A geosynchronous orbit (sometimes abbreviated GSO) is an orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period.[1] The synchronization of rotation and orbital period means that for an observer on the surface of the Earth, the satellite appears to constantly hover over the same meridian(north-south line) on the surface, moving in a slow oscillation alternately north and south with a period of one day, so it returns to exactly the same place in the sky at exactly the same time each day.
However, the term is often popularly used[2] to refer to the special case of a geosynchronous orbit called a geostationary orbit. This is a geosynchronous orbit that is circular and 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 don't 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 once every day. Examples include the Molniya orbit and the orbits of the satellites in the Global Positioning System.
