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A geostationary orbit, also referred to as a geosynchronous equatorial orbit<ref group=lower-alpha>Geostationary orbit and Geosynchronous (equatorial) orbit are used somewhat interchangeably in sources.</ref> (GEO), is a circular geosynchronous orbit Template:Cvt in altitude above Earth's equator, Template:Cvt in radius from Earth's center, and following the direction of Earth's rotation.
An object in such an orbit has an orbital period equal to Earth's rotational period, one sidereal day, and so to ground observers it appears motionless, in a fixed position in the sky. The concept of a geostationary orbit was popularised by the science fiction writer Arthur C. Clarke in the 1940s as a way to revolutionise telecommunications, and the first satellite to be placed in this kind of orbit was launched in 1963.
Communications satellites are often placed in a geostationary orbit so that Earth-based satellite antennas do not have to rotate to track them but can be pointed permanently at the position in the sky where the satellites are located. Weather satellites are also placed in this orbit for real-time monitoring and data collection, as are navigation satellites in order to provide a known calibration point and enhance GPS accuracy.
Geostationary satellites are launched via a temporary orbit, and then placed in a "slot" above a particular point on the Earth's surface. The satellite requires periodic station-keeping to maintain its position. Modern retired geostationary satellites are placed in a higher graveyard orbit to avoid collisions.
HistoryEdit
In 1929, Herman Potočnik described both geosynchronous orbits in general and the special case of the geostationary Earth orbit in particular as useful orbits for space stations.<ref>Template:Cite book</ref> The first appearance of a geostationary orbit in popular literature was in October 1942, in the first Venus Equilateral story by George O. Smith,<ref name="VE">"(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." Template:Cite book</ref> 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.<ref name="VEintro">"It is therefore quite possible that these stories influenced me subconsciously when ... I worked out the principles of synchronous communications satellites ...", Template:Cite book</ref><ref name="clarke"/> The orbit, which Clarke first described as useful for broadcast and relay communications satellites,<ref name="clarke">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> is sometimes called the Clarke orbit.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Similarly, the collection of artificial satellites in this orbit is known as the Clarke Belt.<ref>Template:Cite magazine</ref>
In technical terminology the orbit is referred to as either a geostationary or geosynchronous equatorial orbit, with the terms used somewhat interchangeably.<ref>Template:Cite book</ref>
The first geostationary 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 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.<ref name=dm>Template:Cite magazine</ref>
Conventional wisdom at the time was that it would require too much rocket power to place a satellite in a geostationary orbit and it would not survive long enough to justify the expense,<ref>Template:Cite book</ref> so early efforts were put towards constellations of satellites in low or medium Earth orbit.<ref name="lat"/> The first of these were the passive Echo balloon satellites in 1960, followed by Telstar 1 in 1962.<ref>Template:Cite book</ref> Although these projects had difficulties with signal strength and tracking, issues that could be solved using geostationary orbits, the concept was seen as impractical, so Hughes often withheld funds and support.<ref name="lat">Template:Cite news</ref><ref name=dm/>
By 1961, Rosen and his team had produced a cylindrical prototype with a diameter of Template:Convert, height of Template:Convert, weighing Template:Convert, light and small enough to be placed into orbit. It was spin stabilised with a dipole antenna producing a pancake shaped beam.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In August 1961, they were contracted to begin building the real satellite.<ref name=dm/> 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 in Washington D.C., to phone Nigerian prime minister Abubakar Tafawa Balewa aboard the USNS Kingsport docked in Lagos on August 23, 1963.<ref name="lat"/><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
The first satellite placed in a geostationary orbit was Syncom 3, which was launched by a Delta D rocket in 1964.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> With its increased bandwidth, this satellite was able to transmit live coverage of the Summer Olympics from Japan to America. Geostationary orbits have been in common use ever since, in particular for satellite television.<ref name="lat"/>
Today there are hundreds of geostationary satellites providing remote sensing and communications.<ref name=dm/><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Although most populated land locations on the planet now have terrestrial communications facilities (microwave, fiber-optic), with telephone access covering 96% of the population and internet access 90%,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> some rural and remote areas in developed countries are still reliant on satellite communications.<ref>Template:Cite news</ref><ref>Template:Cite news</ref>
UsesEdit
Most commercial communications satellites, broadcast satellites and SBAS satellites operate in geostationary orbits.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="gmv"/><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
CommunicationsEdit
