Such orbits are desirable for many Earth observation missions such as weather, imagery, and mapping. Alternatively, if the sun lies in the orbital plane, the vehicle will always pass over places where it is midday on the north-bound leg, and places where it is midnight on the south-bound leg (or vice versa). on the south-bound portion (or vice versa). For instance, if the orbit is "square to the sun", the vehicle will always pass over points at which it is 6 a.m. An appropriate inclination (97.8-99.0 degrees) is selected so that the precession of the orbital plane is equal to the rate of movement of the Earth around the sun, about 1 degree per day.Īs a result, the spacecraft will pass over points on the Earth that have the same time of day during every orbit. For these missions, a near-circular orbit with an altitude of 600–900 km is used. For a geostationary spacecraft, correction maneuvers on the order of 40–50 m/s per year are required to counteract the gravitational forces from the sun and moon which move the orbital plane away from the equatorial plane of the Earth.įor sun-synchronous spacecraft, intentional shifting of the orbit plane (called "precession") can be used for the benefit of the mission. They must be counteracted by maneuvers to keep the spacecraft in the desired orbit. 4 Derivation of the closed form expressions for the J 3 perturbationįor most spacecraft, changes to orbits are caused by the oblateness of the Earth, gravitational attraction from the sun and moon, solar radiation pressure, and air drag.
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