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A geosynchronous orbit (sometimes abbreviated GSO) is an orbit around Earth of a satellite with an orbital period that matches Earth's rotation on its axis, which takes one sidereal day (about 23 hours, 56 minutes, and 4 seconds). The synchronization of rotation and orbital period means that, for an observer on Earth's surface, 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 may remain still or trace out a path, typically in a figure-8 form, whose precise characteristics depend on the orbit's inclination and eccentricity. Satellites are typically launched in an eastward direction. A circular geosynchronous orbit is 35,786 km (22,236 mi) above Earth's surface. Those closer to Earth orbit faster than Earth rotates, so from Earth, they appear to move eastward while those that orbit beyond geosynchronous distances appear to move westward.

A special case of geosynchronous orbit is the geostationary orbit, which is a circular geosynchronous orbit in Earth's equatorial plane (that is, directly above the Equator). A satellite in a geostationary orbit appears stationary, always at the same point in the sky, to observers on the surface. Popularly or loosely, the term geosynchronous may be used to mean geostationary. Specifically, geosynchronous Earth orbit (GEO) may be a synonym for geosynchronous equatorial orbit, or geostationary Earth orbit. Communications satellites are often given geostationary or close to geostationary orbits 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 half a sidereal day (i.e., 11 hours and 58 minutes). Relative to Earth's surface, it has twice this period and hence appears to go around Earth once every day. Examples include the Molniya orbit and the orbits of the satellites in the Global Positioning System.

History

The first appearance of a geosynchronous orbit in popular literature was in October 1942, in the first Venus Equilateral story by George O. Smith, but Smith did not go into details. British science fiction author Arthur C. Clarke popularised and expanded the concept in a 1945 paper entitled Extra-Terrestrial Relays – Can Rocket Stations Give Worldwide Radio Coverage?, published in Wireless World magazine. Clarke acknowledged the connection in his introduction to The Complete Venus Equilateral. The orbit, which Clarke first described as useful for broadcast and relay communications satellites, is sometimes called the Clarke Orbit. Similarly, the collection of artificial satellites in this orbit is known as the Clarke Belt.

In technical terminology the geosynchronous orbits are often referred to as geostationary if they are roughly over the equator, but the terms are used somewhat interchangeably.

The first geosynchronous satellite was designed by Harold Rosen while he was working at Hughes Aircraft in 1959. Inspired by Sputnik 1, he wanted to use a geostationary (geosynchronous equatorial) satellite to globalise communications. Telecommunications between the US and Europe was then possible between just 136 people at a time, and reliant on high frequency radios and an undersea cable.

Conventional wisdom at the time was that it would require too much rocket power to place a satellite in a geosynchronous orbit and it would not survive long enough to justify the expense, so early efforts were put towards constellations of satellites in low or medium Earth orbit. The first of these were the passive Echo balloon satellites in 1960, followed by Telstar 1 in 1962. Although these projects had difficulties with signal strength and tracking, that could be solved through geosynchronous satellites, the concept was seen as impractical, so Hughes often withheld funds and support.

By 1961, Rosen and his team had produced a cylindrical prototype with a diameter of 76 centimetres (30 in), height of 38 centimetres (15 in), weighing 11.3 kilograms (25 lb), light and small enough to be placed into orbit. It was spin stabilised with a dipole antenna producing a pancake shaped waveform. In August 1961, they were contracted to began building the real satellite. They lost Syncom 1 to electronics failure, but Syncom 2 was successfully placed into a geosynchronous orbit in 1963. Although its inclined orbit still required moving antennas, it was able to relay TV transmissions, and allowed for US President John F. Kennedy to phone Nigerian prime minister Abubakar Tafawa Balewa from a ship on August 23, 1963.

Since then novel geosynchronous orbits have been used, some highly eliptical, including the Soviet devised Tundra orbit in the 2000s and the Japanese Quazi-Zenith orbit in the 2010s.

Today there are hundreds of geosynchronous satellites providing remote sensing, navigation and communications.

Although most populated land locations on the planet now have terrestrial communications facilities (microwave, fiber-optic), which often have latency and bandwith advantages, and telephone access covering 96% of the population and internet access 90%, some rural and remote areas in developed countries are still reliant on satellite communications.

Types

Geostationary orbit

A geostationary equatorial orbit (GEO) is a circular geosynchronous orbit in the plane of the Earth's equator with a radius of approximately 42,164 km (26,199 mi) (measured from the center of the Earth). A satellite in such an orbit is at an altitude of approximately 35,786 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 motion, while the Sun, Moon, and stars would traverse the skies behind it. 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.

Elliptical and inclined geosynchronous orbits

Elliptical geosynchronous orbits are used in communications satellites to keep the satellite in view of its assigned ground stations and 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.

