In astrodynamics orbital station-keeping 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.
Station-keeping 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 station-keeping is usually performed using attitude control system's thrusters.
Station-keeping is particularly difficult for spin-stabilized satellites.
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.
Station-keeping in low-earth orbit
Station-keeping is necessary for objects in low-earth orbit (LEO) such as the International Space Station, and for satellites for which a precise knowledge of their orbital position is necessary, e.g. earth observation satellites. 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, 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. On 25 April 2008, the Automated Transfer Vehicle "Jules Verne" raised the orbit of the ISS for the first time, thereby proving its ability to replace (and outperform) the Soyuz at this task.
Station-keeping in geostationary orbit
Once a satellite has reached geostationary orbit (GEO), 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 around the Sun (ecliptic) or the Moon's orbit around the Earth, so the gravitational pulls of the Sun and Moon drag 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 longitudinal stable points. Solar radiation pressure, caused by the transfer of momentum from the Sun’s light and infrared radiation, periodically flattens and disturbs the orientation of the orbit. Other factors, such as local irregularities in the gravitational field, also contribute less systematically to drift pressures.
Due to luni-solar perturbations and the ellipticity of the Earth equator, an object placed in a GEO without any station-keeping 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 station-keeping 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. This part of GEO station-keeping, called East-West control, requires significantly less fuel than North-South control, so some aging satellites are only East-West controlled. This still guarantees that the satellite is always visible to a steerable antenna.
The long lifetimes of modern satellites require substantial burdens of fuel for station-keeping, about 46 m/s per year in GEO over the (15 year) period of operation. To save weight, it is 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.
Published - July 2009
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