1、 Recommendation ITU-R P.676-10(09/2013)Attenuation by atmospheric gasesP SeriesRadiowave propagationii Rec. ITU-R P.676-10 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 s
2、ervices, 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 Radiocommunicat
3、ion 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
4、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.itu.
5、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 propagat
6、ion 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 emissio
7、ns 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 p
8、ermission of ITU. Rec. ITU-R P.676-10 1 RECOMMENDATION ITU-R P.676-10 Attenuation by atmospheric gases (Question ITU-R 201/3) (1990-1992-1995-1997-1999-2001-2005-2007-2009-2012-2013) Scope This Recommendation provides methods to estimate the attenuation of atmospheric gases on terrestrial and slant
9、paths using: a) an estimate of gaseous attenuation computed by summation of individual absorption lines that is valid for the frequency range 1-1 000 GHz; and b) a simplified approximate method to estimate gaseous attenuation that is applicable in the frequency range 1-350 GHz. The ITU Radiocommunic
10、ation Assembly, considering a) the necessity of estimating the attenuation by atmospheric gases on terrestrial and slant paths, recommends 1 that, for general application, the procedures in Annex 1 be used to calculate gaseous attenuation at frequencies up to 1 000 GHz; 2 that, for approximate estim
11、ates of gaseous attenuation in the frequency range 1 to 350 GHz, the computationally less intensive procedure given in Annex 2 be used. Annex 1 Line-by-line calculation of gaseous attenuation 1 Specific attenuation The specific attenuation at frequencies up to 1 000 GHz due to dry air and water vapo
12、ur, can be evaluated most accurately at any value of pressure, temperature and humidity by means of a summation of the individual resonance lines from oxygen and water vapour, together with small additional factors for the non-resonant Debye spectrum of oxygen below 10 GHz, pressure-induced nitrogen
13、 attenuation above 100 GHz and a wet continuum to account for the excess water vapour-absorption found experimentally. Figure 1 shows the specific attenuation using the model, calculated from 0 to 1 000 GHz at 1 GHz intervals, for a pressure of 1 013 hPa, temperature of 15 C for the cases of a water
14、-vapour density of 7.5 g/m3(Curve A) and a dry atmosphere (Curve B). 2 Rec. ITU-R P.676-10 Near 60 GHz, many oxygen absorption lines merge together, at sea-level pressures, to form a single, broad absorption band, which is shown in more detail in Fig. 2. This figure also shows the oxygen attenuation
15、 at higher altitudes, with the individual lines becoming resolved at lower pressures. Some additional molecular species (e.g. oxygen isotopic species, oxygen vibrationally excited species, ozone, ozone isotopic species, and ozone vibrationally excited species, and other minor species) are not includ
16、ed in the line-by-line prediction method. These additional lines are insignificant for typical atmospheres, but may be important for a dry atmosphere. For quick and approximate estimates of specific attenuation at frequencies up to 350 GHz, in cases where high accuracy is not required, simplified al
17、gorithms are given in Annex 2 for restricted ranges of meteorological conditions. The specific gaseous attenuation is given by: dB/km)(1820.0 f“Nfwo=+= (1) where oand ware the specific attenuations (dB/km) due to dry air (oxygen, pressure-induced nitrogen and non-resonant Debye attenuation) and wate
18、r vapour, respectively, and where f is the frequency (GHz) and N ( f ) is the imaginary part of the frequency-dependent complex refractivity: +=iDiif“NFSfN“ )()( (2) Siis the strength of the i-th line, Fiis the line shape factor and the sum extends over all the lines (for frequencies, f, above 118.7
19、50343 GHz oxygen line, only the oxygen lines above 60 GHz complex should be included in the summation; the summation should begin at i = 38 rather than at i = 1);)( f“NDis the dry continuum due to pressure-induced nitrogen absorption and the Debye spectrum. The line strength is given by: vapourwater
20、for)1(exp10 oxygenfor)1(exp1025.3112371bebapaSi=(3) where: p : dry air pressure (hPa) e : water vapour partial pressure in hPa (total barometric pressure ptot = p + e) = 300/T T : temperature (K). Rec. ITU-R P.676-10 3 FIGURE 1 Specific attenuation due to atmospheric gases, calculated at 1 GHz inter
