1、 Recommendation ITU-R P.840-6(09/2013)Attenuation due to clouds and fogP SeriesRadiowave propagationii Rec. ITU-R P.840-6 Foreword The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and economical use of the radio-frequency spectrum by all radiocommunication se
2、rvices, 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 Radiocommunicati
3、on Assemblies 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 b
4、y patent 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.i
5、nt/publ/R-REC/en) Series Title BO Satellite delivery BR Recording for production, archival and play-out; film for television BS Broadcasting service (sound) BT Broadcasting service (television) F Fixed service M Mobile, radiodetermination, amateur and related satellite services P Radiowave propagati
6、on RA Radio astronomy RS Remote sensing systems S Fixed-satellite service SA Space applications and meteorology SF Frequency sharing and coordination between fixed-satellite and fixed service systems SM Spectrum management SNG Satellite news gathering TF Time signals and frequency standards emission
7、s V Vocabulary and related subjects Note: This ITU-R Recommendation was approved in English under the procedure detailed in Resolution ITU-R 1. Electronic Publication Geneva, 2013 ITU 2013 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without written pe
8、rmission of ITU. Rec. ITU-R P.840-6 1 RECOMMENDATION ITU-R P.840-6 Attenuation due to clouds and fog (Question ITU-R 201/3) (1992-1994-1997-1999-2009-2012-2013) Scope This Recommendation provides methods to predict the attenuation due to clouds and fog on Earth-space paths. The ITU Radiocommunicatio
9、n Assembly, considering a) that there is a 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-availabil
10、ity systems; c) that for the calculation of 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, recommends 1 that the curves, models and maps given in Annex 1 should be used
11、 for the calculation of attenuation due to clouds 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 clou
12、d liquid water. Annex 1 1 Introduction For clouds 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 s
13、pecific attenuation within a cloud or fog can be written as: c= KlM dB/km (1) where: c: specific attenuation (dB/km) within the cloud Kl: specific attenuation coefficient (dB/km)/(g/m3) M : liquid water density in the cloud or fog (g/m3). 2 Rec. ITU-R P.840-6 At frequencies of the order of 100 GHz a
14、nd above, attenuation due to fog may be significant. The liquid water density in fog is typically about 0.05 g/m3for medium fog (visibility of the order of 300 m) and 0.5 g/m3for thick fog (visibility of the order of 50 m). 2 Specific attenuation coefficient A mathematical model based on Rayleigh sc
15、attering, which uses a double-Debye model for the dielectric permittivity ( f ) of water, can be used to calculate the value of Klfor frequencies up to 1 000 GHz: )1(“819.02+=fKl(dB/km)/(g/m3) (2) where f is the frequency (GHz), and: “2+= (3) The complex dielectric permittivity of water is given by:
16、 221210)/(1)()/(1)()(“ssppfffffffff+= (4) 2221210)/(1)/(1)( +=spfffff (5) where: 0= 77.66 + 103.3 ( 1) (6) 1= 0.0671null(7) 2 = 3.52 (8) = 300 / T (9) with T the temperature (K). The principal and secondary relaxation frequencies are: fp= 20.20 146 ( 1) + 316 ( 1)2GHz (10) fs= 39.8fpGHz (11) 3 Cloud
17、 attenuation along slant paths To obtain the attenuation due to clouds along slant paths for a given probability, the statistics of the total columnar content of liquid water reduced to a temperature of 0C, Lred(kg/m2or, equivalently, mm) for a given site must be known yielding: sinlredKLA = dB for
18、90 5 (12) Rec. ITU-R P.840-6 3 where is the elevation angle and Klis calculated from equations (2) to (11) for a water temperature of 0C. The annual values of total columnar content of reduced cloud liquid water, Lred(kg/m2), exceeded for 0.1, 0.2, 0.3, 0.5, 1, 2, 3, 5, 10, 20, 30, 50, 60, 70, 80, 9
19、0, 95 and 99% of an average year are an integral part of this Recommendation and are available in the form of digital maps. The monthly values of total columnar content of reduced cloud liquid water, Lred(kg/m2), exceeded for 1, 2, 3, 5, 10, 20, 30, 50, 60, 70, 80, 90, 95 and 99% of each average mon
20、th are an integral part of this Recommendation and are available in the form of digital maps. The annual and monthly values of total columnar content are provided in the file R-REC-P.840-6-201309-I!ZIP-E. The data is from 0 to 360 in longitude and from +90 to 90 in latitude, with a resolution of 1.1
21、25 in both latitude and longitude. The total columnar content of reduced cloud liquid water at any desired location on the surface of the Earth can be derived by the following interpolation method: a) determine the two probabilities, paboveand pbelow, above and below the desired probability, p, from
22、 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% for annual statistics and from the set: 1, 2, 3, 5, 10, 20, 30, 50, 60, 70, 80, 90, 95 and 99% for monthly statistics; b) for the two probabilities, paboveand pbelow, determine the total columnar content of reduced
23、cloud liquid water, Lred1, Lred2, Lred3, and Lred4at the four closest grid points; c) determine the total columnar content of reduced cloud liquid water, Lredaboveand Lredbelow, at the probabilities, paboveand pbelow, by performing a bi-linear interpolation of the four values of total columnar conte
24、nt of reduced cloud liquid water, Lred1, Lred2, Lred3, and Lred4at the four grid points, as described in Recommendation ITU-R P.1144; d) determine the total columnar content of reduced cloud liquid water, Lred, at the desired probability, p, by interpolating Lredaboveand Lredbelowvs. paboveand pbelo
25、wto p on a linear Lredvs. log p scale. 3.1 Approximation of Lredby a log-normal distribution The annual statistics of the total columnar content of reduced cloud liquid water content can be approximated by a log-normal distribution. The mean, m, standard deviation, , and probability of reduced liqui
26、d water, Pclw, parameters of the log-normal distribution are an integral part of this Recommendation in the form of digital maps. The total columnar content of reduced cloud liquid water at any desired location on the surface of the Earth can be derived by the following interpolation method: a) dete
27、rmine the parameters, m1, m2, m3, m4, 1, 2, 3, 4, PCLW1, PCLW2, PCLW3 andPCLW4at the four closest grid points; b) determine the total columnar content of reduced cloud liquid water Lred1, Lred2, Lred3, and Lred4for the desired probability, p, at the four closest grid points from the parameters m1, m
28、2, m3, m4, 1, 2, 3,4, PCLW1, PCLW2, PCLW3andPCLW4as follows: +=CLWiiiPPQmiredL1,e for i = 1, 2, 3, 4 (13) 4 Rec. ITU-R P.840-6 where: ()=xttxQ de2122(14) c) determine the total columnar content of reduced cloud liquid water at the desired location by performing a bi-linear interpolation of the four values of total columnar content of reduced cloud liquid water, Lred1, Lred2, Lred3, and Lred4at the four grid points as described in Recommendation ITU-R P.1144.
