Geosynchronous orbit
Encyclopedia
A geosynchronous orbit is an orbit
around the Earth
with an orbital period
that matches the Earth's sidereal rotation period. The synchronization of rotation and orbital period means that for an observer on the surface of the Earth, the satellite
appears to constantly hover over the same meridian
(north-south line) on the surface, moving in a slow oscillation alternately north and south with a period of one day, so it returns to exactly the same place in the sky at exactly the same time each day.
However, the term is often popularly used to refer to the special case of a geosynchronous orbit called a geostationary orbit
. This is a geosynchronous orbit that is circular and at zero inclination
, that is, directly above the equator
. A satellite in a geostationary orbit appears stationary, always at the same point in the sky, to ground observers. Communications satellite
s are often given geostationary orbits, or close to geostationary, so that the satellite antennas that communicate with them don't have to move, but can be pointed permanently at the fixed location in the sky where the satellite appears.
A semisynchronous orbit has an orbital period of 0.5 sidereal days, 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
.
of 42164 km (26,200 mi).
In fact, orbits with the same period share the same semi-major axis:
where a=semi-major axis, P = orbital period
, μ = geocentric gravitational constant ≈ 5.165861726×1012 km3/hr2 .
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 solar day.
, while the Sun, Moon, and stars would traverse the heavens behind it. This is sometimes called a Clarke orbit. Such orbits are useful for telecommunications satellites.
A perfect 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.
s exist around all moons, planet
s, star
s and black hole
s — unless they rotate so slowly that the orbit would be outside their Hill sphere
or so fast that such an orbit would be inside the body. Most inner moons of planets have synchronous rotation
, so their synchronous orbits are, in practice, limited to their leading and trailing ( and ) Lagrange points, as well as the and Lagrange points, assuming they don't fall within the body of the moon. Objects with chaotic
rotations (such as Hyperion
) are also problematic, as their synchronous orbits keep changing unpredictably.
s that 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 apogee 70,600 km and perigee 1,000 km. It was controlled by two ground stations.
An active geosynchronous 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 (higher, lower, more or less elliptical, or some other path) from the conic section
orbit formed by a gravitational body.
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 longer orbital period than by gravity alone if under tension.
The acronym GEO can mean geosynchronous Earth orbit, geostationary Earth orbit or geostationary orbit.
The following orbits are special orbits that are also used to categorize orbits:
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 man 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 (microwave
, fiber-optic), even undersea, with more than sufficient capacity, satellite telephony and Internet access is still the only service available for 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.
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
around the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
with an orbital period
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...
that matches the Earth's sidereal rotation period. The synchronization of rotation and orbital period means that for an observer on the surface of the Earth, the satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
appears to constantly hover over the same meridian
Meridian (geography)
A meridian is an imaginary line on the Earth's surface from the North Pole to the South Pole that connects all locations along it with a given longitude. The position of a point along the meridian is given by its latitude. Each meridian is perpendicular to all circles of latitude...
(north-south line) on the surface, moving in a slow oscillation alternately north and south with a period of one day, so it returns to exactly the same place in the sky at exactly the same time each day.
However, the term is often popularly used to refer to the special case of a geosynchronous orbit called a geostationary orbit
Geostationary orbit
A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...
. This is a geosynchronous orbit that is circular and at zero inclination
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
, that is, directly above the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
. A satellite in a geostationary orbit appears stationary, always at the same point in the sky, to ground observers. Communications satellite
Communications satellite
A communications satellite is an artificial satellite stationed in space for the purpose of telecommunications...
s are often given geostationary orbits, or close to geostationary, so that the satellite antennas that communicate with them don't have to move, but can be pointed permanently at the fixed location in the sky where the satellite appears.
A semisynchronous orbit has an orbital period of 0.5 sidereal days, 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
Molniya orbit
Molniya orbit is a type of highly elliptical orbit with an inclination of 63.4 degrees, an argument of perigee of -90 degree and an orbital period of one half of a sidereal day...
and the orbits of the satellites in the Global Positioning System
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
.
Orbital characteristics
All Earth geosynchronous orbits have a semi-major axisSemi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...
of 42164 km (26,200 mi).
In fact, orbits with the same period share the same semi-major axis:
where a=semi-major axis, P = orbital periodOrbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...
