1、 Recommendation ITU-R P.840-7 (12/2017) Attenuation due to clouds and fog P Series Radiowave propagation ii Rec. ITU-R P.840-7 Foreword The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and economical use of the radio-frequency spectrum by all radiocommunicati
2、on services, 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 Radiocommun
3、ication 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 declarati
4、ons by 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.
5、itu.int/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 prop
6、agation 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 emi
7、ssions 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, 2017 ITU 2017 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without writt
8、en permission of ITU. Rec. ITU-R P.840-7 1 RECOMMENDATION ITU-R P.840-7 Attenuation due to clouds and fog (Question ITU-R 201/3) (1992-1994-1997-1999-2009-2012-2013-2017) Scope This Recommendation provides methods to predict the attenuation due to clouds and fog on Earth-space paths. The ITU Radioco
9、mmunication 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
10、-availability 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; d) that local measured data of the total columnar content of cloud
11、liquid water may not be available, recommends 1 that if local measured data of the total columnar content of cloud liquid water is not available, the method given in 3.1 of Annex 1 should be used for the prediction of the attenuation due to clouds and fog; 2 that if local measured data of the total
12、columnar content of cloud liquid water is available, the method given in 3.2 of Annex 1 should be used for the calculation of the attenuation due to clouds; 3 that the information in 4 of Annex 1 should be used for global calculations of propagation effects, required by, inter alia, space-time chann
13、el models, that require an analytic expression for the statistics of the total columnar content of cloud 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 up to 200 GHz, and t
14、he specific attenuation within a cloud or fog can be written as: 2 Rec. ITU-R P.840-7 (,) = (,) (dB/km) (1) where: c : specific attenuation (dB/km) within the cloud Kl : cloud liquid water specific attenuation coefficient (dB/km)/(g/m3) M : liquid water density in the cloud or fog (g/m3) f: frequenc
15、y (GHz) T : cloud liquid water temperature (K). At frequencies of the order of 100 GHz and above, attenuation due to fog may be significant. The 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 or
16、der of 50 m). 2 Cloud liquid water specific attenuation coefficient A mathematical model based on Rayleigh scattering, which uses a double-Debye model for the dielectric permittivity ( f ) of water, can be used to calculate the value of Kl for frequencies up to 200 GHz: (dB/km)/(g/m3)(2) where f is
17、the frequency (GHz), and: (3) The complex dielectric permittivity of water is given by: (4) (5) where: 0 = 77.66 + 103.3 ( 1) (6) 1 = 0.06710 (7) 2 = 3.52 (8) = 300 / T (9) and T is the liquid water temperature (K). The principal relaxation frequency, fp, and secondary relaxation frequency, fs, are:
18、 fp = 20.20 146 ( 1) + 316 ( 1)2 (GHz) (10) fs = 39.8fp (GHz) (11) K l f , T( ) = 0 .8 1 9 fe “(1 + h 2 )“ 2 221210 )/(1 )( )/(1 )( )(“ sspp fff ffff ff 2221210 )/(1 )/(1 )( sp fffffRec. ITU-R P.840-7 3 3 Cloud attenuation along slant paths If local measured data of the total columnar content of clo
19、ud liquid water, L (kg/m2 or, equivalently, mm), is not available, the method in 3.1 should be used to predict the cloud attenuation along slant paths. This prediction method is based on ERA-40 data where the total columnar content of cloud liquid water reduced to a fixed temperature of 273.15 K, Lr
20、ed (kg/m2 or, equivalently, mm), is used. If local measured data of the total columnar content of cloud liquid water, L, is available from other sources, e.g. from ground radiometric measurements, Earth observation products or meteorological numerical products, the method in 3.2 should be used to ca
21、lculate the cloud attenuation along slant-paths. 3.1 Distribution of cloud attenuation along slant paths based on global digital maps The slant path cloud attenuation, A, for a given probability, p, is: (dB) for 90 5 (12) where Lred is the total columnar content of liquid water reduced to a temperat
22、ure of 273.15 K, Lred (kg/m2 or, equivalently, mm) at the probability p, is the elevation angle, and Kl is calculated using equations (2) to (11) for a water temperature of 273.15 K. The annual values of total columnar content of reduced cloud liquid water, Lred (kg/m2), exceeded for the probability
23、 levels of 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 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 t
24、he probability levels of 1, 2, 3, 5, 10, 20, 30, 50, 60, 70, 80, 90, 95 and 99% of each average month are an integral part of this Recommendation and are available in the form of digital maps. These digital maps are available in the Supplement file R-REC-P.840-7-Maps.zip. The data is from 0 to 360 i
25、n longitude and from +90 to 90 in latitude, with a resolution of 1.125 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
26、, 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% 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 probabilitie
27、s, pabove and pbelow, determine the total columnar content of reduced cloud liquid water, Lred1, Lred2, Lred3 and Lred4 at the four closest grid points; c) determine the total columnar content of reduced cloud liquid water, Lredabove and Lredbelow, at the probabilities, pabove and pbelow, by perform
28、ing a bi-linear interpolation of the four values of total columnar content of reduced cloud liquid water, Lred1, Lred2, Lred3 and Lred4 at 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 pr
29、obability, p, by interpolating Lredabove and Lredbelow vs. pabove and pbelow to p on a linear Lred vs. log p scale. A = L red K l f , 2 7 3 .1 5( )sin j4 Rec. ITU-R P.840-7 3.2 Cloud attenuation along slant paths based on local data The slant path cloud attenuation, A, is: s in 15273 .,fLKA*l (dB) f
30、or 90 5 (13) where L is the total columnar content of liquid water (kg/m2 or, equivalently, mm), is the elevation angle, and is calculated as follow: (dB/km)/(g/m3) (14) where is given in equation (3), “ is given in equation (4), and the liquid water temperature, T, is 273.15 K. 4 Approximation of L
31、red by 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 digital maps of mean, m, standard deviation, , and probability of non-zero reduced total columnar content of cloud liquid w
32、ater, Pclw, parameters of the log-normal distribution are an integral part of this Recommendation. 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 parameters, m1, m2, m
33、3, m4, 1, 2, 3, 4, PCLW1, PCLW2, PCLW3 and PCLW4 at the four closest grid points; b) determine the total columnar content of reduced cloud liquid water Lred1, Lred2, Lred3, and Lred4 for the desired probability, p, at the four closest grid points from the parameters m1, m2, m3, m4, 1, 2, 3, 4, PCLW1
34、, PCLW2, PCLW3 and PCLW4 as follows: for i = 1, 2, 3, 4 (15) where: (16) 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 Lred4 at the four grid points as described in Recommendation ITU-R P.1144. K l* f , T( ) = 0 . 8 1 9 1 . 9 4 7 9 1 0 - 4 f 2 . 3 0 8 + 2 . 9 4 2 4 f 0 . 7 4 3 6 - 4 . 9 4 5 1( )e ( 1 + h 2 ) C L W iii P PQmir e dL 1, e xt txQ de21 22
