1、CCIR RECflN*484-3 92 4855232 0538903 449 Rec. 484-3 4D3: Spacecraft station-keeping - Satellite antenna radiation pattern - Pointing accuracy 281 RECOMMENDATION 484-3 STATION-KEEPING IN LONGITUDE OF GEOSTATIONARY SATELLITES IN THE FIXEDSATELLITE SERVICE (Question 48/4) (1974-1978- 1982- 1992) The CC
2、IR, considering that the geostationary-satellite orbit (GSO) is of unique usefulness to telecommunication services; a) b) that the interference imposes a limit on the number of satellites which may operate in the same frequency band in the same part of the orbit but the number will tend to increase
3、as the degree of accuracy of station-keeping is improved; c) few years; that the number of satellites using this orbit for operational purposes is likely to grow significantly in the next d) achieved by some satellites designed before 1982; that it is technically feasible to maintain satellite posit
4、ion to within fO.lO; although this standardmay not be e) when longitudinal positional drifts approach values comparable with the minimum permissible satellite spacing, that the capacity of the GSO is only slightly impaired by moderate orbital inclinations but is greatly reduced recommends that space
5、 stations on geostationary satellites using frequency bands allocated to the fured-satellite service: 1. of variation; but should maintain their positions within f O. 1 of longitude of their nominal positions irrespective of the cause 2. need not comply with Q 1 as long as the satellite network to w
6、hich the space station belongs does not produce an unacceptable level of interference into any other satellite network whose space station complies with the limits of Q 1; 3. of slightly inclined geostationary orbit; that the position in 0 1 should be the longitude at which the satellite crosses the
7、 equatorial plane for the case 4. that the following Notes will be considered part of this Recommendation. Note I - For experimental stations on board geostationary satellites, the limits specified in Q 1 may be replaced by k0.5“. Note 2 - Annex 1 describes the factors which affect longitudinal stat
8、ion-keeping. 282 1. CCIR RECMN*484-3 72 = 4855212 0518702 385 Rec. 484-3 ANNEX 1 Factors affecting longitudinal station-keeping Perturbations in orbital elements A satellite is ideally geostationary if its orbital elements satisfy the following conditions: - semi-major axis a = synchronous radius =
9、42 165 km, - eccentricity e = O, - inclination i =O. The orbit gradually departs from the geostationary state, however, due to the perturbation forces acting on the satellite. The distortion of the Earths gravitational field due to the non-sphericity of the Earth causes either a steady increase or d
10、ecrease in the semi-major axis, according to the stationary longitude of the satellite. No change however occurs at four special longitudes called the equilibrium points. When the semi-major axis deviates from the synchronous radius by Aa (km) the satellite longitude drifts at the rate of -0.013 Au
11、degrees per day. The solar radiation pressure, acting in proportion to the ratio of satellite cross section to mass, changes the eccentricity with time. There is also a minor effect due to the Moons gravity. A non-zero eccentricity e yields a diurnal libration motion in the satellite longitude of as
12、 much as 12e x 180/n degrees. The gravity of the Moon and the Sun acting as a tidai force on the satellite changes the inclination at a rate that varies from year to year between 0.75“ and 0.95“ per year. A non-zero inclination i (degrees) yields a half-diurnal longitudinal libration motion of as mu
13、ch as +(;L/4) (n/180) degrees. This becomes significant for longitudinal station-keeping when the inclination is greater than a few degrees. 2. Errors in longitudinal station-keeping 2.1 Orbit determination error The accuracy of the station-keeping depends to some extent on the accuracy of orbit det
14、ermination. The orbit determination of geostationary communications satellites is generally based on ranging at two distantly located stations or on ranging and azimuth/elevation angle measurements at a single station; orbit determination based on ranging at only a single station is not feasible. Ap
15、propriate calibration of the ranging and, in particular, of the angle measurement yields an accuracy of satellite longitude determination within a few thousandths of a degree. 2.2 Orbit correction error Longitudinal station-keeping is carried out with East-West orbital corrections that adjust the se
16、mi-major axis and the eccentricity to their target values. In practice, the adjusted orbital elements after a correction have errors due to the efficiency of satellite thruster performance. The North-South orbital correction, which is to keep the inclination within a given limit, is kept from distur
17、bing longitudinal motion by selecting the ideal North-South thrust direction. In practice, however, the thrust has a non-North-South component due to attitude-control errors. An unexpected change in the longitudinal motion is therefore possible. The thrust for attitude control may also have a minor effect on the longitudinal motion. These factors together comprise the longitudinal station-keeping error, but the most significant is usually the coupling between the North-South orbital correction and longitudinal motion.
