ITU-R S 670-1-1992 Flexibility in the Positioning of Satellites as a Design Objective - Section 4D3 - Spacecraft Station-Keeping - Satellite Antenna Radiation Pattern - Pointing Ac.pdf

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1、CCIR RECMN*b70-3 92 = 4855232 0538903 233 Rec. 670-1 283 RECOMMENDATION 670-1 FLEXIBILITY IN THE POSlTIONING OF SATELLITES AS A DESIGN OBJECTIVE (Question 48/4) (1990-1992) The CCIR, considering that flexibility in the positioning of satellites can increase the efficiency of the geostationary orbit;

2、 that flexible satellite positioning can, in some circumstances, increase the Likelihood of successful frequency a) b) coordination; 4 coverage area and performance; d) non-tracking earth stations to be repointed; e) supported; f) onerous than changing their spacings by a smali amount; that repositi

3、oning can impact on the design of satellites as regards lifetime and the trade-off between that the repositioning of satellites will impact on earth segment operations and may require large numbers of that the frequency of the repositioning should be limited so as to minimize disruption to the servi

4、ces being that changing the order of satellites within a given arc of the geostationary-satellite orbit is Likely to be more g) the affected earth and space stations; that the feasibility of accepting repositioning rather than some other means will depend upon the character of h) the analyses contai

5、ned in Annexes 1 and 2, recommends 1. operate within f2“ of their nominal orbital position, or to the extent of their service arc, whichever is less; 2. Note 1 - Administrations are encouraged to develop new satellite technology to permit satellites to operate within f5“ of their nominal orbital pos

6、ition, or to the extent of their service arc, whichever is less; Note 2 - New networks are defined as networks for which the advance publication data is after 1990. that satellites for new networks in the fixed-satellite service should be designed to have the flexibility to that the following Notes

7、be regarded as part of this Recommendation. ANNEX 1 Flexibility in the positioning of satellites 1. htroduction Satellite relocations are technically and operationally feasible and have already been demonstrated on various classes of satellites. However, flexibility of orbit location does present te

8、chnical and operational difficulties. The potential advantages and the difficulties of satellite re-positioning are discussed in this annex. CCIR RECMN*b70-1 72 4855212 0538704 158 284 Rec. 670-1 Two different forms of relocation have been considered, namely: a) a reduction in the existing spacing o

9、f satellites in the orbit; and b) a complete reappraisal of the sequence of satellites in an orbital arc. An advantage of a) is that satellites could, if necessary, continue to transmit to tracked earth stations during the relocation process, since the paths of the satellites would not cross. The ma

10、in disadvantage of this scheme is that it requires several satellites to move. Though scheme b) may require fewer satellites to move, those which did move would likely to be moved further, and would therefore suffer penalties in terms of service outages, fuel burn and change of coverage areas which

11、are unlikely to be operationally acceptable. 2. Orbit efficiency and system considerations It should be noted that it is not possible to say with certainty which geographical areas would need to be covered at some time in the future from a given part of the orbit. Full advantage could therefore be t

12、aken of this means of optimizing the use of the orbit only if networks were designed so that their satellites could be relocated, if necessary, within a service arc after having been put into service. This ability to relocate satellites after they have entered service could also be of great value in

13、 allowing room to be found in orbit for new satellites for unforeseen networks. Such relocation may be more cost-effective than alternative technical solutions - such as improved earth-station antennas or new, sophisticated, modulation techniques. However, it is at present difficult to ascertain the

14、 technical feasibility and cost effectiveness of building satellite system networks to achieve substantial orbit position flexibility. Relocating a satellite within its service arc should be acceptable during the paper design phase, whereas during the development and construction phase, orbit change

15、s should be confined to predetermined limits where the technical and cost penalties are reasonable (and acceptable). This can only be determined on an individual basis. At present, a relocation capability of satellites of 1- 2 from the nominal orbital positions appears feasible. Operational systems

