1、 Rec. ITU-R RA.517-4 1 RECOMMENDATION ITU-R RA.517-4*Protection of the radio astronomy service from transmitters operating in adjacent bands (Question ITU-R 145/7) (1978-1982-1992-2003-2006) Scope This Recommendation provides guidance for administrations and/or operators to protect the radio astrono
2、my service (RAS) from interference by out-of-band emissions that may be caused by active services with allocations adjacent to the RAS bands. The ITU Radiocommunication Assembly, considering a) the value of the scientific results achieved by the radio astronomy service (RAS) through the exploration
3、of the Universe; b) the need for interference-free bands at intervals throughout the radio spectrum in order that radio astronomy measurements can be made; c) that the threshold levels of interference detrimental to the RAS are given in Annex 1 to Recommendation ITU-R RA.769; d) that Recommendation
4、ITU-R RA.1513 provides the acceptable levels of data loss to radio astronomy observations and percentage-of-time criteria resulting from degradation by interference for frequency bands allocated to the RAS on a primary basis; e) that emissions in the out-of-band domain generally result from the modu
5、lation process; f) that in some cases either there are no limits specified in the Radio Regulations for out-of-band emissions, or the limits do not provide the protection required by radio astronomy from transmitters operating in frequency bands adjacent to a band allocated to the RAS, which may cau
6、se interference detrimental to the radio astronomy service due to the unwanted emissions of these transmitters; g) the difficulties experienced by radio services in the design and utilization of transmitters to operate in frequency bands adjacent to a band allocated to the RAS, in such a manner as t
7、o afford adequate protection from interference detrimental to the RAS (see Annex 1); h) the increase in the level of usage of frequency bands adjacent to bands allocated to the RAS, particularly by airborne and satellite transmitters; j) that it is incumbent on both active and passive radio services
8、 to find means to minimize interference that may be detrimental to the services, acting both separately and in cooperation with each other, with due consideration for the efficient use of the radio-frequency spectrum, *NOTE The levels of the detrimental interference to the RAS referred to in Annex 1
9、 to Recommendation ITU-R RA.769 are not accepted by the Arab Administrations, being unrealistic, as confirmed by previous Radiocommunication Conferences in 1995, 1997 and 2000 dealing with Recommendation 66. 2 Rec. ITU-R RA.517-4 recommends 1 that in order to reduce interference detrimental to the R
10、AS, all practical, technical means, for example, the use of filters in transmitters to confine emissions to the allocated band, and in radio astronomy receivers to avoid sensitivity to signals outside the allocated band, should be adopted to the maximum practicable extent; 2 that when frequencies ar
11、e assigned to a station in a service operating in a band adjacent to one allocated on a primary basis to RAS, attempts should be made to limit the edge of the necessary band adjacent to the radio astronomy band, so that the power radiated within this band should produce interference detrimental to a
12、 station of that service; 3 that when future frequency assignments are made by administrations in bands adjacent to those allocated to RAS, account should be taken, to the maximum extent practicable, of the special risk of interference to radio astronomy observations from space-to-Earth and airborne
13、 transmissions, within the adjacent bands; 4 that taking into account 1, 2 and 3 above, practical solutions to limit interference due to unwanted emissions to levels below those detrimental to the RAS be sought by administrations, individually and if necessary in cooperation. Annex 1 Interference to
14、 the RAS from transmitters operating in adjacent bands 1 Introduction The sensitivity limit of most radio astronomy observations is at a flux-density level far below that used for reception of radiocommunication signals. Thresholds of interference that are detrimental to the RAS and criteria for fre
15、quency sharing between RAS and other services are discussed in Annex 1 to Recommendation ITU-R RA.769; in Tables 1, 2 and 3 of the latter, the sensitivity limits are listed for different frequencies. However, interference can also occur from transmitters that do not share the same band. This may be
16、classified as band-edge interference and interference from harmonic and intermodulation signals. Interference to the RAS from emissions in the spurious domain are treated in Recommendation ITU-R RA.611. Additional considerations pertaining to protection of the RAS from unwanted emissions resulting f
17、rom application of wideband digital modulation are given in Recommendation ITU-R RA.1237. Interference problems in a radio astronomy receiver, by a transmitter operating in an adjacent band, can arise by three mechanisms. It can occur if the sensitivity of the radio astronomy receiver to signals out
18、side the radio astronomy band is not sufficiently low. This may be due to the practical limitations on the fall-off of receiver gain at the band edges. Secondly, non-linear effects in the receiver may, in the presence of two or more strong signals at frequencies near the edge of the receiver passban
19、d, give rise to intermodulation products falling within the radio astronomy band. Thirdly, interference may arise through unwanted emission by the transmitter (modulation sidebands, phase-noise in oscillators, etc.) falling within the radio astronomy band. In dealing with band-edge interference, the
20、 problem common to both transmitting and receiving services is the design filters that will adequately suppress the unwanted energy without introducing unacceptable modifications, (e.g. attenuation or phase distortion), into the wanted signals. Rec. ITU-R RA.517-4 3 2 Interference from satellite tra
