1、 Recommendation ITU-R P.1621-2 (07/2015) Propagation data required for the design of Earth-space systems operating between 20 THz and 375 THz P Series Radiowave propagation ii Rec. ITU-R P.1621-2 Foreword The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and e
2、conomical use of the radio-frequency spectrum by all radiocommunication 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
3、 by World and Regional Radiocommunication Conferences and Radiocommunication 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
4、 used for the submission of patent statements and licensing declarations 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.
5、 Series of ITU-R Recommendations (Also available online at http:/www.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, radiod
6、etermination, amateur and related satellite services P Radiowave propagation 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
7、 Satellite news gathering TF Time signals and frequency standards emissions 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, 2015 ITU 2015 All rights reserved. No part of this
8、publication may be reproduced, by any means whatsoever, without written permission of ITU. Rec. ITU-R P.1621-2 1 RECOMMENDATION ITU-R P.1621-2 Propagation data required for the design of Earth-space systems operating between 20 THz and 375 THz (Question ITU-R 228/3) (2003-2005-2015) The ITU Radiocom
9、munication Assembly, considering a) that the spectrum between 20 THz and 375 THz is available for communications in near-Earth and deep space environments; b) that for the proper planning of Earth-space systems operating between 20 THz and 375 THz, it is necessary to have appropriate propagation dat
10、a; c) that methods have been developed that allow the calculation of the most important propagation parameters needed in planning Earth-space systems operating between 20 THz and 375 THz; d) that, as far as possible, these methods have been tested against available data and have been shown to yield
11、an accuracy that is both compatible with the natural variability of propagation phenomena and adequate for most present applications in the planning of systems operating between 20 THz and 375 THz, recognizing a) that No. 78 of Article 12 of the ITU Constitution states that a function of the Radioco
12、mmunication Sector includes, “. carrying out studies without limit of frequency range and adopting recommendations .”, recommends 1 that the methods for predicting the propagation parameters given in Annex 1 be adopted for planning Earth-space systems, in the respective ranges of validity indicated
13、in the Annex. NOTE 1 Supplementary information related to propagation prediction methods for frequencies between 20 THz and 375 THz may be found in Recommendation ITU-R P.1622. Annex 1 1 Atmospheric considerations The performance of a system operating in the 20-375 THz frequency range between the Ea
14、rth and an orbiting spacecraft is affected by the atmosphere of the Earth. These atmospheric effects include: absorption by molecules of atmospheric gasses present along the propagation path resulting in an overall loss in signal amplitude; scattering by particles ranging in size from fractions of a
15、 wavelength to many wavelengths present along the propagation path resulting in an apparent loss in signal amplitude; 2 Rec. ITU-R P.1621-2 refraction of the beam due to changes in atmospheric density along the propagation path resulting in an apparent movement in the position of the transmitting so
16、urce; turbulence due to thermal variations in the atmosphere resulting in fluctuations in the received amplitude and phase of the signal. This Annex deals only with the effects of the troposphere on the wanted signal. As far as possible, the prediction methods in this Annex have been tested against
17、measurements on terrestrial-free space links and astronomical systems. These methods yield results suitable for basic system planning. However, due to the spatial and temporal variability of the atmosphere, local site surveys of propagation characteristics are essential prior to the deployment of an
18、y ground-based system operating between 20 THz and 375 THz. 2 Absorption Figure 1 illustrates the frequency dependence of atmospheric absorption along three zenith paths. The area shaded in light grey illustrates the relatively low absorption associated with a site located 5 km above sea level in an
19、 area of low humidity. The darker grey area shows the additional atmospheric absorption that would occur for a site located 2 km above sea level. The black regions show the further impact of atmospheric absorption for a site located at sea level and transmitting through a standard atmosphere as prov
