1、318 CCIR RECMN*bLS-L 92 4855232 0520634 A12 Rec. 619-1 RECOMMENDATION 619- 1 PROPAGATION DATA REQUIRED FOR THE EVALUATION OF INTERFERENCE BETWEEN STATIONS IN SPACE AND THOSE ON THE SURFACE OF THE EARTH (Question 19/5) (1986-1990-1992) The CCIR, considering a) it is necessary to have appropriate prop
2、agation data that are based on both terrain and atmospheric factors; that for the proper evaluation of interference between stations in space and those on the surface of the Earth, b) adequate accuracy in all regions of the world; that the data available so far do not allow reliable prediction metho
3、ds to be developed which would give Cl that, however, several methods are available which yield sufficient accuracy in at least some regions, recommends that for the evaluation of interference between stations in space and those on the surface of the Earth, administrations use the propagation and ca
4、lculation methods as set out in Annex 1. ANNEX 1 1. Introduction Interference between terrestrial and space stations, between space stations in the same system, and between space stations not in the same system can follow many paths. These can be summarized as follows: 1.1 Interference between space
5、 stations and earth stations not in the same system Mode BI : a transmission from a space station of one space system causing interference to reception by an earth station of another system. Mode B2 : a transmission from an earth sttion of one space system causing interference to reception by a spac
6、e station of another system. 1.2 Mode CI : Mode C2 : Interfereme between space stations and terres fria1 sfations a transmission from a space station causing interference to reception by a terrestrial station. a transmission from a terrestrial station causing interference to reception by a space sta
7、tion. The three principal impairment mechanisms which give rise to these interference paths are those due to clear-air propagation, precipitation scatter, and differential attenuation on adjacent Earth-space paths. These mechanisms are considered below. Ionospheric effects on Earth-space propagation
8、 are discussed in Recommendation 53 1. These effects which depend 011 frequency, involve scintillations (equatorial, mid-latitude, high-latitude), absorption (auroral, polar cap), variations in direction of arrival, propagation delay, frequency change and Faraday rotation. CCIR RECMN*bLS-L 92 = 4855
9、232 0520635 759 = Rec. 619-1 319 2. Clear-air propagation A number of propagation impairment mechanisms can lead to variations in the path loss during conditions of essentially clear air. In general, the individual effects of these mechanisms become increasingly severe as the elevation angle of the
10、Earth-space path decreases. Clear-air effects of interest to low-elevation angle Earth-space paths include: - gaseous absorption, - cloud attenuation, - melting layer attenuation, - tropospheric scintillations, - low-angle fading, - ray bending, - antenna gain degradation, - defocussing (or beam spr
11、eading), - ducting, - groundibuilding reflections. Recommendation 452 gives prediction procedures for calculating the line-of-sight, diffraction, tropospheric scatter, and ducting layer reflection effects for terrestrial systems. Of the effects noted above, only tropospheric scintillations, low-angl
12、e fading, and ray bending give rise to varying interference levels between systems. Reflections from the ground should not occur in a well designed Earth-space system; predicting the amplitude and phase of reflections from buildings is usually only tractable through the use of deterministic models w
13、hich include an accurate knowledge of the urban environment. Before establishing the varying interference levels, it is usual to calculate the basic transmission loss, Lb, along a potential interference path. 2.1 Basic rartsmisswn loss The basic transmission loss Lb can be expressed as: Lb = 92.5 +
14、20logf + 20logd + A, + AD - Gs dB where: f : frequency (GHz) d : path length (km) A, : AD : attenuation due to atmospheric gases (dB) attenuation (dB) due to beam spreading Gs : “gain” (dB) due to scintillation. The attenuation A, due to atmospheric gases is described in Recommendation 676. It is a
15、function of the surface water vapour density p, the effective radius of the Earth Re = 106 (157 - AA)-1 and, to a lesser extent, of the surface air temperature. Average values of the water vapour density p for the months of February and August have been prepared and are reproduced in Figs. 1 and 2 o
16、f Recommendation 836. To a first approximation, the sttistics of the temporal variations of p can be assumed to follow a Gaussian distribution having a standard deviation, o, of about 0.25 times the mean value of p. For example, the absolute humidity likely to be exceeded for 99% of the tie would be
17、 1 - (0.25 x 2.4) = 0.4 times the mean value. Similarly the value exceeded for 99.99% of the time would be 1 - (0.25 x 3.8) = 0.05 times the mean value. is the mean decrease (lapse) of the radio refractivity over a 1 km thick layer based at the surface. World maps of the monthly mean values of are g
18、iven in Figs. 3 to 6 of Recommendation 453. CCIR RECMN*639-3 92 = 4855232 0520636 695 320 Rec. 619-1 The scintillation gain, Gs, is a function of frequency, earth-station antenna diameter, elevation angle, and local climate and can be calculated from the predicted intensity of tropospheric scintilla
19、tion. Application of the prediction methods of Recommendation 530 indicates that in a few regions of the world the combined effects of severe clear-air flat fading and multipath fading on the wanted signal (at fade depths of about 25 dB) can be at least as significant at about 23 GHz as rain attenua
20、tion even on paths as short as 10 km. These regions are locations in which ducts are prevalent but which also have mild rain climates (Le., warm or arctic desert areas). Most other regions subject to the prevalence of ducts are humid regions also subject to severe rain attenuation which would domina
21、te the outage of the wanted signal for margins of about 25 dB. In the United Kingdom, East Anglia is one area at temperate latitudes within which multipath fading of the wanted signal, at the 25 dB level, is known to occur for the same percentage of time as rain attenuation on a 10 km path. 2.2 Trop
