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.     

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.     

Determination of rain attenuation parameters for free space optical link in tropical rain

Free space optics (FSO) is a promising communication technique for various types of services in the optical access network. Single beam FSO system in tropical rainy weather is vulnerable to atmospheric rain attenuation, so it is necessary to have precise power law parameters of rain attenuation in tropical regions. In this study, the power law parameters k, and α are estimated as 2.03 and 0.74, respectively for the FSO applications in tropical South-East Asian weather. These parameters were evaluated by using least square mean equation (LSME) method with Levenberg–Marquardt optimization based on the one year collected heavy rain data. The obtained parameter values for tropical weather are contributed to improve link performance for high-speed networks.     

Statistical Prediction Modeling of Rain Induced Attenuation and Depolarization

Millimeter wave rain co-polarization attenuation, CPA, and cross polarization discrimination, XPD, measurements have been made at 35GHz and 94GHz over a line-of-sight link. On the basis of these experimental results, a study of this rain medium has been made with the method of link measurements. In this paper, we presented a statistical prediction modeling of rain-induced attenuation and depolarization from the statistical distribution of the rain intensity, and the size, and canting angle of raindrops. Our computational results are in good agreement with data of measurements.     

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.     

Calculations of millimeter wave depolarization due to melting layer and rain

A model for calculating the total depolarization due to the melting layer and rain is proposed under the assumption that oblate spheroidal melting particles and raindrops have the same orientation. The melting layer is composed of the melting particles which are made up from the mixture of ice, air and water. The specific attenuation and the specific phase shift both for the melting layer and for rain are given in the power lawaR ^b form for the rain rates 0R12.5 mm/h and the parameters are tabled over the frequency range of 1–100 GHz. Using the model, the numerical calculation of the depolarization is possible for three drop size distributions.     

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.     

Rain induced cross polarization at millimeter waves in a Xi''an environment

The rainfall intensity accumulative distribution, raindrop size and canting angle distributions in Xi'an(108.9 E, 34.3 N) have been obtained from data measured over a long period of time. Rain induced crose polarization in this environment has been studied. The crose polarization discrimination, XPD, was compu ted over a frequency range of 19.3 to 300 GHz for non spherical drops. The variations of XPD with frequency, rainfall rate and copolar attenuation, CPA, were investigated. The computational resulte are in good agreement with experimental data. A mathematical relationship was established between the XPD and CPA, raindrop canting angle, which result is in good agreement with experimental data. The cumulative distribution of rain induced cross polarization for eleven years and in month have been ob tained with the cumulative distribution of the rain rate from the data measured from 1975 to 1985. The role of multiple scattering also has been discussed, it is shown that the multiple scattering play an important part in short millimeter wave.     

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     

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)     

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.     

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.     

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.     

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 =3.2 cm, the radiometers at=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.     

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%.     

The Application of 3D Rain Scatter Model on Horizontally Polarized SHF Signal Propagation in Tropical Location

This paper presents some results of intersystem interference as a result of hydrometeor scattering on horizontally polarized SHF signal propagation in the tropical environment using the 3D rain cell model. The cumulative distribution of transmission loss has been based on the distance dependence (terrestrial station to the center of the common volume distance); frequency dependence and antenna gain dependence. Though there is a less interference (significantly higher transmission loss) at a relatively higher frequency of 20 and 34.8 GHz, the rain-scatter interference problem may not be negligible for a tropical region with high rainfall rate because, for a small percentage of time, the rain scatter power can be received even at a large distance from an interfering station.     

光在雨中传输的研究

概述了雨滴的尺寸分布特性和光波在雨中的传输衰减,介绍并分别讨论了仅考虑雨滴对光传输的散射作用与仅考虑雨滴对光传输的遮挡作用时光的传输衰减模型.重点分析了后者的原理,结合雨滴尺寸分布模型对光在雨中的最大传输距离及其光强的衰减做了计算.     

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.     

Computation of the Backscattering Power Density for Rainfall in Millimeter Waves Band

The size distribution and canting angle distribution for non-spherical rain drops are taken into account. Utilizing the Muller matrix, the backscattering power densities are computed at different rain rate and different polarization in millimeter waves band, according the rainfall condition in Xi'an, China. Finally the results obtained are discussed.