16、should not be moved in orbit unless done voluntarily, and any increased interference imposed by the need for accommodating new systems should be guided by the Recommendations of the CCIR. 3. Potential problems associated with satellite re-positioning 3.1 Service arc limitaiions It may be necessary f

17、or a satellite to be moved from one orbital position to another within its service arc after entry into service in order to permit the access of a new network. The improvement in access to the orbit obtained in this way will be greater if the service arc is long. However, the provision of a long ser

18、vice arc may have a significant effect on the optimum design of a network, particularly satellite spot beam antennas, and it may have some impact on performance. The service arc of satellites serving areas which are very extensive in longitude would be short because their visible arc is small. A pos

19、sible way of extending the service arc is the subdivision of the service area between two satellites, well separated in orbit and connected by inter-satellite links. However, such a method would be costly. It has been observed that multi-purpose satellites may provide services in addition to those o

20、f the fixed- satellite service and these other applications may determine the orbital position required. For meteorological services, for example, the concept of visible arc may not be relevant. CCIR RECMN*b70-3 92 W 4855232 0538905 094 Rec. 670-1 285 3.2 Earth-station antennas A change in satellite

21、 location would require adjustment to earth-station antenna pointing direction and (for linear polarization) adjustment of the plane of polarization. However, with small earth-station antennas and with satellites kept on station to within k0.1“ East-West and North-South, earth-station antennas may n

22、ot need to be steered to track the satellite in normal operation. Many small earth stations will not have technical staff permanently assigned to them. An increasing number of networks will be limited to manual antenna tracking and only a limited range of adjustment of the beam pointing direction. T

23、hus, even a small change in satellite position could present severe mechanical and operational problems and might involve visits by technical staff to all earth stations in the network and interruptions to service. Antenna foundations should be designed to allow for worst-case changes in pointing di

24、rection as appropriate for each earth station site and service arc. Changes in earth-station antenna elevation might affect the clear-sky G/T and the severity of propagation degradations. Such effects may be significant in climates where rain is heavy and the angle of elevation at the earth station

25、is low. The up-link and down-link e.i.r.p. margins should be increased where necessary to take account of the possible need to move the satellite. The impact on system margins due to decreased elevation angles can be significant depending on the frequency of operation. Changes in earth-station anten

26、na elevation, azimuth and e.i.r.p. might invalidate coordination with terrestrial radio stations operating in the same frequency bands. It would, therefore, be desirable to take into account possible future satellite movements in the initial frequency coordination process. Changes in elevation are a

27、ccompanied by a change in azimuth also. The change can range from a very small value, for earth stations at high latitudes, to 180“ for earth stations on the equator and near the sub-satellite point. From a system point of view, however, it is the elevation angle changes which are the more significa

28、nt since these will impact on path loss variations, and fade margins required under fading conditions. 3.3 Earth station repointing 3.3.1 General The most obvious impact on the earth station network would be the need for changes to the antenna pointing and polarization angle settings. This is only c

29、onsidered a significant problem for antennas which lack tracking capability. Movement of the order of 10“ in bands shared with the terrestrial network may cause coordination problems. 3.3.2 Repointing of very small antennas The e.i.r.p. of satellites in the FSS are increasing and soon it may become

30、possible to employ ground receivers with antennas of less than 1 m in diameter. The ease with which people with little or no experience could repoint antennas depends very much on the size and type of antenna used. However, the need for polarization adjustments would present a problem unless circula

31、r polarization were used. 3.3.3 Repointing of 1-2 m antennas In the case of medium power satellites, ground receivers with antennas between 1 and 2 m in diameter which have no tracking capabilities are typically used. With such antennas it would be necessary to change the antenna pointing and polari

32、zation orientation for linearly polarized transmissions. Polar mounted antennas greatly simplify the job of moving between different satellite positions and are also being increasingly used in conjunction with automatic repointing and polarization adjustment equipment. 286 3.3.1 CCIR RECNN*670-3 92

33、4855232 0538906 T20 Rec. 670-1 Repointing of 2-5 m antennas In the 14/11 GHz band an antenna of this size has a narrow beamwidth, and is therefore more likely to have tracking, whereas at 6/4 GHz an antenna of this size is still unlikely to have tracking. Antennas of this size are frequently employe

