Orbital perturbations and their correction

In time, natural forces change the orbits into which satellites are initially put. These perturbations are most significant and most thoroughly characterised for geostationary satellites.

Non-uniformity of the gravitational field of the Earth causes the period of a geostationary satellite, initially exactly equal to one sidereal day, to increase or decrease so as to accelerate the satellite towards either of two stable points, at 77° east longitude or 108° west longitude. In many parts of the orbit a significant drift in location, due to this cause, accumulates within a few weeks if not corrected.

The gravitational fields of the Sun and the Moon cause the orbital plane of a geostationary satellite, initially co-incident with the equatorial plane, to become inclined to it. The inclination grows at about 0.86° per annum if not corrected, causing daily excursions of the satellite north and south of its nominal position.

The pressure of solar radiation and solar wind on a geostationary satellite cause the orbit, initially circular, to become somewhat elliptical. The effect is seasonal, a build up of ellipticity at one time of year being neutralised by a change in the contrary sense at another time of year. Ellipticity causes the satellite to seem to oscillate daily east and west of its nominal position. The effect increases with the area of cross-section of the satellite and it is big enough to be significant if the satellite is large.

Figure 51.2 Flight profile to the goestationary satellite orbit

Significant movement of a satellite, east or west of its nominal position, is likely to cause interference to or from another network operating in the same frequency band and using a neighbouring satellite. Drifts due to the Earth's gravitational field can be corrected throughout the life of a satellite by means of the same Hydrazine thrusters as were used for the final adjustment of the orbit during the launch procedure; the amount of fuel required for this is relatively small. The ITU Radio Regulations require geostationary satellites using frequency bands allocated to the fixed satellite service to be maintained within 0.1° of their nominal orbital longitude, if an east-west perturbation would cause unacceptable interference to another system (ITU, 1990m). The constraint is eased to 0.5° for experimental satellites and satellites which do not use fixed satellite frequency bands (ITU, 1990n). A further relaxation, to 1.0°, applies to certain old satellites, launched before 1987 (ITU, 1990o). Con­straints on broadcasting satellites operating at 12GHz are at least as severe, and they are unconditional (ITU, 1990g).

North-south excursions of satellites due to inclination of the orbital plane can also be corrected by the on board thrusters, but the amount of fuel that is required to maintain low inclination is quite significant; for a 10-year lifetime the mass of hydrazine required for this purpose is about 20% of the total mass of the satellite at start of life. For this reason, bi-propellant thruster systems and electrically powered ion engines which use less payload mass are coming into use (Hayn, et al., 1978; Free, 1980). No regulatory limit is applied to excursions north and south of the equatorial plane, but satellites with inclinations of more than 5° have not been regarded as geosta­tionary for regulatory purposes.

Figure 51.3 Launch sequence of a Hughes spin stabilised geostationary communications satellite using a Delta 3910 expendable launcher

Table 51.5 Characteristics of the major communication satellite launchers

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