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| A '''geosynchronous orbit''' (sometimes abbreviated '''GSO''') is an ] around the ] with an ] |
A '''geosynchronous orbit''' (sometimes abbreviated '''GSO''') is an ] around the ] with an ]. Such a ] could be geosynchronous in an orbit different (higher, lower, more or less elliptical, or some other path) from the ] orbit dictated by the laws of gravity. | ||
| A special case of geosynchronous orbit is the ], which is a circular geosynchronous orbit at zero ] (that is, directly above the ]). 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.<ref>C. D. Brown (1998), ''Spacecraft Mission Design, 2nd Edition,'' AIAA Education Series, p. 81</ref> Specifically, '''geosynchronous Earth orbit''' ('''GEO''') may be a synonym for ''geosynchronous equatorial orbit'',<ref>{{cite web|title=Ariane 5 User's Manual Issue 5 Revision 1|url=http://www.arianespace.com/launch-services-ariane5/Ariane5_users_manual_Issue5_July2011.pdf|work=arianespace|accessdate=28 July 2013|date=July 2011}}</ref> or ''geostationary Earth orbit''.<ref name = NASA2001/> ]s are often given geostationary orbits, or close to geostationary, so that the ]s 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 ] has an orbital period of 1/2 ] 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 ] and the orbits of the satellites in the ]. | |||
| == Orbital characteristics == | |||
| Circular ] geosynchronous orbits have a ] of {{convert|42164|km|mi|0|abbr=on}}. All Earth geosynchronous orbits, whether circular or elliptical, have the same ].<ref> | |||
| {{cite book |title=Fundamentals of Astrodynamics and Applications |last=Vallado |first=David A. |year=2007 |publisher=Microcosm Press |location=Hawthorne, CA |pages=31 }} | |||
| </ref> In fact, orbits with the same period always share the same semi-major axis: | |||
| :<math>a=\sqrt{\mu\left(\frac{P}{2\pi}\right)^2}</math> | |||
| where ''a'' is the semi-major axis, ''P'' is the ], and ''μ'' is the ], equal to 398600.4418 km<sup>3</sup>/s<sup>2</sup>. | |||
| In the special case of a geostationary orbit, the ] of a satellite is a single point on the ]. In the general case of a geosynchronous orbit with a non-zero ] or ], the ground track is a more or less distorted figure-eight, returning to the same places once per sidereal day. | |||
| == Geostationary orbit == | |||
| {{Main | Geostationary orbit}} | |||
| ]A geostationary orbit (GEO) is a circular geosynchronous orbit in the plane of the Earth's equator with a radius of approximately {{convert|42164|km|mi|0|abbr=on}} (measured from the center of the Earth). A satellite in such an orbit is at an altitude of approximately {{convert|35786|km|mi|0|abbr=on}} 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 ], 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 (with respect to an inertial frame in which Newton's laws of motion and gravity hold). However, as a practical device, the geostationary satellite owes much for its realisation to ] who proposed it during the 20th century and in whose honour the orbit is called a Clarke orbit. Such orbits are useful for ]s. | |||
| 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 ], ], variations in the Earth's gravitational field, and the ] effect of the ] and ], and thrusters are used to maintain the orbit in a process known as ]. | |||
| == Other geosynchronous orbits == | |||
| ] geosynchronous orbits can be and are designed for ]s 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 ] (figure 8) in the sky. Satellites in highly elliptical orbits must be tracked by steerable ]s. | |||
| The ] was in a highly-elliptical geosynchronous orbit with an ] of apogee 70,600 km and perigee 1,000 km. It was controlled by two ground stations. | |||
| The ] (QZSS), is a proposed three-satellite regional ] system and enhancement for ] covering ]. | |||
| An active geosynchronous orbit is a hypothetical orbit that could be maintained if forces other than gravity were also used, such as a ]. Such a ] could be geosynchronous in an orbit different (higher, lower, more or less elliptical, or some other path) from the ] orbit dictated by the laws of gravity. | |||
| A further form of geosynchronous orbit is proposed for the theoretical ], in which one end of the structure is tethered to the ground, maintaining a shorter orbital period than by gravity alone if under tension. | A further form of geosynchronous orbit is proposed for the theoretical ], in which one end of the structure is tethered to the ground, maintaining a shorter orbital period than by gravity alone if under tension. | ||
Revision as of 09:30, 7 June 2015
A geosynchronous orbit (sometimes abbreviated GSO) is an orbit around the Earth with an [[orbital it 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 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 (e.g. moons, rocky planets). Without such a surface (e.g. gas giants, black holes) 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 (L4 and L5) 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 (such as exhibited by Hyperion) 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 (microwave, fiber-optic), even undersea, with more than sufficient capacity, telephone 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
- A. C. Clarke, "Extra-Terrestrial Relays," Wireless World, Vol. 51, No. 10, pp. 305-308, 1945
External links
- Satellites currently in Geosynchronous Orbit, list updated daily
- Science@NASA - Geosynchronous Orbit
- NASA - Planetary Orbits
- Science Presse data on Geosynchronous Orbits (including historical data and launch statistics)
- ORBITAL MECHANICS (Rocket and Space Technology)
- fils.html
- Time lapse of Geostationary Satellites Beyond the Alps
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