Inclined orbit

A satellite is said to occupy an inclined orbit around the Earth if the orbit exhibits an angle other than zero degrees with the equatorial plane. This angle is called the orbit's inclination. A planet is said to have an inclined orbit around the Sun if it has an angle other than zero to the plane of the ecliptic.

A geostationary orbit occurs when an object (satellite) is placed approximately 37,000 km (23,000 mi) above the Earth's equator with the characteristic that, from a fixed observation point on the Earth's surface, it appears motionless. A satellite is in an inclined orbit when its orbital plane is tipped some number of degrees from the horizontal defined by the equator. In the case of an inclined geosynchronous orbit, although the satellite remains geosynchronous (that is, completing one orbit around the earth every 24 hours), it is no longer geostationary. From a fixed observation point on Earth, it would appear to trace out a small ellipse as the gravitational effects of other stellar bodies (Sun and Moon) exhibit influence over the satellite, as the effect accumulates over time the trace becomes an analemma with lobes oriented north-southward. The satellite traces the same analemma once each sidereal day.

A geostationary orbit is not stable. It takes regular manoeuvres to actively counteract the above gravitational forces. The majority of the fuel of the satellite, typically hydrazine, is spent for this purpose. Otherwise, the satellite experiences a change in the inclination over time. At the end of the satellite's lifetime, when fuel approaches depletion, satellite operators may decide to omit these expensive manoeuvres to correct inclination and only control eccentricity. This prolongs the life-time of the satellite as it consumes less fuel over time, but the satellite can then only be used by ground antennas capable of following the north-south movement, satellite-tracking Earth stations. Before the fuel runs out, satellites can be moved to a graveyard orbit to keep the geostationary altitude free for subsequent missions.

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