21、vals, including line centres P.0676-0101002003004005006007008009001 000S p e c if ic a t t e n u a tio n (d B / k m )Frequency (GHz)1051041031021011 101102103DryStandard4 Rec. ITU-R P.676-10 FIGURE 2 Specific attenuation in the range 50-70 GHz at the altitudes indicated P.0676-0250103525456586062646
22、66870S p e c if ic a tte n u a tio n ( d B / k m )Frequency (GHz)10210111011020 5 10 15 20Local values of p, e and T measured profiles (e.g. using radiosondes) should be used; however, in the absence of local information, the reference standard atmospheres described in Recommendation ITU-R P.835 sho
23、uld be used. (Note that where total atmospheric attenuation is being calculated, the same-water vapour partial pressure is used for both dry-air and water-vapour attenuations.) Rec. ITU-R P.676-10 5 The water-vapour partial pressure, e, may be obtained from the water-vapour density using the express
24、ion: 7.216Te= (4) The coefficients a1, a2are given in Table 1 for oxygen, those for water vapour, b1and b2, are given in Table 2. The line-shape factor is given by: ()()()() +=2222ffffffffffffffFiiiiii(5) where fiis the line frequency and f is the width of the line: vapourwaterfor)(10oxygenfor)1.1(1
25、0644543)8.0(43bbaebpbepaf+=+=(6a) The line width f is modified to account for Doppler broadening: vapourfor water 101316.2217.0535.0oxygenfor1025.2212262+=+=ifffff(6b) is a correction factor which arises due to interference effects in oxygen lines: vapourfor water 0 oxygenfor )(10)(8.0465=+=epaa(7)
26、The spectroscopic coefficients are given in Tables 1 and 2. 6 Rec. ITU-R P.676-10 TABLE 1 Spectroscopic data for oxygen attenuation f0a1a2a3a4a5a650.474214 0.975 9.651 6.690 0.0 2.566 6.850 50.987745 2.529 8.653 7.170 0.0 2.246 6.800 51.503360 6.193 7.709 7.640 0.0 1.947 6.729 52.021429 14.320 6.819
27、 8.110 0.0 1.667 6.640 52.542418 31.240 5.983 8.580 0.0 1.388 6.526 53.066934 64.290 5.201 9.060 0.0 1.349 6.206 53.595775 124.600 4.474 9.550 0.0 2.227 5.085 54.130025 227.300 3.800 9.960 0.0 3.170 3.750 54.671180 389.700 3.182 10.370 0.0 3.558 2.654 55.221384 627.100 2.618 10.890 0.0 2.560 2.952 5
28、5.783815 945.300 2.109 11.340 0.0 1.172 6.135 56.264774 543.400 0.014 17.030 0.0 3.525 0.978 56.363399 1331.800 1.654 11.890 0.0 2.378 6.547 56.968211 1746.600 1.255 12.230 0.0 3.545 6.451 57.612486 2120.100 0.910 12.620 0.0 5.416 6.056 58.323877 2363.700 0.621 12.950 0.0 1.932 0.436 58.446588 1442.
29、100 0.083 14.910 0.0 6.768 1.273 59.164204 2379.900 0.387 13.530 0.0 6.561 2.309 59.590983 2090.700 0.207 14.080 0.0 6.957 0.776 60.306056 2103.400 0.207 14.150 0.0 6.395 0.699 60.434778 2438.000 0.386 13.390 0.0 6.342 2.825 61.150562 2479.500 0.621 12.920 0.0 1.014 0.584 61.800158 2275.900 0.910 12
30、.630 0.0 5.014 6.619 62.411220 1915.400 1.255 12.170 0.0 3.029 6.759 62.486253 1503.000 0.083 15.130 0.0 4.499 0.844 62.997984 1490.200 1.654 11.740 0.0 1.856 6.675 63.568526 1078.000 2.108 11.340 0.0 0.658 6.139 64.127775 728.700 2.617 10.880 0.0 3.036 2.895 64.678910 461.300 3.181 10.380 0.0 3.968
31、 2.590 65.224078 274.000 3.800 9.960 0.0 3.528 3.680 65.764779 153.000 4.473 9.550 0.0 2.548 5.002 66.302096 80.400 5.200 9.060 0.0 1.660 6.091 66.836834 39.800 5.982 8.580 0.0 1.680 6.393 67.369601 18.560 6.818 8.110 0.0 1.956 6.475 67.900868 8.172 7.708 7.640 0.0 2.216 6.545 68.431006 3.397 8.652
32、7.170 0.0 2.492 6.600 68.960312 1.334 9.650 6.690 0.0 2.773 6.650 118.750334 940.300 0.010 16.640 0.0 0.439 0.079 Rec. ITU-R P.676-10 7 TABLE 1 (end) f0a1a2a3a4a5a6368.498246 67.400 0.048 16.400 0.0 0.000 0.000 424.763020 637.700 0.044 16.400 0.0 0.000 0.000 487.249273 237.400 0.049 16.000 0.0 0.000
33、 0.000 715.392902 98.100 0.145 16.000 0.0 0.000 0.000 773.839490 572.300 0.141 16.200 0.0 0.000 0.000 834.145546 183.100 0.145 14.700 0.0 0.000 0.000 TABLE 2 Spectroscopic data for water-vapour attenuation f0b1b2b3b4b5b622.235080 0.1130 2.143 28.11 .69 4.800 1.00 67.803960 0.0012 8.735 28.58 .69 4.9
34、30 .82 119.995940 0.0008 8.356 29.48 .70 4.780 .79 183.310091 2.4200 .668 30.50 .64 5.300 .85 321.225644 0.0483 6.181 23.03 .67 4.690 .54 325.152919 1.4990 1.540 27.83 .68 4.850 .74 336.222601 0.0011 9.829 26.93 .69 4.740 .61 380.197372 11.5200 1.048 28.73 .54 5.380 .89 390.134508 0.0046 7.350 21.52
35、 .63 4.810 .55 437.346667 0.0650 5.050 18.45 .60 4.230 .48 439.150812 0.9218 3.596 21.00 .63 4.290 .52 443.018295 0.1976 5.050 18.60 .60 4.230 .50 448.001075 10.3200 1.405 26.32 .66 4.840 .67 470.888947 0.3297 3.599 21.52 .66 4.570 .65 474.689127 1.2620 2.381 23.55 .65 4.650 .64 488.491133 0.2520 2.
