Astronomy

How Geostationary Orbit Works



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The term Clarke Belt is an alternative and now little-used term to refer to the range of geostationary and near-geostationary orbital bands lying above the Earth's equator, in which a satellite can circle the Earth while remaining more or less directly above the same physical point on the Earth's surface. Although initially only the subject of speculation by science fiction writers such as Alfred C. Clarke (after whom the belt was named), geostationary orbits now serve vital functions in telecommunications, allowing satellites to provide permanent coverage to a single specific zone on the Earth's surface.

The concept of the geostationary orbit dates to work done in the 1920s by the Slovenian scientist and rocket engineer Herman Potocnik (sometimes published as Hermann Noordung, a pseudonym), who was particularly interested in future methods of supporting human life in space. However, the term Clarke Belt was given in honour of science fiction writer Arthur C. Clarke, who popularized the concept in the 1940s. Later, when satellites became a relatively common technology, Clarke's concept of the geostationary orbit left the realm of science fiction to become an essential tool of modern-day telecommunications.

- About the Geostationary Orbit -

Geostationary orbits are unique because they allow an orbiting object, such as a satellite, to appear to remain fixed above a single point on the surface of the Earth at all times. This is accomplished by setting the object into an orbit at about 22,200 miles above the surface of the Earth, where the time it takes to achieve one orbit is exactly equivalent to one Earth-day. This allows a satellite to maintain continuous coverage of a single region on Earth below, and in particular to maintain continuous contact with a ground tracking and communications station.

In practice, it is impossible to maintain a geostationary orbit permanently, among other reasons because of gravitational disturbances caused by the Moon. For this reason, as with all other satellites, geostationary satellites must occasionally fire stationkeeping thrusters in order to stay in the proper orbital location. Eventually their fuel will be exhausted through these maneuvers, and they will drift out of geostationary orbit.

The geostationary orbit is higher than the low Earth orbit currently used by the Space Shuttle, the International Space Station, and some other satellites.

- Current Applications -

Currently, geostationary orbits are prized for both communications satellites and weather satellites. Weather satellites in geostationary orbits include the GOES system (American), Meteosat (European), GMS (Japanese), and INSAT (Indian). In addition, commercial communications satellites typically make use of geostationary orbits. The sole significant exception to this rule is Russian communications satellites, which instead follow Molniya orbits to maximize northern coverage.

The high demand for spaces in geostationary orbit, and the necessarily limited number of actual geostationary orbit slots (since each possible orbit can only have one satellite at a time), means that there is some concern about how scarce space in the Clarke Belt will be divided up in the event that the use of outer space for communications purposes continues to increase, as it probably will. For the time being, space in the Clarke belt is allocated according to rules set by the International Telecommunications Union, and satellites which are nearing the end of their life expectancy are supposed to be removed from geostationary orbit by their operators, usually to a higher orbit referred to as a disposal orbit or a "graveyard" orbit where they will not interfere with future spacecraft.

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