Modelling of Drop Size Distribution of Rain from Rain Rate and Attenuation Measurements at Millimeter and Optical Wavelengths

This paper describes a technique for modelling of rain drop size distributions at Calcutta in terms of negative exponential function, from the measurements of rain rate and attenuation over a dual wavelength LOS link at millimeter and optical frequencies. The DSD model obtained is then used to determine the attenuation at 94 GHz, for comparison with experimentally obtained attenuation at 94 GHz. This is also compared with the attenuation calculated by considering other experimentally obtained DSD models. The best fit negative exponential distribution function (modified M-P model) is presented along with some other experimentally obtained and reference models.     

Centimeter and millimeter wave attenuation by atmospheric gases and rainfall at a tropical station

The radiowave attenuation due to oxygen and water vapour has been computed over the frequency range 3–350 GHz making use of the mean surface air pressure, temperature and water vapour at Ile-Ife (geog. lat 7.5°N, long 4.5°E) in Southern Nigeria. It is observed that the attenuation at this tropical location is generally higher than at temperate climates. A similar analysis was performed for rainfall attenuation using rainfall intensity measurements. The results obtained with three different expressions for the rain height showed that a rain height of 3 km is a reasonable assumption for estimating earth-space rainfall attenuation at this location. It is found that for frequencies above 200 GHz, the polarization dependence of the specific attenuation due to rainfal becomes negligible. The computed attenuation is lower that that predicted using the corresponding CCIR rain climate data. The results show that whilst the contribution of oxygen and water vapour to the total atmospheric attenuation could be neglected when compared with rainfall attenuation up to about 150 GHz, the contribution becomes significant for frequencies above 190 GHz.     

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)     

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 Attenuation at Millimeter and Submillimeter Wavelengths

The rain attenuation was calculated by using the Marshall-Palmer, Best, Joss-Thomas-Waldvogel and Weibull distributions for raindrop-size. The results were compared with the recent measurements from 8 to 312.5 GHz at the rain rate R = 50mm/hr. Especially, the Weibull distribution has a good agreement with the measurements at 312.5 GHz (0.96 mm) in the submillimeter wavelength. Specific attenuation values from 1 to 1000 GHz were calculated for a rain temperature of –10°C, 0°C and 20°C by using the Weibull distribution.     

Radiometric Measurements of Rain Attenuation at 22.2 and 31.4 GHz Over Calcutta

Measurements of rain attenuation over Calcutta have been made using radiometers operating at frequencies of 22.235 and 31.4 GHz. The results have been correlated with observations of rainfall rates made with a fast-response rain gauge. The distributions of peak rain rates over Calcutta for two different years and the cumulative occurrences of rainfall rates and attenuations at the two frequencies are presented.     

Weibull Raindrop-Size Distribution and its Application to Rain Attenuation from 30 GHz to 1000 GHz

A weibull raindrop-size distribution is fitted to the measurements of rainfall observed using a distrometer in Tokyo. A propagation experiment at 103 GHz is also introduced. The rain attenuation is calculated by considering the Mie scattering for the Marshall-Palmer, Best, Joss-Thomas-Waldvogel, Gamma and Weibull raindrop-size distributions. The results of frequency characteristics from the Weibull raindrop-size distribution agrees well with some experimental data for the millimeter and submillimeter waves above 30 GHz. The quick read table is calculated for the rain attenuation from 30 GHz to 1000 GHz.     

Cloud Characteristics and Cloud Attenuation in Millimeter Wave and Microwave Frequency Bands for Satellite and Remote Sensing Applications over a Northern Indian Tropical Station

Microwave and millimeter wave frequency bands are in demand for requirement of more channels in radio communication systems. It has also been recognized that microwave and millimeterwave frequency radiometers on board satellites as promising tools for remote sensing.The frequency more than 10 GHz is affected by rain and cloud. Though the effects of rain on radiowave is more than cloud but the occurence of cloud is more than rain. Cloud has been found to occur for weeks together over this part of the world. It is therefore essential to study cloud morphology over different geographical region.In this paper, an attempt has been made to the cloud occurrences over an Indian tropical station, Delhi (28.35°N, 77.12°E) observed during different months and daytime and nighttime. It is seen that low clouds occurrence over Delhi is very significant and particularly during July, August and September. The specific attenuation of radiowave due to clouds at various frequencies 10 GHz, 20 GHz, 50 GHz and 100 GHz has been deduced. The specific attenuation of radio wave due to cloud at 10 GHz varies from 0.0608 dB/km to 0.1190 dB/km while at 100 GHz the specific attenuation varies from 6.8460 dB/km to 11.9810 dB/km     

The main results of cloud and rain remote sensing by multichannel microwave radiometry

The basic results have been considered for multiwave remote sounding of the troposphere with clouds and rain from ground-based station in the zenith direction. The radio wave scattering by rain drops as well as variation of vapour content in the atmosphere during measurements are taken into account when processing of the experimental data. A separation of the complete attenuation in clouds with rain has been made over three components: due to vapour, cloud and rain. A relation is considered of millimeter and centimeter wave attenuation in clouds with rain between each other and with the rain intensity. A behaviour of the relation of attenuation structural functions is explained. The Diagnostic problems of millimeter wave attenuation are considered.     

An estimation of S/N degradation at millimeter waves during rain events

The use of Ka Band (20/30 GHz) for future satellite communications has been addressed. The exploitation of Ka band with a bandwidth of 2500 MHz seems to represent the largest significant achievement in satellite communications potential, so far. The problems associated with the use of this frequency band such as attenuation and receiver noise temperature (floor) variation with rain has been addressed. The receiver noise floor variation with rain has so far been ignored. Therefore, in view of propagation and noise study over this Ka Band, both signal attenuation and receiver noise floor variations with rain rate are estimated using dual frequency radiometers operating at 22.235 and 31.4 GHz over a tropical station, Calcutta, India.     

Microwave and Millimeter Wave Characteristics and Attenuation of Clouds over some Malaysian Equatorial Stations

Based on radar range height indicator (RHI) measurements, cloud characteristics in relation to radiowave propagation over three locations in different geographical region in western Malaysia have been presented. It is seen that low cloud occurrence over these locations are quite significant. Cloud attenuation and noise temperature can result in serious degradation of telecommunication link performances. This paper presents cloud coverage in different months, 0°C isotherm height and cloud attenuation results at 12 GHz, 20 GHz, 36 GHz, 50 GHz, 70 GHz and 100 GHz over measurement site. The low level cloud over the measurement sites has been found to occur for many days and nights and particularly in the months of April to May and October to December. Such results are useful for satellite communication and remote sensing application in Malaysia.     

Assessment of rain fade mitigation techniques in the EHF band on a Syracuse 3 20/44-GHz low elevation link

An Earth-to-satellite propagation experiment in the EHF band has been carried out within the framework of the Syracuse 3 program, which is a new generation French military SATCOM system. The originality of this experiment resides in the link's frequencies (20 GHz downlink and 44 GHz uplink) and its low elevation angle (17°). The first part of the article presents a statistical analysis of attenuation data providing the long-term statistics, frequency scaling ratios and fade durations. These results are compared to standard ITU models. The second part of the article is dedicated to the short-term forecasting of rain fade, useful for the implementation of Fade Mitigation Techniques (FMT). Firstly, the downlink attenuation is predicted based on a non-linear ARIMA-GARCH model. The prediction result is then separated into several physical components (gases, clouds and rain) that are scaled to the uplink frequency using specific frequency scaling factors. The performance of the model is assessed based on Syracuse 3 20/44-GHz data collected during a period of 1 year.     

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.     

Characteristics of rain induced attenuation and phase shift at cm and mm waves using a tropical raindrop size distribution model

The tropical raindrop size distribution model developed by Ajayi and Olsen has been employed to study some characteristics of rain induced attenuation and phase shift for a tropical location for spherical, oblate spheroidal and Pruppacher-Pitter drop shapes. Parameters such as the a and b values for the power law relation between the specific attenuation and rainfall rate as well as differential attenuation and phase shift and their normalized values, were computed. A single power law between the specific phase shift and the rain rate was found to be adequate for vertical polarization, whilst a two-segment power law fitting is required for horizontal polarization between 1GHz and about 100GHz. The results were compared in many cases with those obtained with the Laws and Parsons drop size distribution, currently adopted by the CCIR for scattering applications.     

Calculation of Electromagnetic Scattering from a Pruppacher-Pitter Raindrop Using M.A.S. and Slant Path Rain AttenuationPrediction

The combined effects of hydrometeor scattering and absorption result in significant power loss, for Earth-space microwave links operating at frequencies above 10GHz. With the increasing deployment of higher frequencies in commercial wireless networks, the accurate estimation of the specific rain attenuation is very significant for the reliable design of a radio communication system. In the present paper, the scattering of a plane electromagnetic wave from a Pruppacher-Pitter raindrop is treated using the Method of Auxiliary Sources (MAS). The obtained data are compared with those taken from the open literature -in the form of real and imaginary part of the forward scattering amplitude - with excellent results. Then, they are used for the numerical calculation of both the specific rain attenuation and the exceedance probability function, in the case of a hypothetical satellite link located in various climatic regions. The comparison with other models against experimental data has given very encouragingresults.     

