Geostationary orbit

A geostationary orbit (abbreviated GSO) is a circular orbit in the Earth's equatorial plane, any point on which revolves about the Earth in the same direction and with the same period as the Earth's rotation. It is a special case of the geosynchronous orbit, and the one which is of most interest to artificial satellite operators.

Geosynchronous orbits and geostationary orbits were first popularised by science fiction author Arthur C. Clarke in 1945 as useful orbits for communications satellites. As a result they are sometimes referred to as Clarke orbits. Similarly, the "Clarke Belt" is the part of space approximately 35,790 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 35,790 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.

Table of contents

1 Use in artificial satellites
2 Derivation of geostationary altitude
3 References
4 See also

Use in artificial satellites

Geostationary orbits can only be achieved very close to the ring 35,790 km directly above the equator. All other circular non-active geosynchronous orbits will cross the geostationary orbit and possibly collide with satellites there. In practice this means that all geostationary satellites have to exist on this ring, which poses problems for satellites needing to be decommissioned at the end of their service life (for example when they run out of thruster fuel).

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 provides visible and infrared images of Earth's surface and atmosphere. The 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
most commercial communications satellites.

A statite, a hypothetical satellite that uses a solar sail to modify its orbit, can theoretically hold itself in a geostationary orbit with different altitude and/or inclination from the "traditional" equatorial geostationary orbit.

Derivation of geostationary altitude

To calculate Earth's geostationary orbit altitude:

(the distance from the centre of the Earth).

A more exact calculation gives the distance from the Earth's center of 42,164 km. Subtracting the radius of the earth, 6378 km, gives the altitude mentioned above.

See Geosynchronous orbit derivation for the full derivation.