34、d in the case of, either, lower power satellites or, for reception outside the main coverage area of higher power satellites. More importantly in this case, is the fact that antennas with insufficient steering range would have difficulties regarding repointing. Any movement of the satellite(s) outsi

35、de of their nominal station-keeping constraints would lead to substantial operational difficulties for some services, irrespective of how slowly the satellite position was changed. It would be impracticable for antennas in this category to be adjusted by unskilled persons, so sufficient time and mon

36、ey would have to be allowed for trained technicians to attend all the terminals involved, which may run into hundreds to thousands. This would be quite a major undertaking and should not be dismissed lightly. 3.3.5 Multiple-beam earth station Some multiple satellite systems are seeking to employ mul

37、tiple-beam earth stations for simultaneous access to more than one satellite. Should such earth stations proliferate then the allowable movement of one satellite relative to the others in the system would be severely constrained. 3.3.6 Service disruption If the satellite has to pass through the orbi

38、tal position of another satellite operating in the same frequency bands in moving from its old location to the new one, interference may arise if both networks are kept in operation. Further, the mechanics of the move may dictate that some of the major sub-systems of the relocating satellite be temp

39、orarily deactivated, e.g. solar panels or antennas may have to be locked in position during the move; precise and continuous attitude control may be difficult. Thus, there could be circumstances when some or all of the traffic normally carried by the satellite would have to be re-routed during the r

40、elocation period, which may last several days. Some of these problems might not arise if there was a spare satellite in orbit. It would then be possible to locate the spare satellite in the new operational position, and to transfer service from one satellite to the other by pointing over the earth s

41、tation antennas. An important consideration, both operationally and commercially, is the length of time for which an outage would occur during repositioning of a satellite. This will depend on the amount of fuel that can be used for such purposes, whether a spare in-orbit satellite is available and

42、the longitudinal change required. In some cases, the length of time for all the earth stations to be repointed may prove to be the critical factor. 3.4 Coverage areas and satellite antennas A consequence of moving the satellite from one location to another would be to distort the coverage areas on t

43、he ground. The extent to which this impacts on the overall performance will depend on many factors including the use of single or multiple beam satellites, the size of the beams, the extent to which the satellite is moved and the degree of repointability of the satellite antennas. With the satellite

44、 repointed towards the nominal beam centre, the coverage areas differ only slightly at the extremities. In the worst case, near the equator, the satellite antenna beamwidth would need to be increased by about twice as much as at higher latitudes. This would be of greater significance for shaped beam

45、s than for spot beams. CCIR RECMN*670-1 72 4855212 0518907 967 H Rec. 670-1 287 A satellite antenna is usually designed to provide optimum coverage of a given geographical area from a specific location on the geostationary-satellite orbit. If the satellite is required to have a large service arc, it

46、 would usually be necessary to be able to change the pointing angle of the beams while the satellite is in orbit. In addition, it would be necessary for the cross-section of the beams to be shaped so that the footprints covered the service areas from any point on the service arc. The gain of the sat

47、ellite beams is therefore likely to be less than it would be if their shapes were optimized for a single orbital location. In consequence there may also be a greater tendency to interfere with other satellite networks and a greater liability to suffer interference. For large service areas at any mic

48、rowave frequency and for smaller service areas at the higher microwave frequencies, the use of reconfigurable compound antenna systems with beams made up of a large number of high gain “beamlets” provides an efficient solution to the problem of steering beams and adjusting their cross section so tha

49、t the performance of the system is maintained and the interference situation is not aggravated. For smaller service areas, conventional spot beams will be more commonly used. Studies reported so far indicate that, for the assumed satellite systems, a satellite performance penalty not exceeding 1 dB and an interference penalty not exceeding 2 dB would arise for a repositioning capability of f10”. In a practical situation a smaller relocation should prove sufficient to achieve adequate benefit. Keeping within these penalty limits would probably entail the following requirements: - satell