20、ided in Recommendation ITU-R P.835. The Figure clearly shows that the atmosphere, except at some dry, high altitude locations, is opaque to electromagnetic energy at almost all frequencies between about 1 THz and 10 THz (300 m and 30 m). Above 10 THz, the absorptive characteristics of the atmosphere
21、 again become favourable to propagating electromagnetic energy between the surface of the Earth and space. The absorptive characteristics of the spectral region above 10 THz (below 30 m), shown in detail in Fig. 2 for the same three zenith paths, are characterized by a series of regions of low absor
22、ption separated by strong but narrow regions of high absorption. The individual regions of low absorption are limited by a fine structure of many weak absorption lines. Individual absorption lines occur due to the presence of gaseous components in the atmosphere including, but not limited to: NH3, C
23、O2, CO, CH4, NO2, NO, O2, O3, SO2, H2O, and various CFCs. The strength of the absorption lines is dependent on temperature and pressure. Calculations of atmospheric absorption are possible using a line-by-line method similar to Annex 1 of Recommendation ITU-R P.676. However, as thousands of individu
24、al lines are present across the spectral range from 10 THz to 1 000 THz (30 m to 0.3 m), such a method is computationally intensive. Rec. ITU-R P.1621-2 3 FIGURE 1 Atmospheric absorption along a vertical path P. 1 6 2 1 - 0 10 3.Fre q u en cy (T H z)W av el en g t h ( m)6 0 G H zab s o rp t i o nreg
25、 i o n2 7 5 G H zen d o f rad i oal l o cat i o n sAbsorption(dB)504540353025201510501011 1021031011023303 10 33 10 23 1 043 Scattering Atmospheric scatter results in an apparent reduction in signal strength at the receiver due to a redirection of the transmitted energy away from the intended propag
26、ation path. The scattering characteristics of the atmosphere are dependent on the diameter of the scattering particles present along the propagation path. Scattering characteristics take three forms: Rayleigh Mie Wavelength independent. 3.1 Rayleigh scattering The atmosphere exhibits Rayleigh scatte
27、ring characteristics when the scattering particles along the propagation path have a much shorter physical diameter than the wavelength of the electromagnetic wave. At frequencies above 20 THz (wavelength below 15 m), Rayleigh scattering occurs due to interactions between the electromagnetic wave an
28、d polar molecules of atmospheric gases. 4 Rec. ITU-R P.1621-2 FIGURE 2 Absorption above 10 THz (below 30 m) of a standard atmosphere along a vertical path P. 1 6 2 1 - 0 20 . 3Freq u en cy (T H z)W av el en g t h ( m)Absorption(dB)5045403530252015105010210310131030The amount of apparent signal loss
29、at the receiver attributable to Rayleigh scattering is negligible for frequencies below 375 THz (wavelength greater than 0.8 m). However, the magnitude of Rayleigh scattering has a wavelength dependence of 4. At about 1 000 THz (0.3 m) the impact of Rayleigh scattering on a transmitted signal become
30、s comparable to Mie scattering. The most significant result of Rayleigh scattering is the introduction of background noise into receivers. Background noise appears along paths in both the Earth-to-space and space-to-Earth directions. The primary noise source for earth stations operating with spacecr
31、aft comes from Rayleigh scatter of sunlight during daytime operations. Spacecraft pointed at the Earth will also encounter noise from sunlight reflected from the Earths surface, although this is not a Rayleigh effect. Figure 3 provides the radiance, H, of the sky for several conditions each in W/m2/
32、m/sr. For simplicity, Table 1 provides specific values of H for the primary frequencies of interest for space-based communication above 20 THz. A reasonable value of H during night-time operations is 1 109 W/m2/m/sr for most frequencies of interest. Rec. ITU-R P.1621-2 5 FIGURE 3 Radiance of the sky
33、 for various sun conditions P. 1 6 2 1 - 0 3W av el en g t h ( m)Radiance(W/m/m/sr)2Bri g h t s u n s h i n eN o rmal s u n s h i n eO v erc as t0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 2103102101021011TABLE 1 Radiance, H (W/m2/m/sr), of the sky and Earth for several frequencies Sky background Fr
34、equency (THz) Wavelength (m) Bright sunshine Normal sunshine Overcast 566.0 0.530 303.4 101.6 71.75 352.9 0.850 122.3 42.58 30.3 310.9 0.965 64.62 25.12 18.63 283.0 1.06 54.45 25.32 17.99 200.0 1.50 13.01 6.00 4.44 Ignoring atmospheric effects, the background noise power, Pback, arriving at the rece