, μ = geocentric gravitational constant ≈ 5.165861726×1012 km3/hr2 .
In the special case of a geostationary orbit, the ground track
Ground track
A ground track or ground trace is the path on the surface of the Earth directly below an aircraft or satellite. In the case of a satellite, it is the projection of the satellite's orbit onto the surface of the Earth .A satellite ground track may be thought of as a path along the Earth's surface...
of a satellite is a single point on the equator. In the general case of a geosynchronous orbit with a non-zero inclination
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
or eccentricity, the ground track is a more or less distorted figure-eight, returning to the same places once per solar day.
Geostationary orbit
A circular geosynchronous orbit in the plane of the Earth's equator has a radius of approximately 42164 km (26,200 mi) from the center of the Earth. A satellite in such an orbit is at an altitude of approximately 35786 km (22,236 mi) 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 motionDiurnal motion
Diurnal motion is an astronomical term referring to the apparent daily motion of stars around the Earth, or more precisely around the two celestial poles. It is caused by the Earth's rotation on its axis, so every star apparently moves on a circle, that is called the diurnal circle. The time for...
, while the Sun, Moon, and stars would traverse the heavens behind it. This is sometimes called a Clarke orbit. Such orbits are useful for telecommunications satellites.
A perfect 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
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
, radiation pressure
Radiation pressure
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
, variations in the Earth's gravitational field, and the gravitational effect of the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
and Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
), and thrusters are used to maintain the orbit in a process known as station-keeping.
Other synchronous orbits
Synchronous orbitSynchronous orbit
A synchronous orbit is an orbit in which an orbiting body has a period equal to the average rotational period of the body being orbited , and in the same direction of rotation as that body.-Properties:...
s exist around all moons, planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
s, star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s and black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
s — unless they rotate so slowly that the orbit would be outside their Hill sphere
Hill sphere
An astronomical body's Hill sphere is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sphere of its own...
or so fast that such an orbit would be inside the body. Most inner moons of planets have synchronous rotation
Synchronous rotation
In astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting...
, so their synchronous orbits are, in practice, limited to their leading and trailing ( and ) Lagrange points, as well as the and Lagrange points, assuming they don't fall within the body of the moon. Objects with chaotic
Chaos theory
Chaos theory is a field of study in mathematics, with applications in several disciplines including physics, economics, biology, and philosophy. Chaos theory studies the behavior of dynamical systems that are highly sensitive to initial conditions, an effect which is popularly referred to as the...
rotations (such as Hyperion
Hyperion (moon)
Hyperion , also known as Saturn VII, is a moon of Saturn discovered by William Cranch Bond, George Phillips Bond and William Lassell in 1848. It is distinguished by its irregular shape, its chaotic rotation, and its unexplained sponge-like appearance...
) are also problematic, as their synchronous orbits keep changing unpredictably.
Other geosynchronous orbits
Elliptical orbits can be and are designed for communications satelliteCommunications satellite
A communications satellite is an artificial satellite stationed in space for the purpose of telecommunications...
s that 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
Analemma
In astronomy, an analemma is a curve representing the angular offset of a celestial body from its mean position on the celestial sphere as viewed from another celestial body relative to the viewing body's celestial equator...
in the sky. Satellites in highly elliptical orbits must be tracked by steerable ground stations.
The Infrared Space Observatory
Infrared Space Observatory
The Infrared Space Observatory was a space telescope for infrared light designed and operated by the European Space Agency , in cooperation with ISAS and NASA...
was in a highly-elliptical geosynchronous orbit with apogee 70,600 km and perigee 1,000 km. It was controlled by two ground stations.
An active geosynchronous is a hypothetical orbit that could be maintained if forces other than gravity were also used, such as a solar sail
Solar sail
Solar sails are a form of spacecraft propulsion using the radiation pressure of light from a star or laser to push enormous ultra-thin mirrors to high speeds....
. Such a statite
Statite
A statite is a hypothetical type of artificial satellite that employs a solar sail to continuously modify its orbit in ways that gravity alone would not allow. Typically, a statite would use the solar sail to "hover" in a location that would not otherwise be available as a stable geosynchronous...
could be geosynchronous in an orbit different (higher, lower, more or less elliptical, or some other path) from the conic section
Conic section
In mathematics, a conic section is a curve obtained by intersecting a cone with a plane. In analytic geometry, a conic may be defined as a plane algebraic curve of degree 2...
orbit formed by a gravitational body.
