A geosynchronous orbit is an orbit around the Earth with an orbital period of one sidereal
day, intentionally matching the Earth's sidereal rotation period. The synchronization of rotation
and orbital period means that, for an observer on the surface of the Earth, an object in
geosynchronous orbit returns to exactly the same position in the sky after a period of
one sidereal day. Over the course of a day, the object's position in the sky traces out
a path, typically in the form of an analemma, whose precise characteristics depend on the
orbit's inclination and eccentricity. A special case of geosynchronous orbit is
the geostationary orbit, which is a circular geosynchronous orbit at zero inclination.
A satellite in a geostationary orbit appears stationary, always at the same point in the
sky, to ground observers. Popularly or loosely, the term "geosynchronous" may be used to mean
geostationary. Specifically, geosynchronous Earth orbit may be a synonym for geosynchronous
equatorial orbit, or geostationary earth orbit. Communications satellites are often given
geostationary orbits, or close to geostationary, so that the satellite antennas that communicate
with them do not have to move, but can be pointed permanently at the fixed location
in the sky where the satellite appears. A semi-synchronous orbit has an orbital period
of 1/2 sidereal day, i.e., 11 h 58 min. Relative to the Earth's surface it has twice this period,
and hence appears to go around the Earth once every day. Examples include the Molniya orbit
and the orbits of the satellites in the Global Positioning System.
Orbital characteristics Circular Earth geosynchronous orbits have
a radius of 42,164 km. All Earth geosynchronous orbits, whether circular or elliptical, have
the same semi-major axis. In fact, orbits with the same period always share the same
semi-major axis:
where a is the semi-major axis, P is the orbital period, and μ is the geocentric gravitational
constant, equal to 398600.4418 km3/s2.
In the special case of a geostationary orbit, the ground track of a satellite is a single
point on the equator. In the general case of a geosynchronous orbit with a non-zero
inclination or eccentricity, the ground track is a more or less distorted figure-eight,
returning to the same places once per sidereal day.
Geostationary orbit
A geostationary orbit is a circular geosynchronous orbit in the plane of the Earth's equator
with a radius of approximately 42,164 km. A satellite in such an orbit is at an altitude
of approximately 35,786 km above mean sea level. It maintains 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 the
Sun, Moon, and stars would traverse the heavens behind it. The theoretical basis for this
novel phenomenon of the sky goes back to Newton's theory of motion and gravity. In that theory,
the existence of a geostationary satellite is made possible because the earth rotates.
However, as a practical device, the geostationary satellite owes much for its realisation to
Arthur C. Clarke who proposed it during the 20th century and in whose honour the orbit
is called a Clarke orbit. Such orbits are useful for telecommunications satellites.
A perfectly stable geostationary orbit is an ideal that can only be approximated. In
practice the satellite drifts 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. Other geosynchronous orbits
Elliptical geosynchronous orbits can be and are designed for communications satellites
in order to keep the satellite within view of its assigned ground stations or receivers.
A satellite in an elliptical geosynchronous orbit appears 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.
The Infrared Space Observatory was in a highly-elliptical geosynchronous orbit with an orbital height
of apogee 70,600 km and perigee 1,000 km. It was controlled by two ground stations.
The Quasi-Zenith Satellite System, is a proposed three-satellite regional time transfer system
and enhancement for GPS covering Japan. An active geosynchronous orbit is a hypothetical
orbit that could be maintained if forces other than gravity were also used, such as a solar
sail. Such a statite could be geosynchronous in an orbit different from the conic section
orbit dictated by the laws of gravity. A further form of geosynchronous orbit is
proposed for the theoretical space elevator, in which one end of the structure is tethered
to the ground, maintaining a shorter orbital period than by gravity alone if under tension.
Other related orbit types are: Supersynchronous orbit – a disposal / storage
orbit above GSO/GEO. Satellites 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.
Other synchronous orbits
Synchronous orbits can only exist for bodies that have a fixed surface. Without such a
surface there is no fixed point an orbit can be said to synchronise with. No synchronous
orbit will exist if the body rotates so slowly that the orbit would be outside its Hill sphere,
or so quickly that it would be inside the body. Large bodies which are held together
by gravity cannot rotate that quickly since they would fly apart, so the last condition
only applies to small bodies held together by other forces, e.g. smaller asteroids. Most
inner moons of planets have synchronous rotation, so their synchronous orbits are, in practice,
limited to their leading and trailing Lagrange points, as well as the L1 and L2 Lagrange
points, assuming they do not fall within the body of the moon. Objects with chaotic rotations
are also problematic, as their synchronous orbits change unpredictably.
History
At the end of 1928, the Austro-Hungarian rocket engineer Herman Potočnik set out a plan for
a breakthrough into space and the establishment of a permanent human presence there. He conceived
a space station in detail and was the first person to calculate the geostationary orbit,
on which the station would orbit the Earth.
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. However, it was in an inclined orbit, still requiring the use of moving antennas.
The first communications satellite placed in a geostationary orbit was Syncom 3. Geostationary
orbits have been in common use ever since, in particular for satellite television.
Geostationary satellites also carry international telephone traffic but they are being replaced
by fiber optic cables in heavily populated areas and along the coasts of less developed
regions, because of the greater bandwidth available and lower latency, due to the inherent
disconcerting delay in communicating via a satellite in such a high orbit. It takes electromagnetic
waves about a quarter of a second to travel from one end to the other end of the link.
Thus, two parties talking via satellite are subject to about a half second delay in a
round-trip message/response sequence. Although many populated land locations on
the planet now have terrestrial communications facilities, even undersea, with more than
sufficient capacity, telephony and Internet access is still available only via satellite
in many places in Africa, Latin America, and Asia, as well as 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
Low Earth orbit Medium Earth orbit
Geostationary orbit Geosynchronous satellite
Graveyard orbit List of orbits
List of satellites in geosynchronous orbit Molniya orbit
References
External links Satellites currently in Geosynchronous Orbit,
list updated daily Science@NASA - Geosynchronous Orbit
NASA - Planetary Orbits Science Presse data on Geosynchronous Orbits
ORBITAL MECHANICS fils.html
Time lapse of Geostationary Satellites Beyond the Alps