1、Recommendation ITU-R P.840-5(02/2012)Attenuation due to clouds and fogP SeriesRadiowave propagationii Rec. ITU-R P.840-5ForewordThe role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and economical use of the radio-frequency spectrum by all radiocommunication servi
2、ces, including satellite services, and carry out studies without limit of frequency range on the basis of which Recommendations are adopted.The regulatory and policy functions of the Radiocommunication Sector are performed by World and Regional Radiocommunication Conferences and Radiocommunication A
3、ssemblies supported by Study Groups.Policy on Intellectual Property Right (IPR)ITU-R policy on IPR is described in the Common Patent Policy for ITU-T/ITU-R/ISO/IEC referenced in Annex 1 of Resolution ITU-R 1. Forms to be used for the submission of patent statements and licensing declarations by pate
4、nt holders are available from http:/www.itu.int/ITU-R/go/patents/en where the Guidelines for Implementation of the Common Patent Policy for ITU-T/ITU-R/ISO/IEC and the ITU-R patent information database can also be found. Series of ITU-R Recommendations (Also available online at http:/www.itu.int/pub
5、l/R-REC/en)Series TitleBO Satellite deliveryBR Recording for production, archival and play-out; film for televisionBS Broadcasting service (sound)BT Broadcasting service (television)F Fixed serviceM Mobile, radiodetermination, amateur and related satellite servicesP Radiowave propagationRA Radio ast
6、ronomyRS Remote sensing systemsS Fixed-satellite serviceSA Space applications and meteorologySF Frequency sharing and coordination between fixed-satellite and fixed service systemsSM Spectrum managementSNG Satellite news gatheringTF Time signals and frequency standards emissionsV Vocabulary and rela
7、ted subjectsNote: This ITU-R Recommendation was approved in English under the procedure detailed in Resolution ITU-R 1.Electronic PublicationGeneva, 2012 ITU 2012All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without written permission of ITU.Rec. ITU-R
8、P.840-5 1RECOMMENDATION ITU-R P.840-5Attenuation due to clouds and fog(Question ITU-R 201/3)(1992-1994-1997-1999-2009-2012)ScopeThis Recommendation provides methods to predict the attenuation due to clouds and fog on Earth-space paths.The ITU Radiocommunication Assembly,consideringa) that there is a
9、 need to give guidance to engineers in the design of Earth-space telecommunication systems for frequencies higher than 10 GHz;b) that attenuation due to clouds may be a factor of importance especially for microwave systems well above 10 GHz or low-availability systems;c) that for the calculation of
10、the time series of total attenuation and space-time prediction methods, an analytical expression for the statistics of the total columnar content of cloud liquid water is needed,recommends1 that the curves, models and maps given in Annex 1 should be used for the calculation of attenuation due to clo
11、uds and fog;2 that the information in Annex 1 should be used for global calculations of propagation effects, required by, inter alia, space-time channel models, that require an analytic expression for the statistics of the total columnar content of cloud liquid water.Annex 11 IntroductionFor clouds
12、or fog consisting entirely of small droplets, generally less than 0.01 cm, the Rayleigh approximation is valid for frequencies below 200 GHz and it is possible to express the attenuation in terms of the total water content per unit volume. Thus the specific attenuation within a cloud or fog can be w
13、ritten as:dB/km (1)c Kl Mwhere:c : specific attenuation (dB/km) within the cloud;Kl : specific attenuation coefficient (dB/km)/(g/m3);2 Rec. ITU-R P.840-5M : liquid water density in the cloud or fog (g/m3).At frequencies of the order of 100 GHz and above, attenuation due to fog may be significant. T
14、he liquid water density in fog is typically about 0.05 g/m3 for medium fog (visibility of the order of 300 m) and 0.5 g/m3 for thick fog (visibility of the order of 50 m).2 Specific attenuation coefficientA mathematical model based on Rayleigh scattering, which uses a double-Debye model for the diel
