1、 Rec. ITU-R S.1759 1 RECOMMENDATION ITU-R S.1759 Analysis of interference from HEO system space operation transmissions in FSS bands into GSO networks and corresponding guidelines to be used for designing and operating TT b) that TT c) that telecommand signal transmissions originate and terminate un
2、der satellite operator control; d) that TT e) that loss of the uplink command carriers to a satellite and downlink telemetry and ranging carriers during orbital manoeuvres, or when a critical malfunction occurs on-board a spacecraft, could result in the loss of a satellite; f) that TT g) that HEO-ty
3、pe FSS operators should be given some flexibility to operate TT h) that most HEO-type FSS satellites transmit and receive service carriers only while they are within their “active” arcs having large angular separation from GSO links, but it would not be practicable to limit TT j) that there may be s
4、everal possibilities to overcome the challenges of operating TT b) that in certain frequency bands identified in RR Article 22, HEO-type satellites are required to meet epfd limits; c) that in FSS frequency bands other than those identified in recognizing a), sharing between HEO-type FSS systems and
5、 GSO FSS networks is subject to the relevant provisions of Section II of RR Article 9, recommends 1 that the technical and operational analysis and techniques given in Annex 1 may be taken into consideration by operators of HEO-type FSS systems in the design and the operation of their TT during the
6、remaining portion continuity of the service links is maintained by the other satellites following the repeated ground track. But each and every satellite in a system requires its own telemetry and telecommand carriers, on frequencies exclusive to itself (within the system), and these carriers need t
7、o be able to be transmitted at any time during the life of the satellite in orbit. This is true for all types of geostationary and non-geostationary satellite, and is not a feature unique to HEO satellites. Clearly it would not be practicable to operate a satellite whose telemetry and telecommand ca
8、rriers could be transmitted only during one quarter or one third of each period of 12, 18 or 24 h, and it is probable that, unlike the service links of HEO systems, there may be a need for the TT the apogee height is 39 970 km, its latitude is 45 N and its longitude, common to all three planes, is 1
9、35 E; the perigee height is 31 602 km, and the orbit eccentricity is 0.099; this low eccentricity results in each of the satellites being above geostationary altitude throughout the “active” arc, and only about 412 km below geostationary altitude when crossing the Equator; while travelling North to
10、South, the HEO satellites ground track crosses the Equator at 123.7 E, and crosses at 146.3 E while travelling South to North; for the service links the “active” arc extends from 4 h before apogee to 4 h after apogee (i.e. for satellite latitudes above 26.5 N); 4 Rec. ITU-R S.1759 there are four cir
11、cularly polarized telemetry carriers, i.e. one per satellite plus a fourth for back-up, on separate frequencies in the 12 GHz FSS band, and each in a bandwidth of 605 kHz, although for most of the time the majority of the power falls within 20 kHz of the carrier centre frequency; the overall downlin
12、k bandwidth is thus about 2.5 MHz; the satellite e.i.r.p. per telemetry carrier is 7 dBW; the system is designed so that each satellite can transmit its telemetry carrier continuously; there is one circularly polarized telecommand carrier on a separate frequency in the 14 GHz FSS band for each of th
13、e three satellites, in a bandwidth of 600 kHz, although again the majority of the power normally falls within 20 kHz of the carrier centre frequency; the overall uplink bandwidth is thus about 2 MHz; the e.i.r.p. per telecommand carrier in normal mode is 50 dBW (80 dBW in emergency mode); the system
14、 is designed so that each of the three telecommand carriers may be transmitted at any time; for most of each orbit the telecommand carrier to each satellite is transmitted (when necessary) from a TT the TT as Recommendation ITU-R S.580 only provides the pattern from 1 or 100*(/D), whichever is small
15、er (in this case 100*(/D) is smaller), the main-lobe pattern from a different Recommendation needs to be used. The analysis performed for this example used a modified Recommendation ITU-R S.1428 pattern for the main-lobe. on each satellite the beam for transmission of the telemetry carrier and the r
16、eception of the telecommand carrier has a peak gain of 16 dBi, which corresponds to a half-power beamwidth of about 30, and is modelled as meeting Recommendation ITU-R S.672; each satellites TT uplink B representing a link from a GSO earth station at 10 elevation to a satellite at worst-case longitu
17、de; and uplink C representing a link to a GSO satellite just far enough away from one of the two worst-case longitudes to enable the epfdlimit to be met. Note that for uplinks A and C, the GSO earth station is co-located with the HEO TT similarly, Link D is worst-case for the downlink due to the HEO
