1、 Rec. ITU-R S.1586-1 1 RECOMMENDATION ITU-R S.1586-1 Calculation of unwanted emission levels produced by a non-geostationary fixed-satellite service system at radio astronomy sites (Question ITU-R 236/4) (2002-2007) Scope This Recommendation describes a method that could be used to calculate the unw
2、anted emission levels produced by a non-GSO fixed-satellite service system on radio astronomy sites. It also contains a procedure for the calculation of the percentage of time during which a given equivalent power flux-density (epfd) is exceeded when the receiving antenna gain is assumed to be 0 dBi
3、 in the direction of the incoming interference, and a given integration time is considered. The ITU Radiocommunication Assembly, considering a) that, in some cases, the radio astronomy service and space services (space-to-Earth) have been allocated to adjacent or nearby frequency bands; b) that the
4、radio astronomy service is based on the reception of emissions at much lower power levels than are generally used in other radio services; c) that, due to these low received power levels, the radio astronomy service is generally more susceptible to interference from unwanted emissions than other ser
5、vices; d) that several footnotes to the Radio Regulations (RR) (such as RR Nos. 5.149, 5.443B and 5.511A) draw attention to the protection of the radio astronomy service, particularly from space-borne transmitters; e) that due to the characteristics of non-geostationary (non-GSO) satellite systems,
6、and in particular to the time-varying nature of interference, the level of interference from such satellites into radio telescopes cannot be evaluated in the same way as for GSO satellites, recommends 1 that the calculation of unwanted emission levels produced by a non-GSO fixed-satellite service (F
7、SS) system on radio astronomy sites could be conducted by administrations using the method described in Annex 1; 2 that when performing these calculations, the antenna pattern described in Recommendation ITU-R RA.1631 could be used to model radio astronomy antennas; 3 that the percentage of time dur
8、ing which an equivalent power flux-density (epfd) level (defined assuming a 0 dBi receiving antenna gain in the direction of interference and given an integration time) is exceeded could be calculated according to the method described in Annex 2. 2 Rec. ITU-R S.1586-1 Annex 1 Calculation of unwanted
9、 emission levels produced by a non-GSO FSS system at radio astronomy sites The methodology described here, based on the epfd concept defined in RR Article 22, No. 22.5C, is intended for use in calculating the power flux-density (pfd) levels produced by unwanted emissions of a non-GSO FSS satellite s
10、ystem into radio telescopes, taking into account the characteristics of both the satellite system and the radio telescope antenna. The value of the epfd is the aggregate of the contributions from all satellite emissions expressed as the pfd of a single equivalent source on the boresight (peak of mai
11、n beam) of the radio telescope. 1 Required parameters Due to the particular characteristics of non-GSO satellite systems, it is clear that the level of the interference from such satellites into a radio telescope cannot be evaluated in the same way as for GSO satellites. A statistical approach is ne
12、eded which takes into account the dynamic aspect of non-GSO satellites. The evaluation of interference resulting from the satellites at the radio telescope during the integration time (2 000 s) should be based on statistical calculations and should take into account the parameters of both the satell
13、ites and the radio telescope. Non-GSO satellite system parameters: the number of satellites visible in the sky at the radio astronomy station; the pfd at the radio telescope within the radio astronomy band considered, estimated using a dBsd or dBc mask; the distances between the satellites and the r
14、adio astronomy station; the detailed orbital characteristics of the satellites. Radio telescope parameters: the antenna location; the antenna pattern and antenna gain; the practical range of pointing directions; the boresight pointing direction; the off-axis angles between the boresight of the anten
15、na of the radio astronomy station and the directions of the transmitting satellites; the integration time (2 000 s). 2 Calculation of epfd levels at radio astronomy sites The receiving gain of a radio telescope in the direction of a non-GSO satellite (as opposed to a GSO satellite) varies with time
16、chiefly because of the movement of the satellite and the fine angular structure of the radio telescopes side-lobe pattern. There will be times when the telescope gain in the direction of a satellite is much higher than 0 dBi, and other times when it is less. In addition, in the case of multiple sate
17、llites of a non-GSO system, all their contributions must be included and properly taken into account. Rec. ITU-R S.1586-1 3 This may be done using the concept of epfd originally defined to assess possible sharing conditions between GSO and non-GSO systems. In the section below the concept is develop
