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.

Orbital characteristics

Every geosynchronous orbit has a semi-major axis of 42,164 km (26,200 miles). When the orbit has a non-zero inclination or eccentricity, the ground track of a satellite in that orbit is a more or less distorted figure-eight, returning to the same places once per solar day.

Geostationary orbit

Main article: 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 km (22,236 statute miles) 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 objects other than the Earth

Main article: Synchronous orbit

Synchronous orbits exist around all moons, planets, stars and black holes — unless they rotate so slowly that the orbit would be outside their Hill sphere. Most inner moons of planets have synchronous rotation, so their synchronous orbits are, in practice, limited to their leading and trailing Lagrange points. 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. Such devices are still theoretical.

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. 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, including satellite television.

Initially, geostationary satellite also carried cellphone calls but are no longer used so predominantly for voice communication, partly due to the inherent disconcerting delay in getting information to the satellite and back (it takes light or radio about a quarter of a second to make the round trip, thus two parties talking via satellite will perceive about a half second delay in round-trip response). Similarly, international Internet connectivity has shifted away from satellite links.

Nearly all populated land locations on the planet now have terrestrial communications facilities (microwave, fiber-optic), even undersea, with more than sufficient capacity. Satellite telephony is now mainly limited to small, 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.

See also

• Geosynchronous satellite
• Geostationary orbit
• Graveyard orbit
• Molniya orbit
• List of orbits

External links

• Nasa.gov: Geosynchronous Orbit
• Science Presse data on Geosynchronous Orbits (including historical data and launch statistics)
• ORBITAL MECHANICS (Rocket and Space Technology)