a re-entry before the intended end of mission if orbit raising maneuvers 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 average thrust needed to counter-act the air drag and the impulse needed to send payloads and people to the station. GOCE which orbited at 255 km (later reduced to 235 km) used ion thrusters to provide up to 20 mN of thrust to compensate for the drag on its frontal area of about 1 m2. 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's rotation to maintain a fixed ground track, the faint air-drag at this high altitude must also be counter-acted by orbit raising maneuvers in the form of thruster burns tangential to the orbit. These maneuvers 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 maneuvers are sufficient to also control the eccentricity vector. To maintain a fixed ground track it is also necessary to make out-of-plane maneuvers 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. Geostationary orbit For geostationary spacecraft, thruster burns orthogonal to the orbital plane must be executed to compensate for the effect of the lunar/solar gravitation that perturbs the orbit pole with typically 0.85 degrees per year. The delta-v needed to compensate for this perturbation keeping the inclination to the equatorial plane amounts to in the order 45 m/s per year. This