Inclined orbit

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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.

[edit] Special case: geosynchronous inclined orbit

A geostationary orbit occurs when an object (satellite) is placed approximately 37,000 km (22,300 miles) 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 & Moon) exhibit influence over the satellite, as the effect accumulates over time the trace becomes a figure-eight with lobes oriented north-southward once every sidereal day.

A geostationary orbit is not stable. It takes regular manoeuvres to actively counteract the above gravitational forces. The majority of the fuel, typically hydrazine is spent for this purpose. Otherwise the satellite experiences a steady increase in the inclination over time. At the end of the satellite's lifetime, when fuel approaches depletion, satellite operators may decide on omitting inclination manoeuvres and only control eccentricity. This could prolong the life-time of the satellite as it consumes less fuel over time. On the other hand the satellite can only be used by ground antennae capable of following above mentioned north-south movement, Satellite Tracking earth stations. Before the fuel comes to an end, satellites can be moved to a graveyard orbit to keep the geostationary altitude free for subsequent missions.

NASA maintains a Java based real-time display of most commercial satellites which can be helpful in visualising the various orbits.