A further form of geosynchronous orbit is proposed for the theoretical space elevator
Space elevator
A space elevator, also known as a geostationary orbital tether or a beanstalk, is a proposed non-rocket spacelaunch structure...
, in which one end of the structure is tethered to the ground, maintaining a longer orbital period than by gravity alone if under tension.
The acronym GEO can mean geosynchronous Earth orbit, geostationary Earth orbit or geostationary orbit.
The following orbits are special orbits that are also used to categorize orbits:
- Geostationary orbitGeostationary orbitA geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...
(GEO): zero inclination geosynchronous orbit - Supersynchronous orbit - a disposal / storage orbit above GSO/GEO. Satellites drift in a westerly direction.
- Subsynchronous orbitSubsynchronous orbitA subsynchronous orbit is an orbit of a satellite that is nearer the planet than it would be if it were in synchronous orbit, i.e. the orbital period is less than the sidereal day of the planet. An Earth satellite that is in subsynchronous orbit will appear to drift eastward as seen from the...
- a drift orbit close to but below GSO/GEO. Used for satellites undergoing station changes in an eastern direction. - Graveyard orbitGraveyard orbitA graveyard orbit, also called a supersynchronous orbit, junk orbit or disposal orbit, is an orbit significantly above synchronous orbit, where spacecraft are intentionally placed at the end of their operational life...
- a supersynchronous orbit where spacecraft are intentionally placed at the end of their operational life.
History
At the end of 1928 the Austro-Hungarian rocket engineer Herman PotočnikHerman Potocnik
Herman Potočnik was an Austro-Hungarian rocket engineer and pioneer of cosmonautics . He is chiefly remembered for his work addressing the long-term human habitation of space.- Early life :Potočnik was born in Pola, southern Istria, Austria-Hungary...
set out a plan for a breakthrough into space and the establishment of a permanent human presence there. He conceived a space station
Space station
A space station is a spacecraft capable of supporting a crew which is designed to remain in space for an extended period of time, and to which other spacecraft can dock. A space station is distinguished from other spacecraft used for human spaceflight by its lack of major propulsion or landing...
in detail and was the first man to calculate the geostationary orbit
Geostationary orbit
A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...
, on which the station would orbit the Earth.
Author Arthur C. Clarke
Arthur C. Clarke
Sir Arthur Charles Clarke, CBE, FRAS was a British science fiction author, inventor, and futurist, famous for his short stories and novels, among them 2001: A Space Odyssey, and as a host and commentator in the British television series Mysterious World. For many years, Robert A. Heinlein,...
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
Inclined orbit
A satellite is said to occupy an inclined orbit around the Earth if the orbit exhibits an angle other than zero degrees with the equatorial plane. This angle is called the orbit's inclination...
, 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
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
.
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 (microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
, fiber-optic), even undersea, with more than sufficient capacity, satellite telephony and Internet access is still the only service available for many places in Africa, Latin America, and Asia, as well as isolated locations that have no terrestrial facilities, such as Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
's Arctic islands, Antarctica, the far reaches of Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...
and Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
, and ships at sea.
See also
- Low Earth orbitLow Earth orbitA low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
- Medium Earth orbitMedium Earth OrbitMedium Earth orbit , sometimes called intermediate circular orbit , is the region of space around the Earth above low Earth orbit and below geostationary orbit ....
- Geostationary orbitGeostationary orbitA geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...
- Geosynchronous satelliteGeosynchronous satelliteA geosynchronous Satellite is a satellite whose orbit on the Earth repeats regularly over points on the Earth over time. If such a satellite's orbit lies over the equator, the orbit is circular and its angular velocity is the same as the earth's, then it is called a geostationary satellite...
- Graveyard orbitGraveyard orbitA graveyard orbit, also called a supersynchronous orbit, junk orbit or disposal orbit, is an orbit significantly above synchronous orbit, where spacecraft are intentionally placed at the end of their operational life...
- List of orbits
- List of satellites in geosynchronous orbit
- Molniya orbitMolniya orbitMolniya orbit is a type of highly elliptical orbit with an inclination of 63.4 degrees, an argument of perigee of -90 degree and an orbital period of one half of a sidereal day...