Geosynchronous orbit

A geosynchronous orbit (sometimes abbreviated GSO) is an orbit around Earth of a satellite with an orbital period that matches Earth's rotation on its axis, which takes one sidereal day (about 23 hours, 56 minutes, and 4 seconds). 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 traces out a path, typically in a figure-8 form, whose precise characteristics depend on the orbit's inclination and eccentricity. Satellites are typically launched in an eastward direction. A circular geosynchronous orbit is 35,786 km (22,236 mi) above the Earth's surface. Those closer to Earth orbit faster than Earth rotates, so from Earth, they appear to move eastward while those that orbit beyond geosynchronous distances appear to move westward. A geosynchronous orbit (sometimes abbreviated GSO) is an orbit around Earth of a satellite with an orbital period that matches Earth's rotation on its axis, which takes one sidereal day (about 23 hours, 56 minutes, and 4 seconds). 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 traces out a path, typically in a figure-8 form, whose precise characteristics depend on the orbit's inclination and eccentricity. Satellites are typically launched in an eastward direction. A circular geosynchronous orbit is 35,786 km (22,236 mi) above the Earth's surface. Those closer to Earth orbit faster than Earth rotates, so from Earth, they appear to move eastward while those that orbit beyond geosynchronous distances appear to move westward. A special case of geosynchronous orbit is the geostationary orbit, which is a circular geosynchronous orbit in Earth's equatorial plane (that is, directly above the Equator). A satellite in a geostationary orbit appears stationary, always at the same point in the sky, to observers on the surface. Popularly or loosely, the term geosynchronous may be used to mean geostationary. Specifically, geosynchronous Earth orbit (GEO) may be a synonym for geosynchronous equatorial orbit, or geostationary Earth orbit. 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. A semi-synchronous orbit has an orbital period of half a sidereal day (i.e., 11 hours and 58 minutes). Relative to Earth's surface, it has twice this period and hence appears to go around Earth once every day. Examples include the Molniya orbit and the orbits of the satellites in the Global Positioning System. Circular Earth geosynchronous orbits have a radius of 42,164 km (26,199 mi). All Earth geosynchronous orbits, whether circular or elliptical, have the same semi-major axis. In fact, orbits with the same period always share the same semi-major axis: where a is the semi-major axis, P is the orbital period, and μ is the geocentric gravitational constant, equal to 398,600.4418 km3/s2. In the special case of a geostationary orbit, the ground track of a satellite is a single point on the equator. In the general case of a geosynchronous orbit with a non-zero inclination or eccentricity, the ground track is a more or less distorted figure-eight, returning to the same places once per sidereal day. A geostationary equatorial orbit (GEO) is a circular geosynchronous orbit in the plane of the Earth's equator with a radius of approximately 42,164 km (26,199 mi) (measured from the center of the Earth). A satellite in such an orbit is at an altitude of approximately 35,786 km (22,236 mi) above mean sea level. It maintains the same position relative to the Earth's surface. If one could see a satellite in geostationary orbit, it would appear to hover at the same point in the sky, i.e., not exhibit diurnal motion, while the Sun, Moon, and stars would traverse the skies behind it. The theoretical basis for this novel phenomenon of the sky goes back to Newton's theory of motion and gravity. In that theory, the existence of a geostationary satellite is made possible because the Earth rotates (with respect to an inertial frame in which Newton's laws of motion and gravity hold). However, as a practical device, the geostationary satellite owes much for its realisation to Arthur C. Clarke who proposed it during the 20th century and in whose honour the orbit is called the Clarke orbit. Such orbits are useful for telecommunications satellites. A perfectly stable geostationary orbit is an ideal that can only be approximated. In practice the satellite drifts out of this orbit because of perturbations such as the solar wind, radiation pressure, variations in the Earth's gravitational field, and the gravitational effect of the Moon and Sun, and thrusters are used to maintain the orbit in a process known as station-keeping. Elliptical geosynchronous orbits are used in communications satellites to keep the satellite in view of its assigned ground stations and receivers. A satellite in an elliptical geosynchronous orbit appears to oscillate in the sky from the viewpoint of a ground station, tracing an analemma in the sky. Satellites in highly elliptical orbits must be tracked by steerable ground stations.