Rainfall Measurements Due to Radio Frequency Signal Attenuation at 2 GHz

In this paper we present an experimental validated system for measuring rainfall due to radio frequency (RF) signal attenuation at 2 GHz. Measurements took place in Ioannina, NW Greece, starting in April 2015 and lasting for twelve months. The primary acquired extensive results have shown reliable and accurate measurements for rainfall amounts smaller than 1 mm for 5 min periods. The very important innovation is that this paper presents significant earth-to-earth measurements due to rainfall attenuation (at 2 GHz) in order to act as a map for future investigation and as a prior knowledge for the behavior of other systems operating at frequencies around S-band.

Rain Attenuation in the Microwave-to-Terahertz Waveband

In recent years, there has been increased interest in the terahertz waveband for application to ultra-high-speed wireless communications and remote sensing systems. However, atmospheric propagation at these wavelengths has a significant effect on the operational stability of systems using the terahertz waveband, so elucidating the effects of rain on propagation is a topic of high interest. We demonstrate various methods for calculating attenuation due to rain and evaluate these methods through comparison with calculated and experimental values. We find that in the 90 - 225 GHz microwave band, values calculated according to Mie scattering theory using the Best and P-S sleet raindrop size distributions best agree with experimental values. At 313 and 355 GHz terahertz-waveband frequencies, values calculated according to Mie scattering theory using the Weibull distribution and a prediction model following ITU-R recommendations best agree with experimental values. We furthermore find that attenuation due to rain increases in proportion to frequency for microwave-band frequencies below approximately 50 GHz, but that there is a peak at around 100 GHz, above which the degree of attenuation remains steady or decreases. Rain-induced attenuation increases in proportion to the rainfall intensity.

Measurement of Rain Attenuation in Terahertz Wave Range

Rain attenuation at 355.2 GHz in the terahertz wave range was measured with our new 355.2 GHz measuring system under rainfall intensities up to 25 mm/hr. Rain attenuation coefficients were also calculated using four raindrop-size distributions, e Marshall-Palmer (M-P), Best, Polyakova-Shifrin (P-S) and Weibull distributions, and using a specific rain attenuation model for prediction methods recommended by ITU-R. Measurements of a terahertz wave taken at 355.2 GHz were compared with our calculations. Results showed that the propagation experiment was in very good agreement with a calculation from a specific attenuation model for use in prediction method recommended by ITU-R.

Rain Attenuation at Terahertz

Rain attenuation values were calculated using empirical raindrop-size distributions, which were, Marshall-Palmer (M-P), Best, Polyakova-Shifrin (P-S) and Weibull raindrop-size distributions, and also calculated using a specific rain attenuation model for prediction methods recommended by ITU-R. Measurements of Terahertz wave taken at 313 GHz (0.96 mm) were compared with our calculations. Results showed that the propagation experiment was in very good agreement with a calculation from the specific attenuation model for use in prediction methods by ITU-R.

Rain Attenuation Effects on 2.6 GHz WiMAX Networks Deployment in Ghana

WiMAX communication systems operating at 2.6 G frequencies are used for broadband multimedia and internet based services. At these frequencies, the signal will be affected by various propagation impairments such as rain attenuation, cloud attenuation, tropospheric scintillation, ionospheric scintillation, water vapour attenuation, and rain and ice depolarization. Among all the pro-pagation impairments, rain attenuation is the most important and critical parameter. In this research, rain attenuation is calculated at KNUST, Kumasi using ITU-R rain attenuation model. The preliminary results of the work will be used to calculate the attenuation experimentally and comparison can be made, which helps to develop a new rain attenuation model at 2.6 G bands. Rain attenuation is an important aspect of signal propagation above 2.6 GHz frequency. The attenuation time series generation from point rain rate measurement is crucial due to unavailability of actual signal measurements. In this research, a simple and realistic approach has been demonstrated for better estimation of rain attenuation using WiMAX-band signal propagation data and ground rain rate measurements in Ghana. The ITU-R model of rain attenuation has been modified by incorporating an effective slant path model. The effective slant path has been estimated and modeled in terms of a power-law relationship of rain rate data of 2007-2008. The methodology has been validated with the measured data of 2014. Comparison with ITU-R and GMET clearly demonstrates the improved predictability of the proposed model at the present tropical location.

Rain Attenuation Impact on Performance of Satellite Ground Stations for Low Earth Orbiting (LEO) Satellites in Europe

Low Earth Orbits (LEO) satellites are used for public communication and for scientific purposes. These satellites provide opportunities for investigations for which alternative techniques are either difficult or impossible to apply. Ground stations have to be established in order to communicate with such satellites. Usually these satellites communicate with ground stations at S-band. The communication quality depends on the performance of the satellite ground station, in addition to that of satellite. The performance of the satellite ground stations is expressed through Figure of Merit. The aim of this paper is to analyze the rain attenuation impact on the performance of the respective ground station. Rain attenuation depends on geographical location where the satellite ground station is implemented. In order to compare this effect on satellite ground station performance, some cities of Europe are considered. Finally, the rain attenuation impact on the satellite ground station Figure of Merit for the hypothetical satellite ground station installed in Prishtina is analyzed.