15、ectric permittivity ( f ) of water, can be used to calculate the value of Kl for frequencies up to 1 000 GHz:(dB/km)/(g/m3) (2)1(“89.02fKlwhere f is the frequency (GHz), and:(3)“ The complex dielectric permittivity of water is given by:(4)21210)/()/()(“ sspffff (5)21210)/()/()( spfffwhere:0 77.6 103
16、.3 ( 1) (6)1 5.48 (7)2 3.51 (8) 300 / T (9)with T the temperature (K).The principal and secondary relaxation frequencies are:fp 20.09 142 ( 1) 294 ( 1)2 GHz (10)fs 590 1 500 ( 1) GHz (11)Figure 1 shows the values of Kl at frequencies from 5 to 200 GHz and temperatures between 8C and 20C. For cloud a
17、ttenuations, the curve corresponding to 0C should be used.Rec. ITU-R P.840-5 3FIGURE 1Specific attenuation by water droplets at varioustemperatures as function of frequencyFrequency (GHz)Specific attenuationcoefficient, (dB/km)/ (g/m)K l0.010.020.050.10.20.5125105 10 20 50 100 2000 C20 C10 C 8 C3 Cl
18、oud attenuationTo obtain the attenuation due to clouds for a given probability, the statistics of the total columnar content of liquid water L (kg/m2) or, equivalently, mm of precipitable water for a given site must be known yielding:dB for 90 5 (12)sinlKAwhere is the elevation angle and Kl is read
19、from Fig. 1.Statistics of the total columnar content of liquid water may be obtained from radiometric measurements or from radiosonde launches.4 Total columnar content of cloud liquid waterThe total columnar content of cloud liquid water, expressed in kg/m2 or, equivalently, in mm of liquid water, c
20、an be obtained from radiosonde soundings and radiometric measurements. Radiosonde data is widely available; however, it has limited time resolution and is only applicable to zenith paths. The total columnar content of cloud liquid water can be retrieved from radiometric measurements at appropriate f
21、requencies along the desired path.The annual values of total columnar content of cloud liquid water, L (kg/m2), exceeded for 0.1, 0.2, 0.3, 0.5, 1, 2, 3, 5, 10, 20, 30, 50, 60, 70, 80, 90, 95 and 99% of an average year are available in the form of digital maps from the Radiocommunication Study Group
22、 3 website, in the data files 4 Rec. ITU-R P.840-5ESAWRED_xx_v4.TXT, where xx 01, 02, 03, 05, 1, 2, 3, 5, 10, 20, 30, 50, 60, 70, 80, 90, 95 and 99. The data is from 0 to 360 in longitude and from 90 to 90 in latitude, with a resolution of 1.125 in both latitude and longitude. This data is to be use
23、d in conjunction with the companion data files ESALAT_1dot125.TXT and ESALON_1dot125.TXT containing the latitudes and longitudes of the corresponding entries (grid points) in data files ESAWRED_xx_v4.TXT. The total columnar content of cloud liquid water at any desired location on the surface of the
24、Earth can be derived by the following interpolation method:a) determine the two probabilities, pabove and pbelow, above and below the desired probability, p, from the set: 0.1, 0.2, 0.3, 0.5, 1, 2, 3, 5, 10, 20, 30, 50, 60, 70, 80, 90, 95 and 99%;b) for the two probabilities, pabove and pbelow, dete
25、rmine the total columnar content of cloud liquid water, L1, L2, L3, and L4 at the four closest grid points;c) determine the total columnar content of cloud liquid water, Labove and Lbelow, at the probabilities, pabove and pbelow, by performing a bi-linear interpolation of the four values of total co
26、lumnar content of cloud liquid water, L1, L2, L3, and L4 at the four grid points, as described in Recommendation ITU-R P.1144;d) determine the total columnar content of cloud liquid water, L, at the desired probability, p, by interpolating Labove and Lbelow vs. pabove and pbelow to p on a linear L v
27、s. log p scale.Note that the digital maps of total columnar content of cloud liquid water contain the symbol NaN (Not-a-Number) when there is no value of the total content of cloud liquid water corresponding to a given annual probability of exceedance.Example contours of total columnar content of cl
28、oud liquid water are provided in Figures 2, 3, 4, 5, 6, and 7 for exceedance probabilities of 0.1, 0.5, 1, 5, 10 and 20%.FIGURE 24Normalized total columnar content of cloud liquid water (kg/m2) exceeded for 0.1% of the yearLongitude (degrees)80604020020406080Latitude(degrees)150 100 50 0 50 100 1500
