A geosynchronous orbit (sometimes abbreviated GSO) is an Earth-centered orbit with an orbital period that matches Earth's rotation on its axis, 23 hours, 56 minutes, and 4 seconds (one sidereal day). The synchronization of rotation and orbital period means that, for an observer on Earth's surface, an object in geosynchronous orbit returns to exactly the same position in the sky after a period of one sidereal day. Over the course of a day, the object's position in the sky may remain still or trace out a path, typically in a figure-8 form, whose precise characteristics depend on the orbit's inclination and eccentricity. A circular geosynchronous orbit has a constant altitude of 35,786 km (22,236 mi), and all geosynchronous orbits share that semi-major axis.
A special case of geosynchronous orbit is the geostationary orbit, which is a circular geosynchronous orbit in Earth's equatorial plane. A satellite in a geostationary orbit remains in the same position in the sky to observers on the surface. Popularly or loosely, the term geosynchronous may be used interchangeably with geostationary.
Communications satellites are often given geostationary or close to geostationary orbits so that the satellite antennas that communicate with them do not have to move, but can be pointed permanently at the fixed location in the sky where the satellite appears.
Statite proposal
A statite is a hypothetical satellite that uses radiation pressure from the sun against a solar sail to modify its orbit.
It would hold its location over the dark side of the Earth at a latitude of approximately 30 degrees. It would return to the same spot in the sky every 24 hours from an Earth-based viewer's perspective, so be functionally similar to a geosynchronous orbit.
Space Elevator
A further form of geosynchronous orbit is the theoretical space elevator. When one end is attached to the ground, for altitudes below the geostationary belt the elevator maintains a shorter orbital period than by gravity alone.
Period
All geosynchronous orbits have an orbital period equal to exactly one sidereal day.This means that the satellite will return to the same point above the Earth's surface every (sidereal) day, regardless of other orbital properties.This orbital period, T, is directly related to the semi-major axis of the orbit through the formula
where:
a is the length of the orbit's semi-major axis
Inclination
A geosynchronous orbit can have any inclination.
Satellites commonly have an inclination of zero, ensuring that the orbit remains over the equator at all times, making it stationary with respect to latitude from the point of view of a ground observer (and in the ECEF reference frame).
Another popular inclinations is 63.4° for a Tundra orbit, which ensures that the orbit's argument of perigee doesn't change over time.
