Geosynchronous Orbit

A geosynchronous orbit (sometimes abbreviated GSO) is an orbit around the Earth with an orbital period of one sidereal day (approximately 23 hours 56 minutes and 4 seconds), matching the Earth's sidereal rotation period. The synchronization of rotation and orbital period means that, for an observer on the surface of the Earth, an object in geosynchronous orbit returns to the exactly 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 traces out a path, typically in the form of an analemma, whose precise characteristics depend on the orbit's inclination and eccentricity.

A special case of geosynchronous orbit is the geostationary orbit (popularly, the term "geosynchronous" may sometimes be used to mean, specifically, geostationary). This is a circular geosynchronous orbit at zero inclination, that is, directly above the equator. A satellite in a geostationary orbit appears stationary, always at the same point in the sky, to ground observers. Communications satellites are often given geostationary orbits, or close to geostationary, 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.

A semisynchronous orbit has an orbital period of 0.5 sidereal days, i.e., 11 h 58 min. Relative to the Earth's surface it has twice this period, and hence appears to go around the Earth twice every day. Examples include the Molniya orbit and the orbits of the satellites in the Global Positioning System.