NAVIC is a regional — i.e. non-global — Indian navigation system currently operating with 7 satellites, of which 3 are in geostationary orbit and 4 in geosynchronous orbit.

Quasi-Zenith orbit

The Quasi-Zenith Satellite System (QZSS) is a three-satellite regional time transfer system and enhancement for GPS, covering Japan at high elevation. Each satellite dwells over Japan, allowing signals to reach receivers in urban canyons then passes quickly over Australia.

Tunda orbit

The Tundra orbit is an eccentric Russian geosynchronous orbit, which allows the satellite to spend most of it's time over one location. It sits at an inclination of 63.4°, which is a frozen orbit, which reduces the need for stationkeeping. It is used by the Sirius XM Satellite Radio to improve signal strength in northern US and Canada.

Related orbits

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 ends of their operational lives.

Proposed orbits

Statite proposal

A statite is a hypothetical satellite that uses radiation pressure from the sun against a solar sail to modify its orbit.

It would hold its location over the dark side of the Earth at a latitude of approximately 30 degrees. It would return to the same spot in the sky every 24 hours from an Earth based viewer's perspective, so be functionally similar to a geosynchronous orbit.

Space elevator

A further form of geosynchronous orbit is the theoretical space elevator. When one end in attached to the ground, for altitudes below the geostationary belt the elevator maintains a shorter orbital period than by gravity alone.

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 exists 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 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.

Properties

A typical geosynchronous orbit has the following properties:

• Period: 1436 minutes (one sidereal day)
• Semi-major axis: 42,164 km
• Inclinations: 0° (Geostationary), 63.4° (Tundra)

Period

All geosynchronous orbits have an orbital period equal to exactly one sidereal day. This means that the satellite will return to the same point above the Earth's surface every (sidereal) day, regardless of other orbital properties. This orbital period, T, is directly related to the semi-major axis of the orbit through the formula:

${\displaystyle T=2\pi {\sqrt {a^{3} \over \mu }}}$

where:

a is the length of the orbit's semi-major axis

${\displaystyle \mu }$ is the standard gravitational parameter of the central body

Inclination

An inclination of zero is used for geostationary satellites, ensuring that the orbit remains over the equator at all times, making it stationary with respect to latitude from the point of view of a ground observer (and in the ECEF reference frame).

Other popular inclinations include 63.4° for a Tundra orbit, which ensures that the orbit's argument of perigee doesn't change over time.

Ground track

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.

See also

• Geostationary orbit
• Geosynchronous satellite
• Graveyard orbit
• High Earth orbit
• List of orbits
• List of satellites in geosynchronous orbit
• Low Earth orbit
• Medium Earth orbit
• Molniya orbit

References

1. V. Chobotov, ed. (1996). Orbital Mechanics (2nd ed.). AIAA Education Series. p. 304.
2. Brown, C. D. (1998). Spacecraft Mission Design (2nd ed.). AIAA Education Series. p. 81.
3. "Ariane 5 User's Manual Issue 5 Revision 1" (PDF). Ariane Space. July 2011. Archived from the original (PDF) on 4 October 2013. Retrieved 28 July 2013.
4. "What is orbit?". NASA. October 25, 2001. Retrieved 2013-03-10. Satellites that seem to be attached to some location on Earth are in Geosynchronous Earth Orbit (GEO)...Satellites headed for GEO first go to an elliptical orbit with an apogee about 23,000 miles. Firing the rocket engines at apogee then makes the orbit round. Geosynchronous orbits are also called geostationary.
5. "(Korvus's message is sent) to a small, squat building at the outskirts of Northern Landing. It was hurled at the sky. ... It ... arrived at the relay station tired and worn, ... when it reached a space station only five hundred miles above the city of North Landing." Smith, George O. (1976). The Complete Venus Equilateral. New York: Ballantine Books. pp. 3–4. ISBN 978-0-345-28953-7.
6. "It is therefore quite possible that these stories influenced me subconsciously when ... I worked out the principles of synchronous communications satellites ...", McAleer, Neil (1992). Arthur C. Clarke. Contemporary Books. p. 54. ISBN 978-0-809-24324-2.
7. ^ Clarke, Arthur C. (October 1945). "Extra-Terrestrial Relays – Can Rocket Stations Give Worldwide Radio Coverage?" (PDF). Wireless World. pp. 305–308. Archived from the original (PDF) on March 18, 2009. Retrieved March 4, 2009.
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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
• NASA Astronomy Picture of the Day: Time lapse of Geostationary Satellites Beyond the Alps (11 April 2012)

• Artificial satellites in geosynchronous orbit
• Earth orbits
• Satellite broadcasting