1、Rec. ITU-R PN.840-1 RECOMMENDATION ITU-R PN.840- 1 ATTENUATION DUE TO CLOUDS AND FOG (Question ITU-R 201/3) 245 (1 992- 1994) The ITU Radiocommunication Assembly, considering that there is a need to give guidance to engineers in the design of Earth-space telecommunication systems for a) frequencies
2、higher than 10 GHz; b) 10 GHz or low-availability systems; c) the basic phenomena are well understood, that attenuation due to clouds may be a factor of importance especially for microwave systems well above that even if no global prediction of cloud attenuation exists due to the lack of extensive c
3、loud measurements recommends 1. clouds and fog. that the curves and prediction models given in Annex 1 should be used for the calculation of attenuation due to ANNEX 1 1. Introduction For clouds or fog consisting entirely of small droplets, generally less than 0.01 cm, the Rayleigh approximation is
4、valid 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 written as: YC = KlM dB/km (1) where: yc : specific attenuation (dB/km) within the cloud Ki : specific attenuation coefficient (dB/km)/(
5、g/m3) M: liquid water content of the cloud or fog (gim3). At frequencies of the order of 100 GHz and above, attenuation due to fog may be significantThe liquid water content of 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
6、 the order of 50 m). 2. Specific attenuation coefficient A mathematical model based on Rayleigh scattering, which uses a double-Debye model for the dielectric permittivity E (f) of water, can be used to calculate the value of Ki for frequencies up to 1 O00 GHz: 0.819f E“(1 + Tp) Ki = dB/km/g/m3 wher
7、efis the frequency, in GHz, and: 2 + E q=- E“ (3) 236 where: ITU-R RECNN*pM- 840-1 94 48552112 05211773 32T Rec. ITU-R PN.840-1 The complex dielectric permittivity of water is given by: e = OO/T = 77.6 + 103.3(0 - 1) EI = 5.48 2 = 3.51 (9) with T: temperature (K). The principal and secondary relaxat
8、ion frequencies are: Figure 1 shows the values of KI at frequencies from 5 to 200 GHz and temperatures between -8“ C and 20“ C. For cloud attenuations, the curve corresponding to O“ C should be used. FIGURE 1 Specific attenuation by water droplets at various temperatures as function of frequency 10
9、5 . d r: e u e .- E o u 3 u rn .- B 2 1 0.5 0.2 o. 1 0.05 0.02 0.01 I ITU-R RECMNMPN. BYO-1 9Y m 4855232 0523972 266 m Rec. ITU-R PN.840-1 247 3. Cloud attenuation To obtain the attenuation due to clouds for a given probability value, the statistics of the total column of liquid water W, (kg/m2) for
10、 a given site must be known yielding: dB where is the elevation angle and KI is read from Fig. 1. Statistics of the total column of liquid water may be obtained from radiometric measurements or from radiosonde launches. In the absence of statistics of the liquid water column an estimate of the attenuation may be obtained using a value of W, of 0.5 kg/m, corresponding to a value of liquid water content of 0.5 g/m3 and a cloud thickness of 1 km.