21、nsmissions Satellite transmissions have the potential to cause severe interference to the RAS. Whereas terrestrial interference sources are usually in the far side-lobe region of the radio telescope antenna, and possibly further attenuated by the topography of the surroundings of the radio observato
22、ry, interference by satellite transmitters is likely to be received via the main beam and inner side lobes, with considerably higher gain. The nature of the interference depends on the type of transmitter and service provided by the system, whether the satellites are in geostationary or non-geostati
23、onary orbit, and the number of satellites in the system under consideration that are above the horizon at the radio observatory. Some of the situations when a satellite service operates in a band adjacent to a primary radio astronomy band, and that present the greatest potential difficulties for the
24、 RAS are described in Recommendation ITU-R SM.16331. 2.1 Geostationary satellites Multiple geostationary satellites are visible from almost all the radio telescopes currently in operation, and occupy a more-or-less constant range of azimuths and elevations. They have therefore the potential to be tr
25、oublesome sources of interference to radio astronomy observations. The radius of the GSO is approximately 6.6 times the radius of the Earth. At that radial distance a single satellite can illuminate a third of the Earths surface and consequently many radio telescopes with line-of-sight signals. Figu
26、re 1 shows the position of the geostationary satellite belt in celestial coordinates as seen from the latitudes of some major radio astronomy observatories. Plans for the development of some active services call for a large number of geostationary satellites. Such a series of potential sources of in
27、terference that may be received through the near side lobes of the radio telescope antenna pattern could present a unique interference problem to radio astronomers. Threshold levels of interference detrimental to radio astronomy are given in Annex 1 to Recommendation ITU-R RA.769. Listed there is th
28、e level, in each radio astronomy band, of the power into the receiver that is just sufficient to cause interference which is detrimental to the operation of the RAS. Also listed are the equivalent pfd (dB(W/m2) levels associated with this interference, which are calculated with the assumption that t
29、he gain of the radio telescope is 0 dBi in the direction of the interfering source. Such a gain is appropriate for consideration of terrestrial sources of interference confined to the neighbourhood of the horizon. For the case of geostationary sources, the situation is different. If we assume that t
30、he RAS antenna has the side-lobe characteristics assumed in Recommendation ITU-R SA.509, the side-lobe gain would fall to 0 dBi at 19 from the axis of the main beam. For such an antenna the interference level detrimental to the RAS will be exceeded if the main beam is pointed within 19 of a satellit
31、e that produces within the radio astronomy bandwidth a pfd at the radio observatory equal to the threshold level in Annex 1 to Recommendation ITU-R RA.769. A series of satellites spaced at intervals of about 30 along the GSO radiating interference at this level would result in a zone of width approx
32、imately 38 centred on the orbit in which radio astronomy observation free from detrimental interference would be precluded. The width of this precluded zone would increase with the number of interfering satellites in the orbit, and could, in principle, cover the whole sky. The effective number of in
33、terfering satellites will depend upon whether the interfering signals are spot beamed from the satellites transmitting antennas or are more widely radiated. Out-of-band emissions that is not widely separated from the satellites transmitter frequency are likely to be directed by the antennas in a way
34、 similar to that of the intended signals. 1Additional relevant technical material may be found on the Radiocommunication Working Party 7D website. 4 Rec. ITU-R RA.517-4 FIGURE 1 Projection of geostationary-saellite orbit on to the celestial sphere 2.2 Non-geostationary satellites The potential for i
35、nterference detrimental to the RAS from non-geostationary low-Earth orbit satellites is due to their operation in large numbers, which make it possible for many of them to be simultaneously above the horizon at a radio observatory, and in line-of-sight with the radio telescope antenna. This factor l
36、eads to a situation where the radio telescope antenna can receive unwanted emissions from those visible non-geostationary low-Earth orbit satellites through many near and far side lobes of the antenna beam, and also through the main beam. The interference problem is exacerbated by the continually ch
37、anging directions of arrival of the interfering signals, and the need for the radio telescope antenna to track the celestial source under observation. Multiple inputs of strong signals may drive the operating point of the receiver into a non-linear region, resulting in the generation of intermodulat
38、ion products. The impact of unwanted emissions produced at radio astronomy sites by a constellation of satellites in (low) non-geostationary orbits may be determined using the epfd methodology described in Recommendation ITU-R S.1586 Calculation of unwanted emission levels produced by a non-geostati
39、onary fixed-satellite service system at radio astronomy sites, or Recommendation ITU-R M.1583 Interference calculations between non-geostationary mobile-satellite service or radionavigation-satellite service systems and radio astronomy telescope sites, and the antenna gains given in Recommendation ITU-R RA.1631. These Recommendations may be used to determine the percentage of data loss during observations made at a particular radio astronomy site due to interference from a given satellite system. The acceptable percentage of data los is defined in Recommendation ITU-R RA.1513.
