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In astrodynamics orbital station-keeping is the orbital maneuvers made by thruster burns that are needed to keep a spacecraft in a particular assigned orbit. For many Earth satellites the effects of the non-Keplerian forces, i.e. the deviations of the gravitational force of the Earth from that of a homogeneous sphere, gravitational forces from Sun/Moon, solar radiation pressure and air-drag must be counteracted. The deviation of Earth's gravity field from that of a homogeneous sphere and gravitational forces from Sun/Moon will in general perturb the orbital plane. For sun-synchronous orbit the precession of the orbital plane caused by the oblateness of the Earth is a desirable feature that is part of the mission design but the inclination change caused by the gravitational forces of Sun/Moon is undesirable. For geostationary spacecraft the inclination change caused by the gravitational forces of Sun/Moon must be counteracted to a rather large expense of fuel, as the inclination should be kept sufficiently small for the spacecraft to be tracked by a non-steerable antenna. For spacecraft in low orbits the effects of atmospheric drag must often be compensated for. For some missions this is needed simply to avoid re-entry; for other missions, typically missions for which the orbit should be accurately synchronized with Earth rotation, this is necessary to avoid the orbital period shortening. Solar radiation pressure will in general perturb the eccentricity (i.e. the eccentricity vector), see Orbital perturbation analysis (spacecraft). For some missions this must be actively counter-acted with manoeuvres. For geostationary spacecraft the eccentricity must be kept sufficiently small for a spacecraft to be tracked with a non-steerable antenna. Also for Earth observation spacecraft for which a very repetitive orbit with a fixed ground track is desirable, the eccentricity vector should be kept as fixed as possible. A large part of this compensation can be done by using a frozen orbit design, but for the fine control manoeuvres with thrusters are needed. For spacecraft in a halo orbit around a Lagrangian point stationkeeping is even more fundamental as such an orbit is unstable; without an active control with thruster burns the smallest deviation in position/velocity would result in the spacecraft leaving the orbit completely.〔 ==Station-keeping in low-earth orbit== For a spacecraft in a very low orbit the atmospheric drag is sufficiently strong to cause a re-entry before the intended end of mission if orbit raising manoeuvres are not executed from time to time. An example of this is the International Space Station (ISS), which has an operational altitude above Earth's surface of 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 re-entry of the station, it has from time to time been re-boosted to a higher orbit. The chosen orbital altitude is a trade-off between the delta-v needed to counter-act the air drag 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Orbital station-keeping」の詳細全文を読む スポンサード リンク
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