Geostationary communication satellites are useful because they are visible from a large area of the earth's surface, extending 81° away in latitude and 77° in longitude.<ref name="eisemann"/> They appear stationary in the sky, which eliminates the need for ground stations to have movable antennas. This means that Earth-based observers can erect small, cheap and stationary antennas that are always directed at the desired satellite.<ref name="smad"/>Template:Rp However, latency becomes significant as it takes about 240 ms for a signal to pass from a ground based transmitter on the equator to the satellite and back again.<ref name="smad"/>Template:Rp This delay presents problems for latency-sensitive applications such as voice communication,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> so geostationary communication satellites are primarily used for unidirectional entertainment and applications where low latency alternatives are not available.<ref name="wiley">Template:Cite book</ref>
Geostationary satellites are directly overhead at the equator and appear lower in the sky to an observer nearer the poles. As the observer's latitude increases, communication becomes more difficult due to factors such as atmospheric refraction, Earth's thermal emission, line-of-sight obstructions, and signal reflections from the ground or nearby structures. At latitudes above about 81°, geostationary satellites are below the horizon and cannot be seen at all.<ref name="eisemann">Template:Cite journal</ref> Because of this, some Russian communication satellites have used elliptical Molniya and Tundra orbits, which have excellent visibility at high latitudes.<ref name=seh>Template:Cite book</ref>
MeteorologyEdit
A worldwide network of operational geostationary meteorological satellites is used to provide visible and infrared images of Earth's surface and atmosphere for weather observation, oceanography, and atmospheric tracking. As of 2019 there are 19 satellites in either operation or stand-by.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> These satellite systems include:
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- the Meteosat series, launched by the European Space Agency and operated by the European Weather Satellite Organization, EUMETSAT<ref name="auto">{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref>
- the Republic of Korea COMS-1 and<ref name="LK">{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref> GK-2A multi mission satellites.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
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- Chinese Fengyun series<ref>Template:Cite news</ref>
- India's INSAT series<ref name=isro>{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref>
These satellites typically capture images in the visual and infrared spectrum with a spatial resolution between 0.5 and 4 square kilometres.<ref name="bomm"/> The coverage is typically 70°,<ref name="bomm">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and in some cases less.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Geostationary satellite imagery has been used for tracking volcanic ash,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> measuring cloud top temperatures and water vapour, oceanography,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> measuring land temperature and vegetation coverage,<ref>Template:Cite journal</ref><ref name="noaa"/> facilitating cyclone path prediction,<ref name=isro/> and providing real time cloud coverage and other tracking data.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Some information has been incorporated into meteorological prediction models, but due to their wide field of view, full-time monitoring and lower resolution, geostationary weather satellite images are primarily used for short-term and real-time forecasting.<ref>Template:Cite journal</ref><ref name="noaa">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Edit
Geostationary satellites can be used to augment GNSS systems by relaying clock, ephemeris and ionospheric error corrections (calculated from ground stations of a known position) and providing an additional reference signal.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> This improves position accuracy from approximately 5m to 1m or less.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Past and current navigation systems that use geostationary satellites include:
- The Wide Area Augmentation System (WAAS), operated by the United States Federal Aviation Administration (FAA);
- The European Geostationary Navigation Overlay Service (EGNOS), operated by the ESSP (on behalf of EU's GSA);
- The Multi-functional Satellite Augmentation System (MSAS), operated by Japan's Ministry of Land, Infrastructure and Transport Japan Civil Aviation Bureau (JCAB);
- The GPS Aided Geo Augmented Navigation (GAGAN) system being operated by India.<ref>Template:Cite press release</ref><ref>Template:Cite news</ref>
- The commercial StarFire navigation system, operated by John Deere and C-Nav Positioning Solutions (Oceaneering);
- The commercial Starfix DGPS System and OmniSTAR system, operated by Fugro.<ref>Template:Cite conference</ref>
ImplementationEdit
LaunchEdit
