A simple method for estimating rain attenuation distribution

This paper deals with a method for estimating rain attenuation distribution at frequencies above 10 GHz. This method depends only on point rainfall rate. Based on statistical independence among rain cells, a relation connecting cell dimension, path distance, precipitation and attenuation is established. Numerical results have shown good agreement with experimental data.     

Prediction of the effects of rain on satellite communication systems

The major propagation effects for satellite communication systems operating above 4 GHz are caused by rain. With the possible exceptions of depolarization and multiple scattering at frequencies above 20 GHz, these effects may be calculated if the distribution of rain intensity is known in both time and space. The major effects-attenuation and interference-require information about path and volume averaged rain intensities. Current prediction models are not capable of adequately estimating the statistical distributions of path and volume averaged values. Radar observations could provide the required data. The best information currently available for modeling these distributions are statistical cell or storm models derived from radar observations.     

Depolarization Measurements on the ATS-6 20-GHz Downlink: A Description of the VPI & SU Experiment and Some Initial Results (Short Papers)

This paper discusses the depolarizing effects of precipitation at millimeter wavelengths and describes an experiment in which depolarization on the ATS-6 satellite 20-GKz downlink is measured. Data are presented for unexplained clear weather variations in the observed polarization and for depolarization by rain and snow. A preliminary analysis indicates that for a given attenuation level, a satellite path exhibits more severe depolarization than experiments with ground systems would predict.     

Probability distribution of rain-induced cross-polarization

The cross-polarization discrimination of a radiorelay link using two orthogonal polarizations has been calculated from the statistical distribution of the rain intensity, and the size, deformation and canting angle of raindrops. At frequencies of 18.8 GHz and 28.8 GHz and for a path length of 20 km the results are plotted as a function of the path-averaged rain rate. Furthermore, the distributions of the cross-polarization discrimination in the worst case for a path perpendicular to the main wind direction are obtained from the statistical distribution of the path-averaged rain rate for linear and circular polarizations.     

Measurement of rain-induced zenith-path attenuation using 19.9 GHz radiometer at Amritsar (India)

The paper presents the results of 19.9 GHz radiometric propagation studies conducted over a period of one year at Amritsar, for determining rain-induced zenith path attenuation. The zenith path attenuation has been determined by the measurements of sky noise temperature received by the radiometer. The results obtained from the experiment are presented in the form of annual cumulative distributions of rain rate, sky noise temperature, and zenith path attenuation together with worst-month statistics. The rainfall rate cumulative distribution as predicted by ITU-R for our geographical location is lower than the actually measured rainfall rate cumulative distribution. The cumulative distribution of zenith path attenuation predicted by using ITU-R model overestimates the measured cumulative distribution of zenith path attenuation.     

Determination of rain anisotropy and effective spreading of the orientation of ellipsoidal raindrops during intense rainfall

Depolarization measurements performed in circular polarization at centimeter wavelength have frequently shown depolarization values lower than those theoretically expected by application of the Oguchi theory, which assumes equioriented ellipsoidal raindrops. This discrepancy can be explained by considering that in reality the drop-axes are oriented at a given instant according to a certain statistical distribution the spreading of which causes a decrease in the anisotropy (differential propagation constants along the principal planes), which is the prime source of rain depolarization (both in circular and in linear polarization). The reduction of anisotropy with respect to the equioriented drops model is experimentally assessed and the spreading necessary in a particular model distribution for the same reduction is determined in this communication. The study is based upon simultaneous measurements of attenuation and depolarization in circular polarization gathered in 1974-1975 during the Magnola experiment (Italy) on a low-elevation terrestrial path (9deg) at 11.6 GHz. The results confirmed that the equiorientation of raindrops is too severe an assumption, being the true anisotropy 20-30 percent (on average) less than the expected value in case of equiorientation. This figure corresponds to an average spreading of about20-30degwhich is in agreement with the results given by other authors.     

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.     

