# Geosynchronous orbit

Geosynchronous orbit

A geosynchronous orbit is an orbit around the Earth with an orbital period matching the Earth's sidereal rotation period [V. Chobotov, ed., (1996) "Orbital Mechanics, 2nd edition", AIAA Education Series, p. 304.] . 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. In principle, any orbit with a period equal to the Earth's rotational period is technically geosynchronous, however, the term is almost always used ["e.g.", C. D. Brown (1998), "Spacecraft Mission Design, 2nd Edition," AIAA Education Series, p. 81] to refer to the special case of a geosynchronous orbit that is circular (or nearly circular) and at zero (or nearly zero) inclination, that is, directly above the equator. This is sometimes 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, 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.

Orbital characteristics

All geosynchronous orbits have a semi-major axis of convert|42164|km|mi|0|abbr=on. [] In fact, orbits with the same period share the same semi-major axis:$a=sqrt \left[3\right] \left\{muleft\left(frac\left\{P\right\}\left\{2pi\right\} ight\right)^2\right\}$ where a=semi-major axis, P=orbital period, μ=geocentric gravitational constant.

In the special case of a geostationary orbit, the ground track of a satellite is 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 solar day.

Geostationary orbit

A circular geosynchronous orbit in the plane of the Earth's equator has a radius of approximately 42,164 km (from the center of the Earth). A satellite in such an orbit is at an altitude of approximately 35,786 kilometers above mean sea level. It will maintain 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 one would see the Sun, Moon, and stars traverse the heavens behind it. This is sometimes called a Clarke orbit. Such orbits are useful for telecommunications satellites.

A perfect stable geostationary orbit is an ideal that can only be approximated. In practice the satellite will drift 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.

Synchronous orbits around general astronomical objects

Synchronous orbits exist around all moons, planets, stars and black holes &mdash; unless they rotate so slowly that the orbit would be outside their Hill sphere or so fast that such an orbit would be inside the body. Most inner moons of planets have synchronous rotation, so their synchronous orbits are, in practice, limited to their leading and trailing (L4 and L5) Lagrange points, as well as the L1 and L2 Lagrange points, assuming they don't fall within the body of the moon. Objects with chaotic rotations (such as Hyperion) are also problematic, as their synchronous orbits keep changing unpredictably.

Other geosynchronous orbits

"Elliptical orbits" can be and are designed for communications satellites that keep the satellite within view of its assigned ground stations or receivers. A satellite in an elliptical geosynchronous orbit will appear 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.

Theoretically an "active geosynchronous" orbit can be maintained if forces other than gravity are also used to maintain the orbit, such as a solar sail. Such a statite can be geosynchronous in an orbit different (higher, lower, more or less elliptical, or some other path) from the conic section orbit formed by a gravitational body.

Surveillance satellites use active geosynchronous orbits to maintain position and track above a fixed point on the Earth's surface. They are directed by controllers on the ground.

A further form of geosynchronous orbit is obtained by the theoretical space elevator in which one end of the structure is tethered to the ground, maintaining a longer orbital period than by gravity alone if under tension.

Other definitions of geosynchronous orbit
* Geosynchronous orbit (GEO): a circular orbit, 35786 km above Earth's surface

The following orbits are special orbits that are also used to categorize orbits:

* Geostationary orbit (GSO): zero inclination geosynchronous orbit
* Supersynchronous orbit - a disposal / storage orbit above GSO/GEO. Satellites will drift in a westerly direction.
* Subsynchronous orbit - a drift orbit close to but below GSO/GEO. Used for satellites undergoing station changes in an eastern direction.
* Graveyard orbit - a supersynchronous orbit where spacecraft are intentionally placed at the end of their operational life.

History

Author Arthur C. Clarke is credited with proposing the notion of using a geostationary orbit for communications satellites [A. C. Clarke, "Extra-Terrestrial Relays," "Wireless World, Vol. 51", No. 10, pp. 305-308, 1945] . The orbit is also known as the Clarke Orbit. Together, the collection of artificial satellites in these orbits is known as the Clarke Belt.