18、ed for the case of a radio astronomy station subject to interference from non-GSO satellites. The definition is based upon RR No. 22.5C as adopted at the World Radiocommunication Conference (Istanbul, 2000) (WRC-2000). 2.1 Definition of epfd When an antenna receives power, within its reference bandw
19、idth, simultaneously from transmitters at various distances, in various directions and at various levels of incident pfd, the epfd is that pfd which, if received from a single transmitter in the far field of the antenna in the direction of maximum gain, would produce the same power at the input of t
20、he receiver as is actually received from the aggregate of the various transmitters. The instantaneous epfd, expressed in dB(W/m2), is calculated using the following formula: =aiNimaxririitPGGdGepfd1,21010)(4)(10log10 (1) where: Na: number of non-GSO space stations that are visible from the radio tel
21、escope i: index of the non-GSO space station considered Pi:RF power of the unwanted emission at the input of the antenna (or RF radiated power in the case of an active antenna) of the transmitting space station considered in the non-GSO system in the reference bandwidth (dBW) i: off-axis angle betwe
22、en the boresight of the transmitting space station considered in the non-GSO system and the direction of the radio telescope Gt(i): transmit antenna gain (as a ratio) of the space station considered in the non-GSO system in the direction of the radio telescope di: distance (m) between the transmitti
23、ng station considered in the non-GSO system and the radio telescope i: off-axis angle between the pointing direction of the radio telescope and the direction of the transmitting space station considered in the non-GSO system Gr(i): receive antenna gain (as a ratio) of the radio telescope, in the dir
24、ection of the transmitting space station considered in the non-GSO system (see Recommendation ITU-R RA.1631) Gr,max: maximum gain (as a ratio) of the radio telescope epfd: instantaneous epfd in the reference bandwidth at the radio telescope (dB(W/m2). 4 Rec. ITU-R S.1586-1 The epfd calculation in eq
25、uation (1) assumes that the pfd due to all interfering sources is directed at the boresight of the receiving antenna, where the antenna gain is maximum. However, radio astronomy protection criteria are based on a 0 dBi contour of the radio astronomy antenna. The pfd due to all interfering sources di
26、rected at the 0 dBi gain of the receiving antenna, can be determined as follows: From equation (1), the instantaneous epfd directed at the 0 dBi gain of the receiving antenna, expressed in (W/m2), is given by =airNiiriitPGGdGepfd1210dBi0)(4)(10 (2) The instantaneous epfdGr= 0 dBivalues resulting fro
27、m equation (2), averaged over a 2 000 s integration time, can be compared with pfd levels, also expressed in (W/m2) (defined assuming a 0 dBi receiving antenna gain in the direction of interference and given this integration time). NOTE 1 It is assumed that each transmitter is located in the far fie
28、ld of the radio telescope (that is, at a distance greater than 2D2/, where D is the effective diameter of the radio telescope and is the observing wavelength). Though this may not always be satisfied, it is considered to be an adequate approximation. NOTE 2 For some telescopes, the direction of maxi
29、mum gain (boresight direction) may not always coincide with the geometrical axis of the radio telescope. NOTE 3 In the case of active antennas, Pishould be taken as the radiated RF power rather than the power at the input to the antenna. NOTE 4 The antenna gain of the transmitting station, Gt(i), is
30、 taken at the frequency of the radio astronomy band considered. This may differ from the gain at the frequencies of the intended transmissions. Annex 2 Distribution of epfd levels This Annex describes a way to derive epfd statistics over the whole sky. 1 Division of the sky into cells of approximate
31、ly equal solid angle The first step of this approach is to divide the sky into M rings parallel to the horizon and equally spaced in terms of elevation angle, from 0 to 90. The width of each ring is 90/M. The next step is to divide these rings into cells whose azimuth width is chosen to provide an i
32、nteger number of cells per ring and is approximately equal to: )elevation(cos/90 Mdegrees Rec. ITU-R S.1586-1 5 Figure 1 provides an example of division based on a step of 3 width in elevation, this divides the sky into 30 rings of 3 of elevation angle. Then, the azimuth width is approximately equal
33、 to: )elevation(cos30/90degrees Elevation is a mean elevation in a given ring. This leads to a division of the sky into 2 334 cells of approximately 9 square degrees of solid angle each. Table 1 provides the number of cells for each ring corresponding to this example. 2 epfd distribution for a cell
