Station-keeping For Earth Observation Spacecraft
For Earth observation spacecraft typically operated in an altitude above the Earth surface of about 700 - 800 km the air-drag is very faint and a re-entry due to air-drag is not a concern. But if the orbital period should be synchronous with the Earth rotation to maintain a fixed ground track also the faint air-drag at this high altitude must be counter-acted by orbit raising manoeuvres in the form of thruster burns tangential to the orbit. These manoeuvres will be very small, typically in the order of a few mm/s of delta-v. If a frozen orbit design is used these very small orbit raising manoeuvres are sufficient to also control the eccentricity vector.
To maintain a fixed ground track it is also necessary to make out-of-plane manoeuvre to compensate for the inclination change caused by Sun/Moon gravitation. These are executed as thruster burns orthogonal to the orbital plane. For Sun-synchronous spacecraft having a constant geometry relative to the Sun the inclination change due to the solar gravitation is particularly large, a delta-v in the order of 1-2 m/s per year can be needed to keep the inclination constant.
Read more about this topic: Orbital Station-keeping
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