18、 satellite periodically entering the mainbeam of the GSO earth station receive antenna; Links B and E involve GSO satellites in worst-case longitudes insofar as interference from the HEO system is concernned, but the interference path length is a maximum for Link E and in both links the GSO earth st
19、ations are operating at minimum elevation; and Links C and F involve GSO satellites that are just far enough away in longitude from an HEO Equator crossing points for the corresponding epfd limit to be met on the shortest interference path Figure 2 is a time plot of the epfdin uplinks A, B and C as
20、one of the HEO satellites traverses the Equatorial plane, showing that the limit is substantially exceeded in the first two for limited durations but is just met in the case of Link C. Figure 4 is a CDF of the epfdstatistics for the three GSO downlinks over a complete orbit period, and here again th
21、e limit mask can be seen to be exceeded in links D and E but just met in Link F. 6 Rec. ITU-R S.1759 FIGURE 2 Variation of epfdduring “in-line”transitions 3 Impacts of interference from TT no need for remote TT in the present example this is 605 kHz, which would allow about 12 dB of spreading. 4.2.2
22、.1 Implementation This interference mitigation technique could be implemented by adding a spread-spectrum (SS) demodulator to the TT no need for remote TT&C earth stations. 4.2.6.3 Disadvantages The satellites would be burdened with additional equipment for the intersatellite link, and for acquisiti
23、on and tracking control of an intersatellite link antenna. The satellites would need a backup TT&C method in case there is an intersatellite link failure. This technique may pose additional design challenges because the HEO system would be required to avoid the potential for interference into GSO ne
24、tworks that might be caused due to the HEO systems intersatellite link geometry. RR No. 22.5CA, adopted by WRC-2000 and modified by WRC-03, states “The (epfd) limits given in Tables 22-1A to 22-1E may be exceeded on the territory of any country whose administration has so agreed.” This provides for
25、solutions to the downlink problem to be sought through the prior 16 Rec. ITU-R S.1759 agreement between an administration seeking to operate telemetry carriers from HEO satellites in an FSS band subject to the epfdlimits, and the administrations of each of the countries covered by the beam via which
26、 the telemetry carriers are to be transmitted. However, this procedure could take a long time (for concerned administrations to reach agreement). Therefore, it would be preferable to first consider the above possibilities. In any bilateral meetings under the provision of RR No. 22.5CA the above poss
27、ibilities might be also taken into account at the discretion of the parties concerned. 5 Conclusion The foregoing considerations show that there may be several possibilities to overcome the challenges of operating TT&C links for HEO systems operating in bands subject to RR Article 22 epfd limits whi
28、le affording appropriate protection to GSO systems operating in these bands. It is noted that these techniques would also be helpful with respect to other FSS bands shared between non-GSO and GSO networks. Further study will be required to determine which of these potential interference mitigation t
29、echniques will be most appropriate for any specific HEO system. Appendix 1 to Annex 1 Methodology to determine the minimum telemetry carrier switch-off periods of a HEO satellite to comply with RR Article 22 epfdlimits Following is a description of an iterative method that can be used to determine t
30、he precise times/orbit latitudes at which the telemetry carrier of a particular HEO satellite is switched off (and on) in order to meet the relevant epfdlimits. As explained in 4.2.1 of Annex 1, any exceedence with respect to the RR Article 22 epfdlimit mask for a given size of GSO earth station ant
31、enna in a given frequency band may be avoided by switching off each telemetry carrier during a period around each instant when its satellite crosses the Equatorial plane. The minimum duration of the period, and its precise switch-off and switch-on times, to just meet the epfdlimit mask concerned wil
32、l depend on the orbital characteristics of the HEO system and the transmission characteristics of its telemetry carriers. Assuming that, as for the example system described in 2 of Annex 1 (N-SAT-HEO2), each HEO satellite in the system has its own telemetry carrier frequency and otherwise all the te
33、lemetry carriers have identical transmission parameters, it is necessary only to determine the minimum switch-off period duration for one satellite. The period will be the same for each of the other satellites in the system, and the precise switch-off and switch-on times will simply differ by interv