34、First, a random choice is made for a pointing direction of the radio astronomy service antenna which will lie within a specific cell on the sky as defined in the paragraph above. Then, the starting time of the constellation is randomly chosen. The epfdis then evaluated for each time sample over a 2
35、000 s integration time. The average epfd corresponding to this trial is then calculated for the chosen pointing direction and starting time of the constellation. 6 Rec. ITU-R S.1586-1 TABLE 1 Example of division of the sky into square cells of about 9 square degrees solid angle Lower elevation of th
36、e ring (degrees) Ring solid angle (square degrees) Cumulative solid angle (square degrees) Azimuthstep (degrees) Number of cells in the ring Cell solid angle (square degrees) Cumulative number of cells Percentage of solid angle (%) Cumulative solid angle (%) 0 1 079.51 1 079.51 3 120 9 120 5.23 5.23
37、 3 1 076.55 2 156.05 3 120 8.97 240 5.22 10.45 6 1 070.64 3 226.69 3 120 8.92 360 5.19 15.64 9 1 061.79 4 288.49 3 120 8.85 480 5.15 20.79 12 1 050.04 5 338.53 3 120 8.75 600 5.09 25.88 15 1 035.41 6 373.93 3 120 8.63 720 5.02 30.90 18 1 017.94 7 391.87 3 120 8.48 840 4.94 35.84 21 997.68 8 389.55 3
38、 120 8.31 960 4.84 40.67 24 974.68 9 364.23 3 120 8.12 1 080 4.73 45.40 27 949.01 10 313.24 3 120 7.91 1 200 4.60 50 30 920.75 11 233.99 4 90 10.23 1 290 4.46 54.46 33 889.95 12 123.94 4 90 9.89 1 380 4.31 58.78 36 856.72 12 980.66 4 90 9.52 1 470 4.15 62.93 39 821.14 13 801.81 4 90 9.12 1 560 3.98
39、66.91 42 783.31 14 585.12 4 90 8.70 1 650 3.80 70.71 45 743.34 15 328.46 4 90 8.26 1 740 3.60 74.31 48 701.32 16 029.79 5 72 9.74 1 812 3.40 77.71 51 657.39 16 687.17 5 72 9.13 1 884 3.19 80.90 54 611.65 17 298.82 5 72 8.50 1 956 2.97 83.87 57 564.23 17 863.06 6 60 9.40 2 016 2.74 86.60 60 515.27 18
40、 378.33 6 60 8.59 2 076 2.50 89.10 63 464.90 18 843.23 6 60 7.75 2 136 2.25 91.35 66 413.25 19 256.48 8 45 9.18 2 181 2.00 93.36 69 360.47 19 616.95 9 40 9.01 2 221 1.75 95.11 72 306.70 19 923.65 10 36 8.52 2 257 1.49 96.59 75 252.09 20 175.74 12 30 8.40 2 287 1.22 97.81 78 196.79 20 372.53 18 20 9.
41、84 2 307 0.95 98.77 81 140.95 20 513.49 24 15 9.40 2 322 0.68 99.45 84 84.73 20 598.21 40 9 9.41 2 331 0.41 99.86 87 28.27 20 626.48 120 3 9.42 2 334 0.14 100 Rec. ITU-R S.1586-1 7 This operation is repeated to obtain a statistical distribution of the epfd in the considered cell. The methodology inv
42、olves a number of trials, each of which calculates the averaged epfd level over a 2 000 s integration interval. The greater the number of trials, the more accurate this distribution will be. A sufficient number of trials is needed to achieve the required confidence level in the results. In particula
43、r, the number of trials multiplied by the 2 000 s integration time should be significantly higher than the period of the constellation. It is also necessary to ensure adequate statistical sampling over the full period of the constellation. Once it is found that no further significant change occurs i
44、n the distribution, it can be concluded that a sufficient number of trials has been performed. This check can be done either automatically as an integral part of the simulation, or manually, by stopping the simulation at regular intervals. 3 epfd distribution in worst-case pointing directions (to be
45、 applied only if the pfd levels from satellites are constant for a given elevation angle of radio astronomy service antenna) The evaluation of the epfd distributions in cells on the sky may be simplified by first evaluating the epfd distribution in pointing directions corresponding to worst-case poi
46、nting directions. These worst-case pointing directions may be taken as those pointing directions where the probability of visibility of satellites is the highest. These pointing directions may be determined according to Recommendation ITU-R S.1257 Analytical method to calculate short-term visibility
47、 and interference statistics for non-geostationary satellite orbit satellites as seen from a point on the Earths surface (equations (28) and (29). For a given elevation angle and a given constellation of non-GSO satellites, this Recommendation allows the calculation of the worst-case azimuths (there
48、 are usually two worst-case azimuths at a given elevation). For the cells within which these worst-case pointing directions lie, the epfd distribution may be evaluated for a sufficient number of 2 000 s integration times. Then, this epfd distribution may be compared with a pfd threshold level (defin
49、ed assuming a 0 dBi receiving antenna gain in the direction of interference and given a 2 000 s integration time). For a cell, the percentage of time during which a pfd threshold level is exceeded can be calculated as the percentage of 2 000 s integration periods in which the average pfd at the radio telescope exceeds this pfd threshold level. Considering the 2% criterion in recommends 2 of Recommendation ITU-R RA.1513, the comparison of the epfd distribution with the pfd threshold level for cells corresponding
