Rec. Itu-r sf. 1008-1




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Rec. ITU-R SF.1008-1

RECOMMENDATION ITU-R SF.1008-1*

POSSIBLE USE BY SPACE STATIONS IN THE FIXED-SATELLITE SERVICE OF ORBITS
SLIGHTLY INCLINED WITH RESPECT TO THE GEOSTATIONARY-SATELLITE
ORBIT IN BANDS SHARED WITH THE FIXED SERVICE

(1994-1995)

Rec. ITU-R SF.1008-1

The ITU Radiocommunication Assembly,



considering

a) that, after cessation of North-South station-keeping, nominally geostationary space stations have a “natural” drift to a maximum inclination of approximately  15° relative to the equatorial plane at the maximum initial rate of about 0.9° per year;

b) that the use of orbits which are slightly inclined with respect to the geostationary orbit may be attractive for operations in the fixed-satellite service (FSS) for prolonging the useful life of space stations;

c) that inclined orbit usage may be designed at the planning stages of satellite systems;

d) that a satellite may be injected into a pre-inclined orbit in such a way that its inclination first decreases to zero before increasing;

e) that the FSS operations themselves impose constraints which would, in most cases, limit the amount of inclination which would be used by the FSS networks to values considerably less than the natural limit described in § a);

f) that the number of FSS space stations that will utilize slightly inclined orbits will be small in practice;

g) that FSS systems in inclined orbit operating with the power flux-density limits given in Recommen­dation ITU R SF.358 could cause interference to terrestrial systems by exposing a larger number of terrestrial stations to direct interference, but that not all such affected fixed-service stations will be associated with a single terrestrial network;

h) that, for maintenance of service area coverage, space station beam-pointing will most probably remain within a reasonable tolerance of the original beam direction but that, under these circumstances, arrival angles at terrestrial stations and satellite antenna off-axis angles will vary from the geostationary case;

j) that, while the end-to-end performance of terrestrial systems might not be affected in all cases by the degree of inclination, the probability of individual hops being affected will increase with the amount of inclination;

k) that the existing fixed service networks in most bands currently shared with the FSS are in a mature state and in most countries the fixed links are designed to avoid the azimuth directions with potential interference on the basis of assuming that space stations are located at their nominal geostationary-satellite orbit (GSO) locations;

l) that the impact on space stations in inclined orbits of terrestrial stations in the fixed service currently observing the limits in Recommendation ITU-R SF.406 with respect to space stations in the GSO depends on inclination;

m) that any increased orbit avoidance requirements on the fixed service would severely restrict the available horizon for future fixed service installations (see Figs. 4a and 4b);

n) that the use of inclined orbit may result in a larger coordination area of an earth station;

o) that earth stations associated with FSS space stations in slightly inclined orbits may employ tracking,

recommends

1 that a transmitting space station of a network in the FSS having an assignment on the GSO and intended to operate without North-South station keeping during part of its in-orbit life be launched with a pre-inclination of:

1.1 at least N – 5° where N is the number of years by which operation without North-South station keeping has been planned, or

1.2 5°, whichever is the smaller (see Note 3);

2 that when it is anticipated that a transmitting space station of a network in the FSS will operate at orbital inclinations in excess of 5°, agreement should be sought from affected administrations. Provisionally an administration is considered affected if:

2.1 as a direct result of a satellite exceeding 5° of orbital inclination, the satellite could illuminate a fixed station within its territory with an elevation angle below 5°. This does not include fixed stations which see the satellite below 5° when the inclination of that satellite is less than or equal to 5°, and

2.2 the power flux-density due to the radiation of the satellite towards the fixed station falling under § 2.1 is expected to reach the values shown in Fig. 1 under clear air propagation conditions;

3 that FSS space stations in orbits which are slightly inclined with respect to the GSO continue to observe Recommendation ITU-R SF.358 from all positions within their orbit;

4 that fixed service systems continue to observe Recommendation ITU-R SF.406 with respect to the GSO;

5 that earth stations be coordinated or, if necessary, re-coordinated, taking into account the degree of tracking required to accommodate the use of inclined orbits;

6 that the following Notes be considered part of this Recommendation.

NOTE 1 – Recommendations ITU-R SF.358 and ITU-R SF.406 have values similar to those in Article S21 of the Radio Regulations (RR) respectively. The values given in the RR have precedence.

NOTE 2 – Station-keeping and beam pointing information submitted in accordance with RR Appendice S4 should include any planned use of a slightly inclined orbit.

NOTE 3 – Launch with pre-inclination refers to a launch which places a spacecraft in an inclined circular geosyn­chronous orbit having initial parameters which, after injection of the spacecraft into that orbit and without any inclination adjustments, will cause the orbit’s inclination to decrease towards zero before increasing.

NOTE 4 – Section 2 does not apply to those space stations which have started the slightly inclined orbit operation or have been notified as such where appropriate before the end of 1993.

NOTE 5 – Considerations of frequency sharing between the fixed service and the FSS using satellites in orbits slightly inclined with respect to the GSOs are given in Annex 1.

NOTE 6 – Even if the coordination threshold of Fig. 1 is observed, there is still a small but possibly significant risk of high level interference on terrestrial hops with high-gain antennas pointing at the azimuth ranges corresponding to the visibility of inclined orbits with inclinations less than 10°.

NOTE 7 – Recommendation ITU-R S.743 deals with the coordination of satellite networks using slightly inclined GSOs and between such networks and satellite networks using non-inclined GSO satellites.

NOTE 8 – Depending on the number of slightly inclined orbit satellites falling under § 2.1 which interfere into a particular multi-hop radio-relay system, it may be difficult to fulfil the availability and performance objectives of Recommendation ITU R SF.615. Further studies are required on this matter.

FIGURE 1...[D01]  13.5 CM

ANNEX 1

Frequency sharing between the fixed service and the fixed-satellite
service using satellites in orbits slightly inclined with respect
to the geostationary-satellite orbit

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