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Alternatively, if the orbit plane is perpendicular to the Sun, the vehicle will always pass over Earth noon on the north-bound leg, and Earth midnight on the south-bound leg. 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 - or 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 these forces.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 J3 perturbationīackground & Reasons for Selecting a Frozen Orbitįor many spacecraft, changes to the orbit are caused by the oblateness of the Earth, gravitational attraction from the Sun and Moon, solar radiation pressure, and air drag.1 Background & Reasons for Selecting a Frozen Orbit.