Rain Height in Relation to 0°C Isotherm Height Over Some Indian Tropical Locations and Rain Attenuation for an Indian South Coastal Station for Microwave and Millimeter Wave Communication Systems

There is a dearth of results on rain height over Indian tropical stations.The results on rain height in relation to 0°C isotherm height over four stations having different latitudes are presented in this paper. Four stations have been chosen in such a way that all have different latitudes and are located in different geographical regions having different local weather conditions. The seaonal variation of rain height in relation to 0°C isotherm height has been found to be appreciable over the station located in Indian east coast and Gujarat region, while seasonal variation is not significant at lower and intermediate probability levels over the stations located in Indian south-east coast and island. The prevailing local weather conditions over different stations also have been discussed. Based on observed rain heights and rain rates, the attenuation of radio wave at different frequencies lying in the range from 10 GHz to 150 GHz for different probability levels over Indian south-east coastal station have been deduced and presented in this paper.     

Rain Attenuation at Millimeter Wavelength of 1.33 mm

Specific rain attenuation values calculated by using the four empirical raindrop-size distributions, that is, the Marshall and Palmer, the Joss, Thams and Walgvogel, the Ihara, Furuhama and Manabe, and the Weibull raindrop-size distributions. The millimeter wave measurement at 225 GHz (1.33mm) was compared with our calculations. It has been shown that the Weibull distribution has the best agreement with the measurements.     

Rain induced depolarization from 1 GHz to 300 GHz in a tropical environment

The rain induced depolarization in a tropical environment has been studied using a tropical raindrop size distribution developed by Ajayi and Olsen (A-O). The differential attenuation, differential phase shift and cross polarization discrimination, XPD, were computed over a frequency range of 1GHz to 300GHz for spheroidal drops and up to 33GHz for Pruppacher-Pitter drops. The variations of XPD with frequency, rainfall rate and copolar attenuation, CPA, were investigated. A mathematical relationship was established between the XPD and the CPA, canting angle and frequency of propagation from 5GHz to 300 GHz for spheroidal drops and up to 33 GHz for Pruppacher-Pitter drops. The results obtained using the A-O drop size distribution have been compared with those assuming the Laws and Parsons (L-P) distribution. The Pruppacher-Pitter drop shape has been found to give rise to higher XPD, especially at low CPA and high frequencies.     

ON THE ACCURATE MODELING OF MILLIMETER WAVE FIXED WIRELESS CHANNELS EXCEEDANCE PROBABILITY

The ever increasing demand for high date rate multimedia services has led to the deployment of Fixed Wireless Access (FWA) networks operating in frequencies above 10GHz. Propagation characteristics of such networks include line-of-sight (LOS) transmissions highly influenced by the presence of rain. In this paper a methodology for evaluating the outage probability of a FWA channel is introduced, making use of the forward scattering amplitude by distorted raindrops of transmitted signals. Expressions for the imaginary part of the scattering amplitude are derived through a regression fitting analysis on the results of the Method of Auxiliary Sources (MAS) to the problem of electromagnetic scattering from a Pruppacher-Pitter raindrop. These expressions are employed and an analytical method to evaluate the rain attenuation exceedance probability over a fixed wireless access link is presented. The derived exceedance probabilities are compared with experimental data from ITU-R databank with encouraging results.     

Measurements of raindrops sizes and canting angles

Conclusion We have presented a convenient method of measurements in adjusting the parameters for distributions of raindrop sizes and obtaining canting angles to accommodate to the local geographic and meteorologic conditions to get the required data for calculation of AR and XPD in the system planning of a short ground link using millimeter wave band. The measured results had been used in a theoretical formula to compute the XPD induced by rain in ground radio link of path length 2.2km and 35.3GHz with linear polarization, and discovered that the computed XPD agreed well with the measured XPD/6/. The agreement confirms that our methods of measurements are reliable and the measurements described above at one spot in the radio path are feasible in prediction of the AR and XPD for a short ground radio link.Of course, if the accuracy of the discriminating device were improved, it would have smaller errors in discrimination, and the results obtained would be more presise.The assistance of Y.Yan, T.S. Zhang and Y. Huang in making the measurements and calculations is gratefully acknowledged.