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1、Propagation modelV bandAttenuation Due to PrecipitationWhen propagatingthrough rain, snow, hail or ice droplets, radio waves suffer from power loss due to hydrometeor scattering (Fig. 1).The combined effect of hydrometeor scattering and absorption results in a power lossproportional in dB to the squ

2、are of the frequency. This constitutes the main disadvantage of operating at the Ku, Ka or V frequency bands. As far as satellite systems are concerned, the depth of rain fades also depends on the elevation.and polarization angles. On the other hand, as rain attenuation depends unfavorably on the ra

3、infall rate and the raindropsize distribution, it affects heavily tropical and subtropicalregions. An indicative picture of rain fades is obtained fromFig. 2, which illustrates a typical fade incident with a peakvalue of 14dB.Gaseous AbsorptionBesides hydrometeor absorption, gaseous absorption, most

4、ly from oxygen and water vapor, contributes to the total attenuation of radio waves, especially in the case of low elevation angles. However, the contribution ofgaseous absorption to the total attenuation is small comparedto the attenuation due to rain. In Fig. 4 the frequency dependence of oxygen a

5、nd water vapor absorption is presented interms of specific attenuation.Cloud Attenuation: The liquid water content of clouds is the physical cause of cloud attenuation. Prediction models forthis particular attenuation factor have been developed withinthe framework of ITU-R and elsewhere. Figure 5 de

6、picts attenuation values due to clouds and fog exceeded fora certain range of probabilities. The ITU-R model was selected as the underlying prediction method for generating thesecurves, which correspond to the three frequency bands examined in this survey.Melting Layer AttenuationAt a certain height

7、 aboveground level, called the effective rain height, snow and iceprecipitation are converted into rain precipitation. The regionaround this height is called the melting layer. During periodsof light rain and for low elevation angles, the melting layercontributes significantly to the total slant pat

8、h attenuation, as verified by the relevant prediction model.Sky Noise Increase: As attenuation increases, so doesemission noise (.The same factors previously mentioned, i.e. scatter/emission from precipitationhydrometeors, contribute to noise increase, which is moreimportant than attenuation when ea

9、rth stations with low noisefront ends are considered.Signal DepolarizationDifferential phase shift and differential attenuation caused by non-spherical scatterers cause signal depolarization. Although this phenomenon does not affect single polarizedsatellite systems, its effect becomes significant f

10、or systemsreusing frequency by transmitting two orthogonally polarizedsignals for optimum RF spectrum utilization. In this case, depolarization results in cross-polar interference, i.e. part ofthe transmitted power in one polarization interferes with theorthogonally polarized signal. In Fig. 6 the r

11、elevantITU-R method has been employed to demonstrate thelong-term statistics of hydrometeor induced cross-polarization in the Athens, Greece area. The cross-polarization dis -crimination (XPD) not exceeded for various percentages oftime is shown only at the Ku and Ka bands, since validity issues ari

12、se for the proposed model at frequencies above35GHz.Tropospheric ScintillationsVariations in the magnitudeand the profile of the refractive index of the troposphere leadto amplitude fluctuations are known as scintillations. These fluctuations increase with frequency and depend upon the lengthof the

13、slant path decreasing with the antenna beam width. Amplitude fluctuations are also accompanied by a phase fluctuation.W bandRain Rain events are responsible of the highest attenuation values measured at frequency above 10 GHz. When the rain droplets are smaller than the wavelength therain rate is su

14、fficient to estimate rain attenuation. On the other hand, with increasing frequency of operationDrop Size Distribution (DSD) can not be neglected. Statistical models have been developed to take into accountDSD. The most commonly used are the Laws-Parsons and Marshall-Palmer distributions. In particu

15、lar, the parameters of the Marshall-Palmer distribution are only related to thetype of rain (drizzle, widespread or thunderstorm).Gaseous absorptionIn the millimeter wave range dry air attenuation is mainly due to oxygenabsorption. The estimation of the atmospheric gaseous absorption in the W-hand c

16、ould be performed through the Millimetre Propagation Model (MPM), which was found to be in good agreement with experimental results in both horizontal and vertical links test trials. However, this is a physical model, which requires as inputs accurate measurements of the main meteorological paramete

17、rs (pressure, temperature, and humidity percentage) along the vertical path. On a statistical hasis these information can be obtained relying on existing meteorological databases(even those collected through previous propagation experiments). Equipment should be used to characterize the actual chann

18、el such as radio multi-frequency radiometers, able to retrieve the vertical profiles of the quantities of interest.Melting layerThe melting layer is the region around the 0o isotherm, where ice hydrometeors can aggregate, melt and coalesce to form raindrops, in a very narrow range of space, usually

19、in the order of several hundreds of meters. Physical phenomena related to melting layer (such as attenuation, depolarization and scattering) are different from the corresponding rain related ones, therefore requiring a separate treatment. The attenuation due to melting ice particles cannot be neglected, especially for low elevation angle links.T