Attenuation caused by rain at frequencies above 10 GHz

Measurements of the attenuation caused by rain have been performed on a 20 km test path at frequencies of 12, 15, 19, 29 and 39 GHz. Simultaneously the rain intensity has been measured with ten rain gauges installed along the path so that the average spacing of the rain gauges is about 2 km. For a oneyear period the results of the rain gauge network were used to determine the statistical distribution of the pointrain intensity, and the path averaged and timeaveraged rain intensities. Comparisons were made between the experimental rain attenuation and the attenuation calculated from the measured rain intensities using Miescatter theory. In nearly all cases good agreement was obtained. Using the pointrain rate distribution, the prediction method of S. H. Lin for rain attenuation statistics was tested.     

Statistical rainstorm models: Their theoretical and physical foundations

Precipitation has become a serious source of attenuation as higher frequencies are being employed for microwave communications. System performance is strongly influenced by the quantity and character of precipitation that occurs over the links of the system. Rain appears to be the precipitation form that accounts for most of the serious attenuation occurrences. It is also for rain that the Scattering theory is most complete. Rain attenuation can be accurately predicted if the drop-size distribution along the propagation path is known. The drop spectrum determines as well the rainfall rate, radar reflectivity, and microwave emission of rain. Consequently, it is possible to make estimates of rain attenuation through indirect measurements by raingauge, radar, and radiometer. Recent experiments have confirmed that these estimates are sufficiently accurate for practical purposes. From propagation experiments and studies of the fine-scale structure of rain, data are becoming available on the horizontal extent of heavy rain areas and the way this structure influences system performance. These data have been used to formulate statistical raincell models that permit prediction of the performance of single-path and path-diversity systems. The current status of raincell models is reviewed and suggestions for future research are offered.     

The system implications of 6/4 ghz satellite-to-ground signal depolarization results from the intelsat propagation measurement programme

The results of six experiments to measure 6/4 GHz signal depolarization are presented. The measurement sites varied from the tropics to 57°N latitude and elevation angles between 12° and 38° were employed. In two of the experiments, simultaneous measurements at 6 GHz and 4 GHz were made along the same path to measure the correlation between the depolarizations occurring at the two frequencies. It was found that, in all cases, rain was the dominant depolarizing mechanism. Where the occurrence of ice depolarization was significant the magnitude of the ice depolarization occurring was not system limiting on a statistical basis. The correlation between up-link and down-link depolarization occurring along the same path was found to be very good both on an instantaneous basis and a statistical basis. The use of open-loop up-link depolarization compensation mechanisms should therefore be feasible. An examination of the depolarization fluctuation spectra obtained in some severe depolarizing events during one of the experiments would indicate that the settling time for such a mechanism should be on the order of 1 s. The use of simple, ‘non-perfect’, depolarization compensation mechanisms allied to a low-cost, spectrally efficient FEC system may prove to be the best compromise in some locations that experience severe depolarization.     

System Implications of 14/11 GHz Path Depolarization. Part II: Reducing the impairments

Dual-polarized co-channel operation via international communications satellites places stringent requirements on the amount of interference that can be allowed from all sources. System design must allow for the depolarization caused by rain and ice crystals, the principal source of additional interference. In the 14/11 and 14/12 GHz bands, some paths are depolarization limited, particularly at lower path elevation angles, or become so with up-link power control. Others are attenuation limited. Therefore, a means must be found to reduce the level of path depolarization to meet the requirements for path availability. In Part I or this paper, reference joint statistics of cross-polarization discrimination/attenuation statistics were derived for a number of paths and climates to establish the additional fade and cross-polarization discrimination margins required. This second paper addresses methods for reducing the system effects of path depolarization. Appropriate orientation of the polarization vectors at the spacecraft is shown to be an attractive alternative for spot-beam applications. For all other situations, it is concluded that single-parameter, phase-only compensation at the earth terminal will meet all practical system requirements.     

