Station keeping
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Station keeping is the practice of maintaining the orbital position of satellites in geostationary orbit.
Once a satellite has reached geostationary orbit, it seems natural that it should remain there. Life, of course, is not so simple because orbital perturbations cause the satellite to drift.
The principal correction required is to compensate for North-South drift. The geostationary plane (above the equator) is not aligned to the Earth's orbit round the Sun (ecliptic) or the Moon's orbit round the Earth, so the gravitational pull of the Sun and Moon drags satellites off the plane. Uncorrected, this would cause the inclination of the orbit to increase by approximately one degree per year. The average annual velocity change needed to correct this effect is about 50 m/s, which can represent 95% of the total station-keeping propellant budget.
Other drift pressures are also significant if uncorrected. East-West drift occurs because the equator is not perfectly circular, so satellites drift slowly towards one of two stable points. Solar radiation pressure, caused by the transfer of momentum from the Sun’s light and infrared radiation, both flattens the orbit and disturbs the orientation of the satellite. Other factors, such as local irregularities in the gravitational field, also contribute less systematically to drift pressures.
Automatic and Earth-station systems monitor a satellite's position with telemetry instrumentation and make any required corrections using on-board thruster systems. The specifics of these intervals depend on the tolerance of the inclination windows and the propulsion systems used in the thrusters. Ion thruster systems (notably Xenon-Ion Propulsion Systems or XIPS) are being used increasingly for station-keeping, due to their propellant efficiency and suitability for low acceleration applications.
[edit] See also
[edit] External links
- Stationkeeping at the Encyclopedia of Astrobiology, Astronomy, and Spaceflight
- XIPS Xenon Ion Propulsion Systems