Geosynchronous orbit

A geosynchronous orbit is an orbit around the Earth with an orbital period matching the Earth's sidereal rotation period. This synchronization means that for an observer at a fixed location on Earth, a satellite in a geosynchronous orbit returns to exactly the same place in the sky at exactly the same time each day. The special case of a geosynchronous orbit that is circular and directly above the equator is called a geostationary orbit.

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. Examples include the Molniya orbit and the orbits of the satellites in the Global Positioning System.