36、853 26.02 .69 5.040 .72 503.568532 0.0390 6.733 16.12 .61 3.980 .43 504.482692 0.0130 6.733 16.12 .61 4.010 .45 547.676440 9.7010 .114 26.00 .70 4.500 1.00 552.020960 14.7700 .114 26.00 .70 4.500 1.00 556.936002 487.4000 .159 32.10 .69 4.110 1.00 620.700807 5.0120 2.200 24.38 .71 4.680 .68 645.86615
37、5 0.0713 8.580 18.00 .60 4.000 .50 658.005280 0.3022 7.820 32.10 .69 4.140 1.00 752.033227 239.6000 .396 30.60 .68 4.090 .84 841.053973 0.0140 8.180 15.90 .33 5.760 .45 859.962313 0.1472 7.989 30.60 .68 4.090 .84 8 Rec. ITU-R P.676-10 TABLE 2 (end) f0b1b2b3b4b5b6899.306675 0.0605 7.917 29.85 .68 4.5
38、30 .90 902.616173 0.0426 8.432 28.65 .70 5.100 .95 906.207325 0.1876 5.111 24.08 .70 4.700 .53 916.171582 8.3400 1.442 26.70 .70 4.780 .78 923.118427 0.0869 10.220 29.00 .70 5.000 .80 970.315022 8.9720 1.920 25.50 .64 4.940 .67 987.926764 132.1000 .258 29.85 .68 4.550 .90 1 780.000000 22 300.0000 .9
39、52 176.20 .50 30.500 5.00 The dry air continuum arises from the non-resonant Debye spectrum of oxygen below 10 GHz and a pressure-induced nitrogen attenuation above 100 GHz. +=5.155.112252109.11104.111014.6)(fpdfdpff“ND(8) where d is the width parameter for the Debye spectrum: ()8.04106.5 +=epd (9)
40、2 Path attenuation 2.1 Terrestrial paths For a terrestrial path, or for slightly inclined paths close to the ground, the path attenuation, A, may be written as: () dB00rrAwo+= (10) where r0is path length (km). 2.2 Slant paths This section gives a method to integrate the specific attenuation calculat
41、ed using the line-by-line model given above, at different pressures, temperatures and humidities through the atmosphere. By this means, the path attenuation for communications systems with any geometrical configuration within and external to the Earths atmosphere may be accurately determined simply
42、by dividing the atmosphere into horizontal layers, specifying the profile of the meteorological parameters pressure, temperature and humidity along the path. In the absence of local profiles, from radiosonde data, for example, the reference standard atmospheres in Recommendation ITU-R P.835 may be u
43、sed, either Rec. ITU-R P.676-10 9 for global application or for low (annual), mid (summer and winter) and high latitude (summer and winter) sites. Figure 3 shows the zenith attenuation calculated at 1 GHz intervals with this model for the global reference standard atmosphere in Recommendation ITU-R
44、P.835, with horizontal layers 1 km thick and summing the attenuations for each layer, for the cases of a moist atmosphere (Curve A) and a dry atmosphere (Curve B). The total slant path attenuation, A(h, ), from a station with altitude, h, and elevation angle, , can be calculated as follows when 0: (
45、)()=hHHhA dsin, (11) where the value of can be determined as follows based on Snells law in polar coordinates: +=)()(arccosHnHrc(12) where: += cos)()( hnhrc (13) where n(h) is the atmospheric radio refractive index, calculated from pressure, temperature and water-vapour pressure along the path (see
46、Recommendation ITU-R P.835) using Recommendation ITU-R P.453. On the other hand, when 0, there is a minimum height, hmin, at which the radio beam becomes parallel with the Earths surface. The value of hmincan be determined by solving the following transcendental equation: ()()chnhrminmin=+ (14) This
47、 can be easily solved by repeating the following calculation, using hmin= h as an initial value: ()rhnchminmin= (15) Therefore, A(h, ) can be calculated as follows: ()() ()HHHHhAhhhminmindsindsin,+=(16) In carrying out the integration of equations (11) and (16), care should be exercised in that the
48、integrand becomes infinite at = 0. However, this singularity can be eliminated by an appropriate variable conversion, for example, by using u4= H h in equation (11) and u4= H hminin equation (16). A numerical solution for the attenuation due to atmospheric gases can be implemented with the following
49、 algorithm. To calculate the total attenuation for a satellite link, it is necessary to know not only the specific attenuation at each point of the link but also the length of path that has that specific attenuation. To derive the path length it is also necessary to consider the ray bending that occurs in a spherical Earth. 10 Rec. ITU-R P.676-1
