Orbital stationkeeping
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In astrodynamics orbital stationkeeping is a term used to describe a particular set of orbital maneuvers used to keep a spacecraft in assigned orbit, either low earth orbit (LEO), or geostationary orbit (GEO). It is especially important for satellite communications systems since maintaining proper satellite position over long periods of time is crucial for the operation of those systems.
Stationkeeping maneuvers require inclusion of a particular delta-v in the mission's delta-v budget. Due to its usually low requirements for propulsive impulses the stationkeeping is usually performed using attitude control system's thrusters.
Stationkeeping is particularly hard for spin-stabilized satellites.
[edit] Stationkeeping in LEO
Stationkeeping is necessary for some objects such as the International Space Station or the Mir or Salyut stations. The International Space Station has an operational altitude above Earth between 330 and 410 km. Due to atmospheric drag, the space station is constantly losing orbital energy. In order to compensate for this loss, which would eventually lead to a reentry of the station (time depends on solar activity), it is being reboosted to a higher orbit from time to time. The chosen orbital altitude is a trade-off between the delta-v needed to reboost the station and the delta-v needed to send payloads and people to the station. The upper limitation of orbit altitude is due to the constraints imposed by the Soyuz spacecraft.
[edit] Stationkeeping in GEO
Due to luni-solar perturbations and the ellipticity of the Earth equator, an object placed in a GEO without any stationkeeping would not stay there. It would start building up inclination at an initial rate of about 0.85 degrees per year. After 26.5 years the object would have an inclination of 15 degrees, decreasing back to zero after another 26.5 years. Therefore, a lot of energy has to be devoted to maneuvers that compensate this tendency. This part of the GEO stationkeeping is called North-South control. The ellipticity of the Earth equator is causing an East-West drift if the satellite is not placed in one of the stable (75 degrees longitude east, 105 degrees longitude west) or unstable (15 degrees longitude west, 165 degrees longitude east) equilibrium points. Nevertheless, this part of GEO stationkeeping, called East-West control requires significantly less amount of fuel than North-South control. Therefore, in some cases aging satellites are only East-West controlled. This would still guarantee that the satellite is always visible to a steerable antenna.
Taking into consideration the relatively long periods of operation of modern GEO satellites (up to 15 years) the delta-v expended over such a period can be substantial (about 46 m/s per year). It is therefore crucial for GEO satellites to have the most fuel-efficient propulsion system. Some modern satellites are therefore employing a high specific impulse system like plasma or ion thrusters.
