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A geostationary orbit (GEO) is a geosynchronous orbit directly above the
Earth's equator (0° latitude), with orbital eccentricity of zero. From the
ground, a geostationary object appears motionless in the sky and is therefore
the orbit of most interest to operators of artificial satellites (including
communication and television satellites). Due to the constant 0° latitude,
satellite locations may differ by longitude only.
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.
Introduction
Geostationary orbits can only be achieved very close to the ring 35,786 km
directly above the equator. This equates to an orbital velocity of 3.07 km/s or
a period of 1436.06 minutes which equates to almost exactly one earth day or
23.934 hours. This makes sense considering that the satellite must be locked to
the earth's rotational period in order to have a stationary footprint. This can
be calculated and verified here: . 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
centered on the Equator) or be raised to a "graveyard" disposal orbit.
Satellites with bad figure 8 movements that wobble, may cause the tracking
actuators on antennas that have an autotracking pointing and control unit to
fail prematurely. This is due to the fact the actuators that position the
antenna are in continuous motion while they are always positioning to seek the
strongest signal from the satellite.
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, broadcast satellites and SBAS
satellites operate in geostationary orbits. (Russian television satellites have
used elliptical Molniya and Tundra orbits due to the high latitudes of the
receiving audience.) The first satellite placed into a geostationary orbit was
Syncom-3, launched by a Delta-D rocket in 1964.
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
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