Analysis of the Effects of Rainfall Intensity into the Performance of Video Conferencing

Attenuation due to rain is an important limitation in the designing of the microwave communication links operating at frequencies above 10 GHz. Video conferencing performs over high-speed link via high-speed satellite (WINDS satellite) transmission in the Ka-band which is vulnerable to attenuation due to rain. Using the ITU-R proposed models for the prediction of rain attenuation gives a good estimation for the microwave propagation loss caused by rain for the temperate regions, but it underestimates the rain attenuation prediction for the monthly variation of rain rate by averaging annual rain rate for particular region. This study was carried out at the University of Rwanda/Huye Campus. The rain rates were predicted for 0.01 percentage of time from rainfall data measured during two months of July and August using the RD-80 Joss and Waldvogel Disdrometer. From the data collected and analyzed, compared with ITU-R model, we can see that where Rwanda region is situated in region K, what is different from the obtained results because the data are collected during months that coincide with a dry season.

The Effects of Power Control on Free-Space Optical Communications during Snowfall and Rainfall

Free-space optical (FSO) communication requires a line-of-sight connection between a transmitter and a receiver in which the information signal is modulated by an optical carrier that propagates in free space. The FSO channel is greatly affected by weather conditions such as fog, rain, and snow. In the literature, several adaptive techniques, such as power control (PC), have been suggested to mitigate channel link degradations. In this paper, we investigate the effects of snow and rain attenuation on the bit error rate (BER) of the FSO system using two types of modulations, the on-off keying (OOK) modulation and the pulse-position modulation (16-PPM). The effect of PC on the performance of FSO communications is also examined in this study. We evaluated the system’s performance with two types of snow, wet snow and dry snow, as well as with different rain regions. Results show that PC improves the BER of the FSO system;a high rate of improvement is found for wet snow and rain. PC has almost no effect with dry snow because of the high attenuation and the limitations on transmitted power. The BER for 16-PPM is better than that for OOK modulation.

用TOGA/COARE试验中机载雷达资料进行衰减效应订正的个案研究

为了最大限度地利用星载测雨雷达，必须实现一些主要条件：高的空间分辨率、宽阔的覆盖面积、大的动态降雨测量范围以及精确的点测量。但满足这些条件的雷达的工作频率段的电磁波穿过雨区就会受到衰减，再加上回波涨落的不确定性，因此必须修正这些因素所导致的误差。本文用1993年2月在西太平洋进行的国际大型科学考察实验TOGA／COARE中取得机载雷达（和星载雷达频率相近）实测数据进行衰减效应订正的个案研究，取得了和以往模拟相一致的结果。

基于商用微波链路反演降雨的空间插值方法对比研究

为探究适宜于商用微波链路降雨监测数据的空间插值方法,基于无线微波雨衰特性经验模型,分析不同线状降雨数据转换为点状降雨数据情景下,反距离权重法、克里金法和趋势面法对降雨空间插值的反演精度。研究结果表明:微波链路越密集,条数越多,其趋势的一致性越好;在微波线状数据转换为点状数据的五种情景中,将微波链路以200m为间隔概化为具体的降雨监测点时,空间插值效果较好;趋势面法的空间插值效果较好,反距离权重法次之,克里金法最差。研究结果明晰了线状降雨数据转换为格网型空间数据的原则,为商用微波链路组网构建空间二维雨量场提供了支撑,对提高线状降雨监测产品兼容性具有重要意义。

地空路径太赫兹波雨天衰减特性分析

雨滴的散射和吸收作用会严重增加地空链路上太赫兹波的传输损耗,降低无线通信的性能。为实现太赫兹波在地空链路上的传输应用,必须对太赫兹波在降雨环境中的传输特性进行深入研究。本文对原有的雨衰模型进行了修正,基于Mie理论,分析了降雨率的变化对地空链路上太赫兹波传输的影响,并与原有模型的计算结果进行了对比。结果表明:在整个太赫兹频段,雨衰减损耗会随降雨率的增加而增大,随频率的增加先增大后减小,且高频太赫兹波段相对0.1~1 THz频段范围的雨衰损耗更小;同时,当频率超过1 THz时,大气窗口越靠近10 THz,损耗越小,在降雨天气环境进行无线通信传输时将更具有通信优势,且频率越低,天顶角越大,模型修正前后的差异性更加明显。

