1、206 Rec. ITU-R PN.453-4 RECOMMENDATION ITU-R PN.453-4 THE RADIO REFRACTIVE INDEX: ITS FORMULA AND REFRACTIVITY DATA (Question ITU-R 201/3) (1970-1986-1990- 1992-1994) The ITU Radiocommunication Assembly, considering the necessity of utilizing a single formula for calculation of the index of refracti
2、on of the atmosphere; a) b) the need for reference data on refractivity and refractivity gradients all over the world, recommends that the atmospheric radio refractive index, n, be computed by means of the formula given in Annex 1; that refractivity data given on world charts in Annex 1 should be us
3、ed, except if more reliable local data are 1. 2. available. ANNEX 1 1. The formula for the radio refractive index The atmospheric radio refractive index, n, can be computed by the following formula: n = 1 + N x lo4 where: N: radio refractivity expressed by: N = 77.6 T (P + 4810 +) P : atmospheric pr
4、essure (hPa)* e : T: absolute temperature (K). This expression may be used for all radio frequencies; for frequencies up to 100 GHz, the error is less For ready reference, the relationship between water vapour pressure e and relative humidity is given by: water vapour pressure (hPa)* than 0.5%. es e
5、 1 O0 with: e, = nexp(-) t+c where: H: relative humidity (70) r : Celsius temperature (OC) (3) (4) * It should be noted that the World Meteorological Organization (WMO) has recommended the adoption of hPa, which is numerically identical to mb, as the unit of atmospheric pressure. ITU-R ITU-R PN-45
6、3-4 94 = 4855232 0522522 TT4 Rec. ITU-R PN.453-4 e, : saturation vapour pressure (hPa) at the temperature r (OC) and the coefficients a, b, c, are: for water: for ice: a = 6.1121 a = 6.1115 b = 17.502 b = 22.452 c = 240.97 c = 272.55 (valid between -20“ to +50“, with an accuracy of _+0.20%) (valid b
7、etween -50“ and O“, with an accuracy of _+0.20%) Vapour pressure e is obtained from the water vapour density p using the equation: hPa 207 where p is given in g/m3. Representative values of p are given in Recommendation ITU-R PN.836. 2. Surface radio refractivity World charts of monthly mean values
8、of No, extrapolated to sea level for reference purposes, are presented in Figs. 1 and 2 for February and August respectively. In accordance with Recommendation IT-R PN.369, the value of refractivity at the Earths surface may be computed by means of the formula: where: h, : height of the Earths surfa
9、ce above sea level (km), and ho : constant usually taken equal to 7.35 km. It is to be noted, however, that the contours of Figs. 1 and 2 were derived using a value of ho = 9.5 km. 3. Vertical refractivity gradients The vertical gradient of radio refractivity in the lowest layer of the atmosphere is
10、 an important parameter for the estimation of path clearance and propagation effects such as ducting, surface reflection and multipath on terrestrial line-of-sight links. Figures 3 to 6 present isopleths of monthly mean decrease (Le. lapse) in radio refractivity over a 1 km layer from the surface. T
11、he change in radio refractivity, LW, was calculated from: where NI is the radio refractivity at a height of 1 km above the surface of the Earth. The AN values were not reduced to a reference surface. Refractivity gradient statistics for the lowest 100 m from the surface of the Earth are used to esti
12、mate the probability of occurrence of ducting and multipath conditions. Where more reliable local data are not available, the charts in Figs. 7 to 10 give such statistics for the world. ITU-R ITU-R PN-453-4 94 4855212 0522523 930 = 208 Rec. ITU-R PN.453-4 FIGURE 1 Monthly mean values of No: February
13、 1 . N 80 8 10 10 20 30 1 - N IO 10 60 so 40 30 20 10 N 0 5 10 20 3c 4C 5( 6( 71 8( 5 - ITU-R ITU-R PN-453-4 9La U 4855232 0522524 877 - n io o io lo 80 10 20 IO N O S 10 20 30 40 50 60 70 80 S - Rec. ITU-R PN.453-4 209 FIGURE 2 Monthly mean values of No: August - N 30 TO 60 50 40 30 20 10 N 0 5 10
14、20 3c 4c 5( 61 7 80 5 - 210 ITU-R ITU-R PN.453-4 94 4855232 0522526 64T Rec. ITU-R PN.453-4 -. FIGURE 3 Monthly mean values of M. February FIGURE 4 Monthly mean values of AI% May 1TU-R ITU-R PN.453-4 94 Y855212 0522527 586 Rec. ITU-R PN.453-4 FIGURE 5 Monthly mean values of August FIGURE 6 Monthly m
15、ean values of W. November 211 212 ITU-R ITLI-R PN.453-4 94 4855212 0522528 412 Rec. ITU-R PN.453-4 FIGURE 7 Percentage of time gradient 2-100 (Nlkm): February FIGURE 8 Percentage of time gradient 5-100 (N/km): May FGURE 9 Percentage of time gradient -100 (N/km): August FIGURE 10 Percentage of time gradient I-100 (N/km): November 213
