Geostationary Orbit
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Geostationary Orbit (GEO)
A geostationary orbit (GEO) is an orbit directly above the Earth's equator (0º latitude). It is a special case of the geosynchronous orbit (abbreviated GSO), and is the one which is of most interest to operators of artificial satellites (including communication and television satellites). Satellite locations may differ by longitude only (remember, in geostationary orbit, latitude is zero).
The idea of a geosynchronous satellite for communication purposes was first published in 1928 by Herman Potočnik. The geostationary orbit was first popularised by science fiction author Arthur C. Clarke in 1945 as a useful orbit for communications satellites. As a result this is sometimes referred to as the Clarke orbit. Similarly, the Clarke Belt is the part of space approximately 35,786 km above mean sea level in the plane of the equator where near-geostationary orbits may be achieved.
Geostationary orbits are useful because they cause a satellite to appear stationary with respect to a fixed point on the rotating Earth. As a result, an antenna can point in a fixed direction and maintain a link with the satellite. The satellite orbits in the direction of the Earth's rotation, at an altitude of approximately 35,786 km (22,240 statute miles) above ground. This altitude is significant because it produces an orbital period equal to the Earth's period of rotation, known as the sidereal day.
Use in artificial satellites
Geostationary orbits can only be achieved very close to the ring 35,786 km directly above the equator. In practice this means that all geostationary satellites have to exist on this ring, which poses problems for satellites that will be decommissioned at the end of their service life (e.g. when they run out of thruster fuel). Such satellites will either continue to be used in inclined orbits (where the orbital track appears to follow a figure-of-eight loop centred on the Equator) or be raised to a "graveyard" disposal orbit.
A geostationary transfer orbit is used to move a satellite from low Earth orbit (LEO) into a geostationary orbit. A worldwide network of operational geostationary meteorological satellites are used to provide visible, as well as infrared images of Earth's surface and atmosphere. These satellite systems include:
- the US GOES.
- Meteosat, launched by the European Space Agency and operated by the European Weather Satellite Organization, EUMETSAT.
- the Japanese GMS.
- India's INSAT series.
Most commercial communications satellites and television satellites operate in geostationary orbits. (Russian television satellites have used elliptical Molniya and Tundra orbits due to the high latitudes of the receiving audience.)
A statite, a hypothetical satellite that uses a solar sail to modify its orbit, could theoretically hold itself in a "geostationary" orbit with different altitude and/or inclination from the "traditional" equatorial geostationary orbit. However, this would rely on using the solar wind at high altitude outside the Earth's magnetosphere.
Practical limitations
While a geostationary orbit should hold a satellite in fixed position above the equator, orbital perturbations cause slow but steady drift away from the geostationary location. Satellites correct for these effects with station keeping manoeuvres. In the absence of servicing missions, consumption of thruster propellant for station keeping places a limitation on the lifetime of a satellite. [1]
References
[1] Wikipedia
http://en.wikipedia.org/wiki/Geostationary_orbit