面向降雨环境的激光雷达衰减模型研究

针对现有激光雷达衰减模型依赖统计生成点云、缺乏噪点解释性等问题,提出了一种面向降雨环境的激光雷达衰减模型。首先,建立激光雷达发射-接收模型,并根据雨滴尺寸分布模型模拟获得雨滴空间分布数据;其次,耦合散射模型与噪点模型,获得激光传播过程的光强变化,得到点云的仿真图像;最后,采集正常天气和降雨天气下的点云图像,仿真生成不同降雨量下的衰减点云。将衰减模型获得的点云与对应实际降雨天气点云图像进行对比,结果表明:所提出的模型在各评价指标上明显优于现有模型,有效解释了降雨过程对激光雷达产生的衰减影响。

对流层电波传播研究与展望

对流层电波传播特性是影响无线电系统性能的重要因素,是电波传播研究的主要领域.本文系统综述了近60年来中国电波传播研究所在无线电气象、地面无线电业务电波传播、空间无线电业务电波传播、电波传播效应对无线电系统影响评估等对流层电波传播不同方向开展的研究和取得的研究进展.在此基础上,最后给出了对对流层电波传播研究的展望.

基于复合机器学习模型的无线微波降雨反演研究

为实现精细化的无线微波水文气象密集监测,提出基于无线微波链路的降雨增强反演方案。利用极值梯度提升(XGBoost)算法进行干湿判别,并结合支持向量回归机(SVR)和高斯过程回归(GPR)两种机器学习算法,构建XG_SVR与XG_GPR复合反演模型,与传统ITU-R模型反演结果比较分析。实验结果表明,XGBoost算法的干湿判别结果优于滑动标准差法,平均分类准确率为88%,1 h以下高时间分辨率的准确率可达90%以上;两种复合模型整体反演效果良好,XG_SVR与ITU-R模型均适用于高时间分辨率,相关性系数均值皆高于0.80;而XG_GPR模型在反演1 h及以上低时间分辨率降雨时有显著优势,相关性系数为0.95左右,且均方误差远小于其他两种模型。利用复合机器学习模型改进传统无线微波测雨的方案具有可行性及发展潜力。

雨衰时间序列的混沌识别与预测

针对传统模型在高频段雨衰预测时存在参数计算复杂、实时性差的问题,设计了一种新的非线性动态预测模型.通过混沌识别证明了雨衰时序具备混沌的动力学特性及采用混沌预测方法的可行性.该方法以雨衰前导数据为训练样本建立非线性自适应滤波器,可忽略不同地理区域降雨分布差异性的影响.仿真结果表明,嵌入维数是影响预测精度的最主要因素,在满足嵌入维数为8、重构时延为3 s、采样间隔为1 s条件时预测相对误差可达0.05以下.同时预测的雨衰时间概率分布与ITU-R模型相比有较好的一致性,验证了所提方法具备参数配置简单、可用度高的优点.

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。

南京地区基于E频段毫米波链路的降雨监测研究

近年来,利用微波链路监测降雨这项新技术越来越受关注,准确的降雨观测数据对气候监测至关重要。该研究利用在南京地区搭建的E频段毫米波链路反演降雨强度信息,对降雨引起的衰减进行分离并实时确定参考基线,基于雨滴尺寸分布计算降雨强度,校正了湿天线效应的影响。实验结果表明,基于E频段链路反演获得的降雨强度在71GHz频段的相关系数约达到0.90,均方误差约为0.11mm/h,在81GHz频段约达到0.91,均方误差约为0.15mm/h。通过与激光雨滴谱仪的实测降雨强度作比较,验证了E频段毫米波链路监测降雨的可靠性。

降雨衰减对毫米波雷达有源干扰效果分析

介绍了复杂气象环境对毫米波信号传播衰减的影响,重点分析了降雨对毫米波信号传播附加衰减的影响。在此基础上,定量分析了降雨对毫米波信号侦察距离、有源压制式干扰效果和有源欺骗式干扰效果的影响,研究结论对于复杂气象环境下毫米波导引头的作战使用具有重要意义。

X频段星地数据传输链路降雨及沙尘衰减特性研究

降雨衰减(雨衰)和沙尘衰减是引起高频卫星通信链路损耗的重要因素。本文基于国际电信联盟ITU-R模型,利用MATLAB仿真对全国十个典型站点降雨衰减量进行了计算,对在99.99%可用度情况下的降雨衰减进行了分析,计算得到X频段星地数据传输链路降雨衰减特征;利用Goldhirsh模型计算了地面站不同仰角下衰减量和能见度的关系。本文研究可以为卫星地面站开展数据传输、测量控制等工作提供参考。

太赫兹电磁波的传播特性

文章叙述了太赫兹波的频谱分布,物理特性及其在大气中的传播,其中包括降雨、障碍物和地面、室内等不利条件的影响。