35、iver is given by: W42 HAP rrb a c k (1) where: r: field of view of the receiver (rad) Ar: area of the receiver (m2) : bandwidth of the receiver (m) H: radiance (W/m2/m/sr). Apart from background noise due to Rayleigh scatter, other sources of background noise may also appear. These sources include b
36、ut are not limited to planets, bright stars, and reflections from natural or artificial objects. During periods of intense rain, systems operating between 20 THz and 375 THz are not considered operational. Therefore, lightning is not considered a noise source along Earth-space paths. Depending on th
37、e orientation and motion of the propagation path as well as that of background noise sources, the magnitude and duration of noise events may vary by many orders of magnitude. 6 Rec. ITU-R P.1621-2 3.2 Mie scattering The atmosphere exhibits Mie scattering characteristics when the scattering particles
38、 along the propagation path have roughly the same physical diameter as the wavelength of the electromagnetic wave. Mie scattering is a complex function of the size, shape and number of particles along the propagation path. The distribution of particle sizes and shapes along the propagation path are
39、a function of the path profiles of both water vapour content and wind speed. Aerosols and microscopic water particles are the predominant components of Mie scattering at frequencies between 20 THz and about 375 THz (15 m and 0.8 m). In this frequency range, Mie scattering is a significantly greater
40、effect than Rayleigh scattering. For comparison, Fig. 4 shows the specific attenuation (dB/km) of Rayleigh and Mie scattering for a standard atmosphere at sea level. FIGURE 4 Specific attenuation for a standard atmosphere at sea level P. 1 6 2 1 - 0 4W av el en g t h ( m)Specificattenuation(dB/km)Mi
41、 eR ay l ei g h0 . 80 . 70 . 60 . 50 . 40 . 30 . 20 . 100 . 3 0 . 7 1 2 3 4F re q u en cy (T H z)103300 100 753.3 Wavelength independent scattering The atmosphere exhibits wavelength independent scattering characteristics when the scattering particles along the propagation path have a much larger ph
42、ysical diameter than the wavelength of the electromagnetic wave. Wavelength independent scattering is most accurately described by diffraction theory. The particles most frequently occurring on Earth-space paths are hydrosols and hydrometeors. The effects of wavelength independent scattering are sig
43、nificant. Clouds, fog, rain, or snow can effectively prohibit the propagation of electromagnetic radiation above 20 THz (below 15 m). 4 Refraction Atmospheric refraction occurs when electromagnetic energy propagates through media with varying densities. The effect on a system operating between 20 TH
44、z and 375 THz along an Earth-space path is an angular shift in the direction of the propagation path. Refraction is a function of wavelength and elevation angle as well as the temperature and pressure profile along the propagation path. Rec. ITU-R P.1621-2 7 4.1 Formula for the effective atmospheric
45、 refractive index The effective atmospheric refractive index, neff, for frequencies above 150 THz (wavelengths 2 m) at temperature T 15 C and pressure P 1 013.25 hPa for a vacuum wavelength, vac, is approximated by: 228 41 54025146 81094928.4326101 v a cv a ce f fn(2) where: vac : wavelength (m). Th
46、e effective atmospheric refractive index can be adjusted for other temperatures and pressures using the expression: )0366.01(4696.760 )10)0113.07868.0(1(162.1)1(1),(6T TPPnPTn e f fe f f (3) where: T : temperature (C) P : atmospheric pressure (hPa). Water vapour has only a very slight influence (les
47、s than 1%) on the atmospheric refractive index in the frequency range mentioned above. The effective atmospheric refractive index, neff, differs from the actual atmospheric refractive index, n, by taking into account the vertical path profile. The value of neff allows calculations of the apparent ch
48、ange in elevation angle to be conducted using Snells law as given in (4). The use of neff is satisfactory because, in practice, values of n present along the propagation path will fluctuate rapidly. The acquisition and tracking systems must automatically adjust in real-time to account for these fluc
49、tuations. Therefore, systems operating between 150 THz and 375 THz only require an approximation for the initial acquisition. 4.2 Apparent change in elevation angle Refraction will cause the apparent elevation angle to a spacecraft to differ from its true elevation angle. The amount of refraction occurring in the atmosphere is calculated using Snells law and the value of neff calculated in (2) and (3). The observed elevation angle is calculated by: ),( )c o s (c o s 1 PT
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