1、Rec. ITU-R PN.835-1 221 RECOMMENDATION ITU-R PN.835-1 REFERENCE ST.410ARD ATMOSPHERE FOR GASEOUS ATTENUATION (Question ITU-R 201/3) (1992-1994) The ITU Radiocommunication Assembly, corzsidei-irzg the necessity for a reference standard atmosphere for use in calculating gaseous attenuation along an a)
2、 Earth-space path, reconzmends 1. as a function of altitude, for calculating gaseous attenuation, except when more reliable local data are available. that the standard atmosphere in Annex 1 be used to determine temperature, pressure and water-vapour pressure ANNEX I 1. Temperature and pressure atmos
3、phere is divided into seven successive layers showing linear variation with temperature, as given in Fig. 1. The reference standard atmosphere is based on the United States Standard Atmosphere, 1976, in which the 90 80 70 60 s. E 50 h e, U 1 .z 40 2 30 20 10 O FiGRE 1 Reference profile of atmospheri
4、c temperature 160 180 200 220 240 260 280 300 Temperature (K) “ MI1 ITU-R RECMNaPN. 835-1 94 m 4855212 0523968 835 m Subscript, i O 1 2 3 4 5 6 7 222 Rec. ITU-R PN.835-1 The temperature Tat height h is given by: T(h) = Ti + Li (It - Hi) K where: Ti = T(HJ and Li is the temperature gradient starting
5、at altitude Hi and is given in Table 1. Altitude, Hj km) O 11 20 32 47 51 71 85 TABLE 1 I Temperature gradient, Li mm) -6.5 0.0 +1 .o +2.8 0.0 -2.8 -2.0 When the temperature gradient Li # O, pressure is given by the equation: Ti 134.163 I Li hPa t 3) Tj + Li(h - Hi) P(h) = Pi and when the temperatur
6、e gradient Li = O, pressure is obtained from the equation: hPa 1 - 34.163; - Hi) P(h) = Piexp (4) The ground-level standard temperature and pressure are: To = 288.15 K Po = 1013.25 hPa Note that above about 85 km altitude, local thermodynamic equilibrium of the atmosphere starts to break down, and t
7、he hydrostatic equation, on which the above equations are based, is no longer valid. , . 2. Water-vapour pressure the equation: The distribution of water vapour in the atmosphere is generally highly variable, but may be approximated by p(h) = poexp(-h/ho) gim3 (6) where the scale height ho = 2 km, a
8、nd the standard ground-level water-vapour density is: po = 7.5 gim3 (7) Vapour pressure is obtained from the density using the equation (see Recommendation ITU-R PN.453): hPa ITU-R RECMNrPN. 835-L 94 4855212 05219b9 771 Rec. ITU-R PN.835-1 223 Water-vapour density decreases exponentially with increa
9、sing altitude, up to an altitude where the mixing ratio e(h)/P(h) = 2 x 1k6. Above this altitude, the mixing ratio is assumed to be constant. 3. Dry atmosphere for attenuation calculations The profile of the density of atmospheric gases other than water vapour (the “dry atmosphere”) may be found from the temperature and pressure profiles given in 0 1 above. For attenuation calculations, this density profile may be approximated by an exponential profile according to equation (6) with:
