1、Rec. ITU-R PN.618-3 329 SECTION 5F: ASPECTS RELATIVE TO SPACE TELECOMMUNICATION SYSTEMS RECOMMENDATION ITU-R PN.618-3 PROPAGATION DATA AND PREDICTION METHODS REQUIRED FOR THE DESIGN OF EARTH-SPACE TELECOMMUNICATIONS SYSTEMS (Question ITU-R 20613) (1986-1990- 1992- 1994) The ITU Radiocommunication As
2、sembly, considering 4 prediction techniques; . that for the proper planning of Earth-space - systems it is necessary to have appropriate propagation data and b) needed in planning Earth-space systems; that methods have been developed that allow the prediction of the most important propagation parame
3、ters cl that as far as possible, these methods have been tested against available data and have been shown to yield an accuracy that is both compatible with the natural variability of propagation phenomena and adequate for most present applications in system planning, recommends 1. space radiocommun
4、ication systems, in the respective ranges of validity indicated in the Annex. Note I - Supplementary information related to the planning of broadcasting-satellite systems as well as maritime, land, and aeronautical mobile-satellite systems, may be found in Recommendations ITU-R PN.679, ITU-R PN.680,
5、 IT-R PN.681 and ITU-R PN.682, respectively. that the methods for predicting the propagation parameters set out in Annex 1 be adopted for planning Earth- ANNEX 1 1. Introduction In the design of Earth-space links for communication systems, several effects must be considered. Effects of the non-ioniz
6、ed atmosphere need to be considered at all frequencies, but become critical above about 1 GHz and for low elevation angles. These effects include: a) absorption in atmospheric gases; absorption, scattering and depolaiization by hydrometeors (water and ice droplets in precipitation, clouds, etc.): an
7、d emission noise from absorbing media; all of which are especially important at frequencies above about 10 GHz; loss of signai due to beam-divergence of the earth-station antenna, due to the normal refraction in the atmosphere; a decrease in effective antenna gain, due to phase decorrelation across
8、the antenna aperture, caused by irregularities in the refractive-index structure; b) c) d) relatively slow fading due to beam-bending caused by large-scale changes in refractive index; more rapid fading (scintillation) and variations in angle of arrival, due to small-scale variations in refractive i
9、ndex; COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesITU-R RECMN+PN. 638-3 94 H 4855232 0523373 934 330 Rec. ITU-R PN.618-3 e) possible limitations in bandwidth due to multiple scattering or multipath effects, especially in high- capa
10、city digital systems; attenuation by the local environment of the ground terminal (buildings, trees. etc.); short-term variations of the ratio of attenuations at the up- and down-link frequencies, which may affect the accuracy of adaptive fade countermeasures. f) g) This Annex deals with the propaga
11、tion data required in system planning. Effects of the Earths ionosphere and of extra-terrestrial ionized media which may need to be considered for systems operation are treated in Recommendation ITU-R PIS3 1 and include: h) rotation of the plane of polarization (Faraday rotation), particularly if fr
12、equency re-use and linear polarization are employed; dispersion, which results in a differential time delay across the bandwidth of the transmitted signal; j) k) excess time delay; i) This Annex deals only with the effects of the troposphere on the wanted signal. Interference aspects are treated sci
13、ntillation, which affects amplitude, phase and angle-of-arrival of the received signal. - in separate Recommendations and Report: - - - interference between earth stations and terrestrial stations (Recommendation ITU-R PN.452); interference from and to space stations (Recommendation ITU-R PN.619); b
14、idirectional coordination of earth stations (Report ITU-R PN. 1010). An apparent exception is path depolarization which, although of concern only from the standpoint of interference (e.g. between orthogonally-polarized signal transmissions), is directly related to the propagation impairments of the
15、Co-polarized direct signal. . The information is arranged according to the link parameters to be considered in actual system planning, rather than according to the physical phenomena causing the different effects. As far as possible, simple prediction methods covering practical applications are prov
16、ided, along with indications of their range of validity. These relatively simple methods yield satisfactory results in most practical applications, despite the large variability (from year to year and from location to location) of propagation conditions. As far as possible, the prediction methods in
17、 this Annex have been tested against measured data from the data banks of Radiocommunication Study Group 3 (see Recommendation ITU-R PN.31 i). 2. Propagation loss The propagation loss on an Earth-space path, relative to the free-space loss, is the sum of different contributions as follows: - attenua