应用Weibull分布计算毫米波降雨去极化

用Ｗｅｉｂｕｌｌ分布密度函数表示雨滴尺寸分布，计算毫米波通过雨区的去极化分辨率（ＸＢＤ），在本地区的降雨条件下所得的结果比以前其他已知的雨滴尺寸分布的计算结果更符合实际。     

Radio propagation for space communications systems

This paper presents a review of the most recent information on the effects of the earth's atmosphere on space communications systems. The design and reliable operation of satellite systems which provide the many applications in space and rely on the transmission of radio waves for communications and scientific purposes are dependent on the propagation characteristics of the transmission path. The presence of atmospheric gases, clouds, fog, precipitation, and turbulence cause uncontrolled variations in the signal characteristics which can result in a reduction of the quality and reliability of the transmitted information. Models and techniques used in the prediction of atmospheric effects as influenced by frequency, geography, elevation angle, and type of transmission are discussed. Recent data on performance characteristics obtained from direct measurements on satellite links operating to above 30 GHz are reviewed. Particular emphasis is placed on the effects of precipitation on the earth-space path, including rain attenuation, and rain and ice-particle depolarization. Sky noise, antenna gain degradation, scintillations, and bandwidth coherence are also discussed. The impact of the various propagation factors on communications system design criteria is presented. These criteria include link reliability, power margins, noise contributions, modulation and polarization factors, channel crosstalk, error-rate, and bandwidth limitations.     

A new method of estimating microwave attenuation over a slant propagation path based on rain gauge data

A basic problem in estimating the microwave attenuation over a satellite-earth propagation path on the basis of rain gauge data is that, for a given precipitation event, the attenuation and the rainfall records very often do not have consistent detailed correlation. Such inconsistencies can be greatly reduced if the falling speed of the rain drops, i.e., 2 to 9 m/s for drop sizes of 0.05 to 0.7 cm, is taken into account. Adjustments in time and drop size spectrum are needed for data collected from field rain gauges to realize the actual rainfall along the slant path for correlating rain gauge data with measured satellite signals.     

一种改进气象雷达测雨性能的新方法

常规气象雷达无法分离雷达反射率和雨衰减率，因此用它测雨具有一些原理上的误差。本文提出一种雷达和卫星系统反演沿地空路径降雨率分布的新方法。推导出反演沿路径雨衰减率的递推方程组。给出了实现该方法的雷达和卫星联合测雨系统，以及应用该系统测量得到沿地空路径降雨率分布的典型结果。     

A model for calculating the depolarization of microwave signals propagating through rain

A model for calculating the amount of depolarization for linearly polarized electromagnetic waves propagating through rain is described. The presented method provides mathematical expressions for the co-polar and the cross-polar components of the received signal emerging from a rain cell. Calculations are carried out in two ways: with a general distribution of canting angles as measured by Saunders and with an equivalent canting angle together with the associated percentages of the positively and the negatively canted raindrops in a rain cell. The predicted attenuation and the cross polarization as calculated by both methods are found to be in good agreement with the published data measured by various workers.     

神经网络方法在降雨去极化效应预报中的应用

利用神经网络中的ＢＰ算法，通过在已知的传播路径上降雨同极化衰减ＣＰＡ来预算降雨去极化分辨率ＸＰＤ，且将计算结果与其他算法进行了对比。本文的工作是神经网络方法在建立电波大气传输效应预报模型应用中的有益尝试。     

Statistical variations in forward propagation rain depolarization due to drop size fluctuations

Observed electromagnetic wave depolarization in forward propagation due to precipitation tends to distribute statistically about some mean level of depolarization. This is due to random rain parameters that cause this depolarization. Using a drop-size modified depolarization model, we have analyzed the statistical scatter of the cross polarization ratio due to random distributions of drop population and size and found a standard deviation of approximately 2.5 dB about the mean, independent of frequency.     

Theory for measuring the effective polarization parameters of rain from orthogonal linearly-polarized transmissions

The theory is presented for one method of determining the effective polarization parameters of rain from propagation measurements over a line-of-sight link. The method requires the transmission of orthogonal linearlypolarized signals and a set of three amplitude and two relative phase measurements. Approximate equations are given for determining both instantaneous and «statistical parametric» values of the effective parameters in a twoparameter Gaussian model of the raindrop canting angle distribution. The instantaneous values could be useful for comparison with in-situ measurements, while the statistical parametric values are required for prediction of rain depolarization statistics.     

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.