制的模型,获得了降雨强度与相位调制之间的关系.研制了雨滴碰击光缆相位调制实验室验证系统,对比了模拟降雨强度分别为3,5,7,10,15,18,22,30 mm/h时的实验测试与仿真结果,两者变化趋势一致,误差在9%以内.该模型可用于仿真获得不同降雨强度下雨滴碰击光缆引起的光相位调制,为进一步研究降雨对光纤振动传感系统性能的影响,优化光纤振动传感工程应用系统,提出可行的雨量补偿方案提供了理论参考.     

一种基于雨声谱形状的水面降雨强度反演方法

本文依据雨声谱的不同特征,结合最小二乘法和非线性方法,分频段使用不同的模型反演水面降雨的强度。文中首先研究雨声谱在15~30 kHz的对数线性变化规律,并利用模型中的参数值将降雨类型分为无雨、小雨(＜10 mm/h)及中雨或大雨(＞10 mm/h);其次采用S型增长模型(Logistic模型)拟合小雨时的雨声谱,获得小雨期间降雨强度的量化算法;第三,研究2~10 kHz雨声谱的对数线性规律,利用回归中的参数值获得大雨期间降雨强度的量化算法。反演精度的分析结果表明,利用本算法反演的水面降雨强度与实测的水面降雨强度比较一致,每分钟平均降雨量的误差只有1%~4%,因此,本算法可以较好地反演水面降雨的强度。      

降雨对C波段散射计测风的影响及其校正

传统观点认为C波段散射计工作波长大于雨滴直径,受降雨散射衰减的影响很小, 因此往往忽略降雨对C波段散射计测风的影响.本文基于降雨引起C波段散射计信号的衰减、 后向体散射及雨滴落入海面后的扰动作用,推导了降雨条件下的雷达方程, 构建了2010年全年的ASCAT散射计、降雨雷达和欧洲中期天气预报中心数值预报的匹配数据集, 定量分析了降雨对C波段散射计测风的影响,发现其信号衰减随降雨强度和入射角的增大而增强; 后向体散射和雨表面扰动作用随降雨强度的增大而增强、随入射角的增大而减小, 其中雨表面扰动作用对散射计测风的影响大于后向体散射.另外, 利用降雨条件下的雷达方程和匹配数据集,本文建立了降雨条件下的C波段主动微波辐射传输模型, 实验表明,该模型能够改善降雨条件下C波段散射计测风的精度.     

星间微波光子链路调制方式的优化

针对外调制星间微波光子链路输出信噪比优化问题,建立了基于双电极马赫 曾德尔调制器的强度调制直接探测星间微波光子链路模型,通过优化调制器调制方式来提高链路性能。用数值模拟方法得到了单边带、双边带和推挽式3种调制方式下链路输出信噪比,利用曲面投影法求得了最优调制方式时一定信噪比要求下发射端所需最小光放大器增益和对应的调制器直流偏置相位。结果表明:相同输入射频信号功率和发射光功率情况下,双边带调制输出信噪比比单边带调制高3 dB,低直流偏置相位推挽调制可以进一步优化输出信噪比。输入射频信号功率为-20 dBm,输出信噪比为17.3 dB时,所需最小光放大器增益为43.9 dB,对应的直流偏置相位为0.87。     

Temperature Effects on Attenuation Due to Rain for Millimeter and Centimeter Waves

Radio wave operating in millimetrewave and microwave frequency bands are adversely affected due to rain. Particularly the attenuation is of immense significance for sensitive remote measurements by satellites using frequencies greater than 10 GHz. Maintenance of an uninterrupted communication link requires a precise knowledge of the attenuation effect due to rain for commissioning right kind of transmitting sources for various purposes required in present day situation. Precise measurement of attenuation at various frequencies will enable us to choose the right frequency, polarization, incident angle and power of the source for different purposes. In this paper we have compared the results of earlier works using aR^b Olsen et al, (1) and the formulation by Moupfouma, (2) on the basis of theoretical analysis for explaining the observed results. Effect of temperature, considered in detail in this communication, has contributed the necessary correction factor of the rain attenuation for explaining the observed results. Theoretical analyses to measure the attenuation of the propagating wave due to temperature variation in the rain path have been presented. Correction factor due to temperature profile (temperature from the ground to the rain height within which the radio wave traces its path) has been incorporated in two models by using the concept of dipole energy changes. The effect of this temperature is noted to be quite significant and incorporates an error to the extent of 7–8%.     

Rain Rate Statistics and Fade Distribution of Millimeter Waves in Indian Continents

For any communication service operating in the Microwave/ Millimeter wave region, statistical information characterising the attenuation due to rain along satellite slant path would be required for the design of satellite communication links and for the broadcasting network above 20 Ghz. It is necessary to have a prior knowledge of the probability of exceeding different levels of rain attenuation in order to design appropriate fade margins into systems and establishing estimates of the year to year variability of rain fade margin for particular geographic regions of India so that the communication system reflects the extremes of these variabilities. Direct measurement of beacon signals from geostationary satellites have been a mean to determine the above information and experiments can be pursued with satellite such as INSAT. [1]Attenuatiuon of Millimeter Waves by rainfall restricts the path length of a communication system. A knowledge of the rain attenuation at such frequencies is therefore desirable in designing a reliable communication system. Signal level fading over line-of-sight links strongly depends on the hop length, frequency and climate. For short hops, the probability of occurance of deep fades becomes diminishingly small. However, since an extended hop length is possible for regions with little rain activity, clear weather fading can affect the link reliability in a similar way ti a rain.[2]     

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.     

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.     

Rain Attenuation Ratios on Short Microwave and Millimeter Wave Bands Earth-Space Paths

The knowledge of the ratio of rain attenuation at one frequency to that at another on slant paths is useful for the design of satellite-to-Earth communication links and up-link power control systems. It is well known that the rain attenuation is influenced by parameters of precipitation along the slant path such as DSD (raindrop size distribution), raindrop temperature, rainfall rate, and so on. In this paper, based on several DSDs applied to various climate zones, at short microwave and long millimeter wave bands, the attenuation ratios are estimated on Earth-space paths. A comparison of the prediction results with the experiment data in Boston and Kashima areas is carried out. It is shown that the M-P and Weibull DSD applied to rain attenuation ratios estimation are better DSD at higher latitude regions. The Guangzhou DSD applied to rain attenuation ratios prediction is better in tropical and subtropical areas in China. The lognormal DSD may be a appropriate DSD applied to predict rain attenuation ratios in tropical areas at A_down>1dB or R>15mm/h. However, the attenuation ratios predicted by the Guangzhou DSD disagree with by the lognormal DSD, it requires that the DSD applied to predict rain attenuation ratios are further studied in tropical areas.