29、.20.511222333444 444 40.50.511112222233 4441111112 2222233 344 444 4 44440.2 0.20.5 0.51 12 22 33334444444440.5112222233333 444440.20.20.5 0.50.50.51 1111222223333340.22344 4Rec. ITU-R P.840-5 5FIGURE 3Normalized total columnar content of cloud liquid water (kg/m2) exceeded for 0.5% of the yearLongi
30、tude (degrees)80604020020406080Latitude(degrees)150 100 50 0 50 100 1500.20.50.51 111111122233333333444444444444440.20.20.50.50.5111111222333340.20.20.20.50.50.50.50.50.511111111112 22223 33333334 4440.20.50.51111111122222 222233333440.20.5123FIGURE 4Normalized total columnar content of cloud liquid
31、 water (kg/m2) exceeded for 1% of the yearLongitude (degrees)80604020020406080Latitude(degrees)150 100 50 0 50 100 1500.2 0.50.50.511112222233333333444444440.20.50.50.50.51 111111113330.2 0.20.20.20.5 0.50.50.50.50.50.51111111111222222223333334 40.20.20.20.50.51111122222233330.50.512236 Rec. ITU-R P
32、.840-5FIGURE 5Normalized total columnar content of cloud liquid water (kg/m2) exceeded for 5% of the yearLongitude (degrees)80604020020406080Latitude(degrees)150 100 50 0 50 100 1500.20.20.20.50.50.50.5112222233333330.2 0.20.20.20.5 0.50.50.50.50.50.51 11111112222230.20.20.20.20.20.20.5 0.50.50.5111
33、111 2233330.50.510.20.51122 2 330.2 0.20.50.50.50.50.51111112 22310.50.50.2FIGURE 6Normalized total columnar content of cloud liquid water (kg/m2) exceeded for 10% of the yearLongitude (degrees)80604020020406080Latitude(degrees)150 100 50 0 50 100 1500.2 0.20.20.20.20.20.20.20.20.20.20.20.20.20.5 0.
34、50.5 0.50.50.50.50.50.50.51111111222222233330.20.20.20.20.20.20.20.50.50.50.50.50.50.50.5 11111111122 22 2 33 30.20.20.20.20.20.20.20.20.20.20.20.50.50.5 0.50.50.50.50.511 11111222 33 30.20.5Rec. ITU-R P.840-5 7FIGURE 7Normalized total columnar content of cloud liquid water (kg/m2) exceeded for 20%
35、of the yearLongitude (degrees)80604020020406080Latitude(degrees)150 100 50 0 50 100 1500.20.20.2 0.20.20.20.20.20.20.20.50.50.50.50.511111 22220.20.20.20.20.20.50.511112 20.20.20.20.20.20.50.50.5 1 10.20.20.20.20.20.20.20.5 120.20.20.50.510.20.20.225 Statistical distribution of total columnar conten
36、t of cloud liquid waterThe statistics of the total columnar content of cloud liquid water can be approximated by a log-normal distribution. The mean, m, standard deviation, , and probability of liquid water, Pclw, parameters of the log-normal distribution are available in the form of digital maps fr
37、om the Radiocommunication Study Group 3 website in the data files WRED_LOGNORMAL_MEAN_v4.TXT, WRED_LOGNORMAL_STDEV_v4.TXT, and WRED_LOGNORMAL_PCLW_v4.TXT. The data is from 0 to 360 in longitude and from 90 to 90 in latitude, with a resolution of 1.125 in both latitude and longitude. This data is to
38、be used in conjunction with the companion data files ESALAT_1dot125.TXT and ESALON_1dot125.TXT containing the latitudes and longitudes of the corresponding entries (grid points) in data files WRED_LOGNORMAL_MEAN_v4.TXT, WRED_LOGNORMAL_STDEV_v4.TXT, and WRED_LOGNORMAL_PCLW_v4.TXT. The total columnar
39、content of cloud liquid water at any desired location on the surface of the Earth can be derived by the following interpolation method:a) determine the parameters, m1, m2, m3, m4, 1, 2,3, 4, PCLW1, PCLW2, PCLW3 and PCLW4 at the four closest grid points;b) determine the total columnar content of clou
40、d liquid water L1, L2, L3, and L4 for the desired probability, p, at the four closest grid points from the parameters m1, m2, m3, m4, 1, 2, 3, 4, PCLW1, PCLW2, PCLW3 and PCLW4 as follows:for i = 1, 2, 3, 4 (13)CLWiiQmiL1e8 Rec. ITU-R P.840-5where:(14)xtQde212c) determine the total columnar content o
41、f cloud liquid water at the desired location by performing a bi-linear interpolation of the four values of total columnar content of cloud liquid water, L1, L2, L3, and L4 at the four grid points as described in Recommendation ITU-R P.1144.Note that the digital maps of log-normal parameters contain the symbol NaN (Not-a-Number) when there is no value of total content of cloud liquid water corresponding to a given annual probability of exceedance in the digital maps of total columnar content of cloud liquid water.