18、tion by atmospheric gases: - - focusing and defocusing; - - scintillation and multipath effects; attenuation by rain, other precipitation and clouds; decrease in antenna gain due to wave-front incoherence; - attenuation by sand and dust storms. Each of these contributions has its own characteristics
19、 as a function of frequency, geographic location and elevation angle. As a rule, at elevation angles above lo“, only gaseous attenuation, rain attenuation and possibly scintillation will exceed a few tenths of a decibel, depending on propagation conditions. COPYRIGHT International Telecommunications
20、 Union/ITU RadiocommunicationsLicensed by Information Handling Services ITU-R RECMN*PN= bL6-3 94 4855212 0521372 670 Rec. ITU-R PN.618-3 33 1 (In certain climatic zones, snow and ice accumulations on the surfaces of antenna reflectors and feeds can produce prolonsed periods with severe attenuation,
21、which might dominate even the annual cumulative distribution of attenuation.) 2.1 Attenuation due to atmospheric gases Attenuation by atmospheric gases which is entirely caused by absorption depends mainly on frequency, elevation angle, altitude above sea level and water vapour density (absolute hum
22、idity). At frequencies below 10 GHz. it may normally be neglected. Its importance increases with frequency above 10 GHz, especially for low elevation angles. This effect is discussed in detail in Recommendation ITU-R PN.676. 2.1.1 Procedure for calculating gaseous attenuation The method described be
23、low should be used to calculate the median gaseous absorption loss expected for a given value of surface water vapour density, pw, for frequencies up to 350 GHz (excluding the 57-63 GHz band for which information may be obtained from Recommendation ITU-R PN.676). Parameters required for the method i
24、nclude: - f : frequency (GHz) 8 : path elevation angle h, : height (km) above mean sea level of the Earth terminal; if unknown, a value of h, = O will give somewhat conservative results p, : water vapour density (g/m3) at the surface sin3 8 c 100 mmh, use the value 100 mm/h in place of R0.01. Step 6
25、: dation ITU-R PN.838 and the rainfall rate, R0.01, determined from Step 4, by using: Obtain the specific attenuation, YR, using the frequency-dependent coefficients given in Recommen- dBkm Step 7: The predicted attenuation exceeded for 0.01% of an average year is obtained from: (9) Step 8: is deter
26、mined from the attenuation to be exceeded for 0.01% for an average year by using: The estimated attenuation to be exceeded for other percentages of an average year, in the range 0.001% to 1%, - A - 0.12-(0.546 + 0.04310gp) Ao.01 This interpolation formula gives factors of 0.12, 0.38, 1 and 2.14 for
27、1%, 0.1%, 0.01% and 0.001%, respectively. Step 9: equation (1 1): If desired, the value of p corresponding to a given value of Ap may be computed from the inverted form of 11.628(-0.546 + d0.298 + 0.172 log(0.12 . A0.01 /Ap) PR = 10 with the constraint that: Ao,ol IAp 2 0.15 When the complete predic
28、tion method above was tested using the procedure set out in Annex 1 to Recommendation ITU-R PN.311, the results differed between high and low latitudes. For latitudes above 30, the prediction was found to be in good agreement with available measurement data in the range 0.001% to 0.1%, with a standa
29、rd deviation of some 35%, when used with concurrent rain rate measurements. This method provides an estimate of the long-term statistics of attenuation due to rain. When comparing measured statistics with the prediction, allowance should be given for the rather large. year-to-year variability in rai
30、nfall rate statistics (see Recommendation ITU-R PN.678). Attenuation due to water cloud or fog of known liquid water content can be calculated from Recommendation ITU-R PN.840. Except for clouds of high water content, total attenuations due to cloud will not be great at frequencies below 30 GHz and
31、are likely in any case to be included in the measured statistics. The effects of ice cloud, dry hail and dry snow can be generally neglected for these frequencies. As the frequency increases, the effect of clouds becomes steadily more important. However, recent measurements show that in some types o
32、f cloud (e.g. strato-cumulus) the additional attenuation produced at vertical incidence is no more than 0.5 to i dB, even at 150 GHz. On the other hand, clouds with a high liquid-water content, such as cumulo-nimbus, can cause an additional attenuation of about 4 to 5 dB at 100 GHz and up to 8 dB at
33、 150 GHz. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services-Rec. ITU-R PN.618-3 335 2.2.1.2 Long-term frequency and polarization scaling of rain attenuatiort statistics The method of $ 2.2.1 .I may be used to investigate the dependence