The first communications satellite placed in a geosynchronous orbit was Syncom 2, launched in 1963. Geosynchronous orbits have been in common use ever since, in particular for satellite television.

Geostationary satellites also carry international telephone traffic but they are being replaced by fiber optic cables in heavily populated areas and along the coasts of less developed regions, because of the greater bandwidth available and lower latency, due to the inherent disconcerting delay in communicating via a satellite in such a high orbit. It takes electromagnetic waves about a quarter of a second to travel from one end to the other of the link, thus two parties talking via satellite will be subject to about a half second delay in round-trip response.

Although many populated land locations on the planet now have terrestrial communications facilities (microwave, fiber-optic), even undersea, with more than sufficient capacity, satellite telephony and Internet access is still the only service available for many places in Africa, Latin America, and Asia, as well as isolated locations that have no terrestrial facilities, such as Canada's Arctic islands, Antarctica, the far reaches of Alaska and Greenland, and ships at sea.

ee also

*Geostationary orbit
*Geosynchronous satellite
*Graveyard orbit
*List of orbits
*List of satellites in geosynchronous orbit
*Molniya orbit

References

* [http://www.sciencepresse.qc.ca/clafleur/Scfam-geostationary.html Science Presse data on Geosynchronous Orbits (including historical data and launch statistics)]
* [http://www.braeunig.us/space/orbmech.htm ORBITAL MECHANICS] (Rocket and Space Technology)

Wikimedia Foundation. 2010.

### Look at other dictionaries:

• Geosynchronous Orbit —    A spaceship or satellite in orbit 35,900 kilometers above the equator with an orbital period of 24 hours. This orbit keeps the object over a specific Earth location at all times. This type of orbit is used most often with communication… …   The writer's dictionary of science fiction, fantasy, horror and mythology

• geosynchronous orbit — noun a circular orbit around the Earth having a period of 24 hours • Hypernyms: ↑orbit, ↑celestial orbit • Hyponyms: ↑geostationary orbit …   Useful english dictionary

• geosynchronous orbit —    The orbit of a satellite in which the speed and path are precisely timed to position it over a fixed location within a narrow equatorial band of the planet around which it revolves (i.e., synchronized to the planet s geometry) …   IT glossary of terms, acronyms and abbreviations

• Inclined geosynchronous orbit — Inclined Orbit bezeichnet ganz allgemein jede Umlaufbahn mit einer Bahnneigung (Inklination) gegenüber der Äquatorebene des umkreisten Himmelskörpers. Inclined geosynchronous orbit Normalerweise wird mit dem Begriff jedoch eine spezielle, aus dem …   Deutsch Wikipedia

• List of satellites in geosynchronous orbit — TOC This is the list of satellites in geosynchronous orbit. Those satellites are commonly used for communication purposes, such as radio and television networks, backhaul, and direct broadcast. A couple planned global navigation systems by China… …   Wikipedia

• Geosynchronous satellite — A geosynchronous satellite is a satellite whose orbital track on the Earth repeats regularly over points on the Earth over time. If such a satellite s orbit lies over the equator and the orbit is circular, it is called a geostationary satellite.… …   Wikipedia

• Orbit phasing — In astrodynamics orbit phasing is the adjustment of the time position of spacecraft along its orbit, usually described as adjusting the orbiting spacecraft s true anomaly. This is predominantly used in satellite positioning, especially if the… …   Wikipedia

• Geosynchronous Satellite Launch Vehicle — Infobox rocket name = GSLV imsize = 250 caption = Geosynchronous Satellite Launch Vehicle function = Expendable launch vehicle manufacturer = ISRO country origin = IND height = 49 m alt height = 160 ft diameter = 2.8 m alt diameter = 9.1 ft mass …   Wikipedia

• Geosynchronous Satellite Launch Vehicle — GSLV Mk I/II GSLV MkIII Das Geosynchronous Satellite Launch Vehicle (GSLV) …   Deutsch Wikipedia

• Orbit — This article is about orbits in celestial mechanics, due to gravity. For other uses, see Orbit (disambiguation). A satellite orbiting the Earth has a tangential velocity and an inward acceleration …   Wikipedia