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Geostationary satellites are launched to the east into a prograde orbit that matches the rotation rate of the equator. The smallest inclination that a satellite can be launched into is that of the launch site's latitude, so launching the satellite from close to the equator limits the amount of inclination change needed later.<ref name="conf"/> Additionally, launching from close to the equator allows the speed of the Earth's rotation to give the satellite a boost. A launch site should have water or deserts to the east, so any failed rockets do not fall on a populated area.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Most launch vehicles place geostationary satellites directly into a geostationary transfer orbit (GTO), an elliptical orbit with an apogee at GEO height and a low perigee. On-board satellite propulsion is then used to raise the perigee, circularise and reach GEO.<ref name="conf">Template:Cite conference</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Orbit allocationEdit
Template:See also Satellites in geostationary orbit must all occupy a single ring above the equator. The requirement to space these satellites apart, to avoid harmful radio-frequency interference during operations, means that there are a limited number of orbital slots available, and thus only a limited number of satellites can be operated in geostationary orbit. This has led to conflict between different countries wishing access to the same orbital slots (countries near the same longitude but differing latitudes) and radio frequencies. These disputes are addressed through the International Telecommunication Union's allocation mechanism under the Radio Regulations.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In the 1976 Bogota Declaration, eight countries located on the Earth's equator claimed sovereignty over the geostationary orbits above their territory, but the claims gained no international recognition.<ref>Template:Cite journal</ref>
Statite proposalEdit
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. A statite is stationary relative to the Earth and Sun system rather than compared to surface of the Earth, and could ease congestion in the geostationary ring.<ref>Template:Cite patent</ref><ref>Template:Cite magazine</ref>
Retired satellitesEdit
Geostationary satellites require some station keeping to keep their position, and once they run out of thruster fuel they are generally retired. The transponders and other onboard systems often outlive the thruster fuel and by allowing the satellite to move naturally into an inclined geosynchronous orbit some satellites can remain in use,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> or else be elevated to a graveyard orbit. This process is becoming increasingly regulated and satellites must have a 90% chance of moving over 200 km above the geostationary belt at end of life.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Space debrisEdit
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Space debris at geostationary orbits typically has a lower collision speed than at low Earth orbit (LEO) since all GEO satellites orbit in the same plane, altitude and speed; however, the presence of satellites in eccentric orbits allows for collisions at up to Template:Cvt. Although a collision is comparatively unlikely, GEO satellites have a limited ability to avoid any debris.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
At geosynchronous altitude, objects less than 10 cm in diameter cannot be seen from the Earth, making it difficult to assess their prevalence.<ref name="telk1">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Despite efforts to reduce risk, spacecraft collisions have occurred. The European Space Agency telecom satellite Olympus-1 was struck by a meteoroid on August 11, 1993, and eventually moved to a graveyard orbit,<ref name="The Olympus failure">"The Olympus failure" ESA press release, August 26, 1993. Template:Webarchive</ref> and in 2006 the Russian Express-AM11 communications satellite was struck by an unknown object and rendered inoperable,<ref name=srdc20060419>"Notification for Express-AM11 satellite users in connection with the spacecraft failure" Russian Satellite Communications Company, April 19, 2006.</ref> although its engineers had enough contact time with the satellite to send it into a graveyard orbit. In 2017, both AMC-9 and Telkom-1 broke apart from an unknown cause.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="telk1"/><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
PropertiesEdit
A typical geostationary orbit has the following properties:
- Inclination: 0°
- Period: 1436 minutes (one sidereal day)<ref name="smad">Template:Cite book</ref>Template:Rp
- Eccentricity: 0
- Argument of perigee: undefined
- Semi-major axis: 42,164 km
InclinationEdit
An inclination of zero ensures 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 Earth-centered Earth-fixed reference frame).<ref name="smad"/>Template:Rp
PeriodEdit
The orbital period is 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. For a geostationary orbit in particular, it ensures that it holds the same longitude over time.<ref name="smad"/>Template:Rp This orbital period, T, is directly related to the semi-major axis of the orbit through Kepler's Third Law:
- <math>T = 2\pi\sqrt{a^3 \over \mu}</math>
where:
- Template:Mvar is the length of the orbit's semi-major axis
- Template:Mvar is the standard gravitational parameter of the central body<ref name="smad"/>Template:Rp
EccentricityEdit
The eccentricity is zero, which produces a circular orbit. This ensures that the satellite does not move closer or further away from the Earth, which would cause it to track backwards and forwards across the sky.<ref name="smad"/>Template:Rp
StabilityEdit
A geostationary orbit can be achieved only at an altitude very close to Template:Convert and directly above the equator. This equates to an orbital speed of Template:Convert and an orbital period of 1,436 minutes, one sidereal day. This ensures that the satellite will match the Earth's rotational period and has a stationary footprint on the ground. All geostationary satellites have to be located on this ring.