34、of attenuation statistics on elevation angle. polarization and frequency, and is therefore a useful general tool for scaling of attenuation according to these parameters. If reliable attenuation data measured at one frequency are available, the following empirical formula giving an attenuation ratio
35、 directly as a function of frequency and attenuation may be applied for frequency scaling on the same path in the frequency range 7 to 50 GHz: where: Al and A2 are the equiprobable values of the excess rain attenuation at frequencies fi and f2 (GHz), respectively. Frequency scaling from reliable att
36、enuation data is preferred, when applicable, rather than the prediction methods starting from rain data. When polarization scaling is required, it is more appropriate to use directly the parameters k and a as given in Recommendation ITU-R PN.838. These parameters also provide a radiometeorological b
37、asis for frequency scaling. 2.2.2 Seasonal variations - worst-month System planning often requires the attenuation value exceeded for a time percentage, pw, of the worst month. The following procedure is used to estimate the attenuation exceeded for a specified percentage of the worst month. Step I:
38、 the equation specified in Recommendation ITU-R PN.581 and by applying any adjustments top as prescribed therein. Obtain the annual time percentage, p, corresponding to the desired worst-month time percentage, pw, by using Step 2: For the path in question obtain the attenuation, A (dB), exceeded for
39、 the resulting annual time percentage, p. from the method of $ 2.2.1.1, or from measured or frequency-scaled attenuation statistics. This value of A is the estimated attenuation for pw per cent of the worst month. Curves giving the variation of worst-month values from their mean are provided in Reco
40、mmen- dation ITU-R PN.58 1. 2.2.3 Variability in space and time of statistics Precipitation attenuation distributions measured on the same path at the same frequency and polarization may show marked year-to-year variations. In the range 0.001% to 0.1% of the year, .the attenuation values at a fixed
41、probability level are observed to vary by more than 20% r.m.s. When the models for attenuation prediction or scaling in $ 2.2.1 are used to scale observations at a location to estimate for another path at the same location, the variations increase to more than 25% r.m.s. 2.2.4 Site diversi0 Intense
42、rain cells that cause large attenuation values on an Earth-space link often have horizontal dimensions of no more than a few kilometres. Diversity systems able to re-route traffic to alternate earth stations, or with access to a satellite .with extra on-board resources available for temporary alloca
43、tion, can improve the system reliability considerably. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services ITU-R RECMN*PN* b38-3 94 W 4855232 0523377 352 W 336 Rec. ITU-R PN.618-3 Two concepts exist for characterizing diversity performanc
44、e: the “diversity improvement factor” is defined as the ratio of the single-site time percentage and the diversity time percentage, at the same attenuation level. “Diversity gain” is the difference (dB) between the single-site and diversity attenuation values for the same time percentage. Both param
45、eters are important. depending on the system design approach, and prediction procedures for both are given below. The procedures have been tested at frequencies between 10 and 30 GHz, which is the recommended frequency range of applicability. The diversity prediction procedures are only recommended
46、for time percentages less than O. 1 %. At time percentages above O. 196, the rainfall rate is generally small and the corresponding site diversity improvement is not significant. 2.2.4.1 Diversi0 improvement factor The diversity improvement factor, I, is given by: 100 2 1+- =1+- Pi I=-= P2 (1 + 2) (
47、 10;l2) P1 where pl and p2 are the respective single-site and diversity time percentages, and is a parameter depending on link characteristics. The approximation on the right-hand side of equation (16) is acceptable since 2 is generally small. From a large number of measurements carried out in the 1
48、0-20 GHz band, and mainly between 11 and 13.6 GHz, it has been found that the value of 2 depends basically on the distance, d, between the stations, and only slightly on the angle of elevation and the frequency. It is found that 2 can be expressed by the following empirical relationship: (17) 2 - 10
49、-4dl.33 - Figure 2 shows p2 versus pi on the basis of equations (16) and (17). 2.2.4.2 Diversiy gain The diversity gain, G (dB), between pairs of sites is calculated with the empirical expression given below. d : separation (km) between the two sites A : path rain attenuation (dB) for a single site f : frequency (GHz) 8 : path elevation angle (degrees) y: angle (degrees) made by the azimuth of the propagation path with respect to the baseline between sites, chosen such that y 5 90”. Parameters required for the calculation of diversity gain are: Step I: Calculate the gain