34、als determined by the orbit characteristics and the number of satellites following the same ground-track. Taking N-SAT-HEO2 as an example, a computer simulation may be set up to model a worst-case interference scenario as illustrated in Fig. 3 of the Annex, and used to calculate the epfdlevel into R
35、ec. ITU-R S.1759 17 a reference (e.g., 1.2 m) GSO earth station antenna at a worst-case location and operating to a GSO satellite at a worst-case longitude. (As explained in the Annex, for N-SAT-HEO2 the two worst-case situations are when both the GSO satellite and its reference earth station are at
36、 0 latitude and a longitude corresponding to one of the HEO equator-crossings, either 123.7 E or 146.3 E.) This simulation is run for one complete orbit period (86 163 s = 1 sidereal day) with the epfdcalculated for each time step (10 s time steps are sufficiently accurate for this example). The res
37、ults are then plotted in the form of a cumulative distribution function (CDF) of time percentage against epfd. In the present case this is the curve for Link D in Fig. 4 of the Annex. Noting that the Equator-crossing at 123.7 E occurs when the satellite (starting at perigee) is about three-quarters
38、of the way around its 23-h 56-min orbit, the above simulation may be re-run using a large time-step until about 17 h into the orbit. From that point, the simulation can then be stepped using a small time-step until the epfdlevel reaches the value at which the Link D curve intersects the Article 22 m
39、ask (i.e. about 179 dBW/m2per 40 kHz in this example). The time at which this “mask intersection” occurs may be noted. The simulation may then continue to be advanced using a small time-step until the epfdlevel reaches a peak when the satellite is directly over the equator and then returns once more
40、 to the Link D “mask intersection value” and that time noted. The simulation may then be stopped and the model modified to effectively switch-off the telemetry carrier between the two times noted. (In the simulation, the switching off of the telemetry carrier may be modelled by setting the EIRP to a
41、 negligible value for the interval between the two times.) The simulation may then be run in the form thus modified, and the results converted to a new CDF. It will be found that this CDF is significantly below the RR Article 22 mask at all points. (See curve (c) in Fig. 5.) The model may then be fu
42、rther modified to reduce the switch-off period to about one-third of the period in the previous paragraph, but with approximately the same time centre, and the simulation re-run to produce the corresponding CDF. It is likely that this CDF will exceed the RR Article 22 mask for significant epfd range
43、s. (See curve (d) in Fig. 5.) The results from the foregoing two paragraphs will enable the approximate switch-off and switch-on times to produce a CDF just meeting the RR Article 22 mask to be estimated by selecting times that are in-between those resulting in curves (c) and (d). A further run usin
44、g these times may then be made. If the resulting CDF is still not optimum, a further iteration of the process should yield a sufficiently accurate result. The above steps were carried out for N-SAT-HEO2 and a 1.2m GSO earth station antenna, for a telemetry carrier at 12.25 GHz. The resulting CDFs ar
45、e shown in Fig. 5, where curve (b) is the same as the curve for Link D in Fig. 4 (i.e. worst case geometry with no switching off of the telemetry carrier). It will be seen that (e) is the optimum curve since it just barely stays within the epfdlimit mask. Although only three iterations were needed t
46、o produce the optimum curve in this case, it is unlikely that more than four or at most five iterations would be needed for any other practical case. Thus, taking both Equator crossings into account, the switch-off periods for the present example would be as shown in Table 1. 18 Rec. ITU-R S.1759 FI
47、GURE 5 1.2 m epfdstatistics for various telemetry carrier switch-off periods TABLE 1 Switch-off periods to ensure example HEO systems telemetry carriers would just meet epfd limits for 1.2 m in the 12-18 GHz band (Ku-band) GSO earth station antenna Satellite Perigee time (s) 1st switch-off time (s)
48、1st switch-on time (s) 2nd switch-off time (s) 2nd switch-on time (s) 1 0 18 290 19 280 66 850 67 840 2 28 680 9 370 10 360 47 000 47 990 3 57 490 38 180 39 170 75 810 76 800 Note from Table 1 that each of the three satellites in the example HEO system would have to switch off for the same duration
49、in the vicinity of its two crossings of the equatorial plane per orbit to meet the epfd limit mask. Each switch-off period is 990 s = 16.5 min corresponding to 1.15% of the orbital period (one sidereal day). Taking account of both switch-off periods per orbit results in a total “telemetry outage” corresponding to 2.30% of each satellites orbit. Each switch-off period would occur while the satellite concerned is within the latitude range of approximately 1.487. For each successive orbit the switch-off and switch-o