A combination of lunar gravity, solar gravity, and the flattening of the Earth at its poles causes a precession motion of the orbital plane of any geostationary object, with an orbital period of about 53 years and an initial inclination gradient of about 0.85° per year, achieving a maximal inclination of 15° after 26.5 years.<ref name=Anderson2015>Template:Cite conference</ref><ref name="smad"/>Template:Rp To correct for this perturbation, regular orbital stationkeeping maneuvers are necessary, amounting to a delta-v of approximately 50 m/s per year.<ref name="iop">Template:Cite conference</ref>
A second effect to be taken into account is the longitudinal drift, caused by the asymmetry of the Earth – the equator is slightly elliptical (equatorial eccentricity).<ref name="smad"/>Template:Rp There are two stable equilibrium points sometimes called "gravitational wells"<ref>Template:Cite book</ref> (at 75.3°E and 108°W) and two corresponding unstable points (at 165.3°E and 14.7°W). Any geostationary object placed between the equilibrium points would (without any action) be slowly accelerated towards the stable equilibrium position, causing a periodic longitude variation.<ref name=Anderson2015/> The correction of this effect requires station-keeping maneuvers with a maximal delta-v of about 2 m/s per year, depending on the desired longitude.<ref name="iop"/>
Solar wind and radiation pressure also exert small forces on satellites: over time, these cause them to slowly drift away from their prescribed orbits.<ref>Template:Cite conference</ref>
In the absence of servicing missions from the Earth or a renewable propulsion method, the consumption of thruster propellant for station-keeping places a limitation on the lifetime of the satellite. Hall-effect thrusters, which are currently in use, have the potential to prolong the service life of a satellite by providing high-efficiency electric propulsion.<ref name="iop"/>
DerivationEdit
For circular orbits around a body, the centripetal force required to maintain the orbit (Fc) is equal to the gravitational force acting on the satellite (Fg):<ref name="Pople" />
- <math>F_\text{c} = F_\text{g}</math>
From Isaac Newton's universal law of gravitation,
- <math>F_\text{g} = G \frac{M_\text{E} m_\text{s}}{r^2}</math>,
where Fg is the gravitational force acting between two objects, ME is the mass of the Earth, Template:Val, ms is the mass of the satellite, r is the distance between the centers of their masses, and G is the gravitational constant, Template:Val.<ref name=Pople>Template:Cite book</ref>
The magnitude of the acceleration, a, of a body moving in a circle is given by:
- <math>a = \frac{v^2}{r}</math>
where v is the magnitude of the velocity (i.e. the speed) of the satellite. From Newton's second law of motion, the centripetal force Fc is given by:
- <math>F_\text{c} = m_\text{s}\frac{v^2}{r}</math>.<ref name="Pople" />
As Fc = Fg,
- <math>m_\text{s}\frac{v^2}{r} = G \frac{M_\text{E} m_\text{s}}{r^2}</math>,
so that
- <math>v^2 = G \frac{M_\text{E}}{r}</math>
Replacing v with the equation for the speed of an object moving around a circle produces:
- <math>\left(\frac{2\pi r}{T}\right)^2 = G \frac{M_\text{E}}{r}</math>
where T is the orbital period (i.e. one sidereal day), and is equal to Template:Val.<ref>Edited by P. Kenneth Seidelmann, "Explanatory Supplement to the Astronomical Almanac", University Science Books,1992, p. 700.</ref> This gives an equation for r:<ref>Template:Cite book </ref>
- <math>r = \sqrt[3]{\frac{GM_\text{E} T^2}{4\pi^2}}</math>
The product GME is known with much greater precision than either factor alone; it is known as the geocentric gravitational constant μ = Template:Val. Hence
- <math> r = \sqrt[3]{\frac{\mu T^2}{4\pi^2}}</math>
The resulting orbital radius is Template:Convert. Subtracting the Earth's equatorial radius, Template:Convert, gives the altitude of Template:Convert.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
The orbital speed is calculated by multiplying the angular speed by the orbital radius:
- <math>v = \omega r \quad \approx 3074.6~\text{m/s}</math>
In other planetsEdit
By the same method, we can determine the orbital altitude for any similar pair of bodies, including the areostationary orbit of an object in relation to Mars, if it is assumed that it is spherical (which it is not entirely).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The gravitational constant GM (μ) for Mars has the value of Template:Val, its equatorial radius is Template:Val and the known rotational period (T) of the planet is Template:Val (Template:Val). Using these values, Mars' orbital altitude is equal to Template:Val.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
See alsoEdit
- List of orbits
- List of satellites in geosynchronous orbit
- Orbital station-keeping
- Space elevator, which ultimately reaches to and beyond a geostationary orbit
Explanatory notesEdit
ReferencesEdit
Template:Reflist Template:FS1037C MS188
External linksEdit
- How to get a satellite to geostationary orbit
- Orbital Mechanics (Rocket and Space Technology)
- List of satellites in geostationary orbit
- Clarke Belt Snapshot Calculator
- 3D Real Time Satellite Tracking
- Geostationary satellite orbit overview
- Daily animation of the Earth, made by geostationary satellite 'Electro L' photos Satellite shoots 48 images of the planet every day.
- Orbital Mechanics for Engineering Students