22、ospheric scintillation A prediction method for tropospheric scintillation applicable down to elevation angles of about 4“ is given in Recommendation 618. This procedure aiiows the calculation of both the scintillation intensity and the cumulative distribution of effective fading. Tropospheric scinti
23、llation also produces signai enhancements which will lead to transmission loss values less than those of free space. Significant levels of tropospheric scintillation can exist for percentage times well above 1% of a year. On very low elevation angle paths, small-scale changes in the refractive index
24、 coupled with a wave-like movement of the atmosphere can give rise to severe low-angle fading. 2.3 Lo w-angle fading For elevation angles below 5“ and for small percentage times, substantial signal enhancements and fading can occur. Measurements at 7 GHz have indicated up to 6 dB enhancements of the
25、 signal at elevation angles of between 1“ and 2“, while on a 3.3“ elevation angle path, enhancements of up to 8 dB and fading down to 16 dB were measured in essentially clear-air conditions. No recommended model exists as yet to predict these levels of low-,mgle fading. 2.4 Ray bending Ray bending i
26、s caused by variations in the refractive index along the path. Such variations are usually only significant in a vertical direction, since the lower atmosphere at any given altitude tends to be fairly stratified. The main beam of an antenna is not infinitely thin and so the upper and lower edges of
27、the beam will undergo different amounts of ray bending due to their different elevation angle paths through the lower atmosphere. The differential ray bending will therefore lead to beam spreading. Beam spreading in a vertical plane is given in Fig. 1. 3. Precipitation scatter Interference can be ca
28、used when energy from one system is scattered by precipibtion aid enters the (mtenna beam of another system. This situation is only of significance when the main beams of the two systems intersect within that portion of the atmosphere in which hydrometeors can exist. Under these conditions, a common
29、 volume can be set up in which hydrometeors are present for appreciable periods and enhanced levels of unwanted signals may result. While such interference can be significant, it is usually not severe enough to be system-limiting and, by a judicious selection of path geometries to prevent the likeli
30、hood of common volumes occurring, can usually be avoided altogether. The procedure for calculating precipitation scatter is given in Recommendation 452. - CCIR RECMN*b19-3 92 m 4855232 O520637 523 m Rec. 619-1 FIGURE 1 Beam spreading of near-horizontai radio waves 321 20 30 40 50 60 70 80 90 Value o
31、f An important problem in this context is possible interference from a solar power satellite. Using av,lilable dat? on likely harmonic content, it can be shown that - even at the 4th harmonic - the interfering signal, at a distice of 50 km from the rain cell, can be comparable with the level of the
32、received signal in the fixed-satellite service. At the fundamental frequency, however, direct radiation from the side lobes of the power satellite to the terrestrial station will probably exceed the signal due to precipitation scatter. For limitations on the power-flux density on the surface of the
33、Earth, reference is made to Recommendation 369. 4. Differential attenuation on adjacent paths 4.1 Rain effects If hV0 space stations operating at the same frequency are separated from one another by a small angle, situations can arise when rain attenuation on the path from a space station reduces th
34、e signal sufficiently to allow interference from a neighbouring space station into the receiving earth station. In this case it is essential to consider the joint effects of the rain attenuation of Ac of the wanted signal and the rain attenuation of Ai of the interfering signal. , CCIR RECMN*bLS-L 9
35、2 Y855212 0520b38 4b8 - 322 Rec. 619-1 For interference calculations, the conditional distributions of AA = A, - Ai and A, must be used. AS a basis for generating total exceedance statistics for various regions, in which the single path attenuation statistics are different, AA statistics exceeded fo
36、r 1 % of the time have been calculated under the condition: 0.5 dB A, Am, where A, is the mwimum allowed attenuation of the wanted signal. The empirical formula: AA(l%) = 0.036(0.45f + cosec) . loge(cp + i) . log,(A,n + l)i.i5 dB (2) where: f : frequency (GHz): 11 I f I 30 GHz 8 : earth-station elev
37、ation angle (degrees): 5“ I8 530“ cp : angular separation (degrees) of the two paths in the azimuth direction: O“ I cp I 10“ was found to give a good fit to the data in the parameter ranges noted. In order to determine the percentage of time that a particular value of AA is exceeded for a given valu
38、e of A, the 1% figure must be multiplied by the probability that 0.5 dB Ac Am. The latter can be obtained from a measured or predicted cumulative distribution for a single path (see Recommendation 618). Equation (2) has yet to be tested over a larger range of frequencies and elevation angles. Nevert
39、heless, results obtained over a relatively large azimuth separation (31.5“) appear to be in general agreement with the predictions of equation (2). On this basis, differential rain attenuation along separate Earth-space paths is not considered to be a likely cause of unacceptable interference. 4.2 S
40、cintillation effects Height diversity measurements at an elevation angle of 3.3“ and a frequency of 11.198 GHz showed that a separation of 15.1 m resulted in essentially uncorrelated scintillation at the two antennas. This implies that two separate Earth-space paths need only have an elevation angle
41、 difference of the order of 0.1“ at an elevation angle of 3.3“ to be uncorrelated in terms of scintillation. Closely spaced satellites, and even some which are Co-located, could therefore be subjected to essentially uncorrelated scintillation and care should be taken in evaluating the potential interference between such systems, particularly when up-link power control is to be implemented.
