Wikipedia

Geostationary orbit

(Redirected fromGeostationary satellite)
For broader coverage of this topic, seeGeosynchronous orbit.

Ageostationary orbit,also referred to as ageosynchronous equatorial orbit[a](GEO), is acirculargeosynchronous orbit35,786 km (22,236 mi) in altitude above Earth'sequator,42,164 km (26,199 mi) in radius from Earth's center, and following thedirectionofEarth's rotation.

Two geostationary satellites in the same orbit
A 5° × 6° view of a part of the geostationary belt, showing several geostationary satellites. Those with inclination 0° form a diagonal belt across the image; a few objects with smallinclinationsto theEquatorare visible above this line. The satellites are pinpoint, while stars have createdstar trailsdue toEarth's rotation.

An object in such an orbit has anorbital periodequal to Earth's rotational period, onesidereal 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 writerArthur C. Clarkein the 1940s as a way to revolutionise telecommunications, and the firstsatelliteto be placed in this kind of orbit was launched in 1963.

Communications satellitesare often placed in a geostationary orbit so that Earth-basedsatellite antennasdo not have to rotate to track them but can be pointed permanently at the position in the sky where the satellites are located.Weather satellitesare also placed in this orbit for real-time monitoring and data collection, andnavigation satellitesto provide a known calibration point and enhance GPS accuracy.

Geostationary satellites are launched via atemporary orbit,and placed in a slot above a particular point on the Earth's surface. The orbit requires some stationkeeping to keep its position, and modern retired satellites are placed in a highergraveyard orbitto avoid collisions.

History

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Syncom 2, the first geosynchronous satellite

In 1929,Herman Potočnikdescribed both geosynchronous orbits in general and the special case of the geostationary Earth orbit in particular as useful orbits forspace stations.[1]The first appearance of a geostationaryorbitin popular literature was in October 1942, in the firstVenus Equilateralstory byGeorge O. Smith,[2]but Smith did not go into details. Britishscience fictionauthorArthur C. Clarkepopularised and expanded the concept in a 1945 paper entitledExtra-Terrestrial Relays – Can Rocket Stations Give Worldwide Radio Coverage?,published inWireless Worldmagazine. Clarke acknowledged the connection in his introduction toThe Complete Venus Equilateral.[3][4]The orbit, which Clarke first described as useful for broadcast and relay communications satellites,[4]is sometimes called the Clarke orbit.[5]Similarly, the collection of artificial satellites in this orbit is known as the Clarke Belt.[6]

In technical terminology the orbit is referred to as either a geostationary or geosynchronous equatorial orbit, with the terms used somewhat interchangeably.[7]

The first geostationary satellite was designed byHarold Rosenwhile he was working atHughes Aircraftin 1959. Inspired bySputnik 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 onhigh frequencyradios and anundersea cable.[8]

Conventional wisdom at the time was that it would require too muchrocketpower to place a satellite in a geostationary orbit and it would not survive long enough to justify the expense,[9]so early efforts were put towards constellations of satellites inlowormediumEarth orbit.[10]The first of these were the passiveEcho balloon satellitesin 1960, followed byTelstar 1in 1962.[11]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.[10][8]

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 wasspin stabilisedwith a dipole antenna producing a pancake shaped beam.[12]In August 1961, they were contracted to begin building the real satellite.[8]They lostSyncom 1to electronics failure, butSyncom 2was successfully placed into a geosynchronous orbit in 1963. Although itsinclined orbitstill required moving antennas, it was able to relay TV transmissions, and allowed for US PresidentJohn F. Kennedyin Washington D.C., to phone Nigerian prime ministerAbubakar Tafawa Balewaaboard theUSNS Kingsportdocked inLagoson August 23, 1963.[10][13]

The first satellite placed in a geostationary orbit wasSyncom 3,which was launched by aDelta D rocketin 1964.[14]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.[10]

Today there are hundreds of geostationary satellites providingremote sensingand communications.[8][15]

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%,[16]some rural and remote areas in developed countries are still reliant on satellite communications.[17][18]

Uses

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See also:Geosynchronous satellite

Most commercialcommunications satellites,broadcast satellitesandSBASsatellites operate in geostationary orbits.[19][20][21]

Communications

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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.[22]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.[23]: 537 However,latencybecomes 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.[23]: 538 This delay presents problems for latency-sensitive applications such as voice communication,[24]so geostationary communication satellites are primarily used for unidirectional entertainment and applications where low latency alternatives are not available.[25]

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 asatmospheric refraction,Earth'sthermal 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.[22]Because of this, someRussiancommunication satellites have usedellipticalMolniyaandTundraorbits, which have excellent visibility at high latitudes.[26]

Meteorology

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See also:Weather satellite

A worldwide network of operationalgeostationary meteorological satellitesis used to provide visible andinfrared imagesof 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.[27]These satellite systems include:

These satellites typically capture images in the visual and infrared spectrum with a spatial resolution between 0.5 and 4 square kilometres.[35]The coverage is typically 70°,[35]and in some cases less.[36]

Geostationary satellite imagery has been used for trackingvolcanic ash,[37]measuring cloud top temperatures and water vapour,oceanography,[38]measuring land temperature and vegetation coverage,[39][40]facilitatingcyclonepath prediction,[34]and providing real time cloud coverage and other tracking data.[41]Some information has been incorporated intometeorological 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.[42][40]

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Further information:GNSS augmentation
Service areas of satellite-based augmentation systems (SBAS).[20]

Geostationary satellites can be used to augmentGNSSsystems by relayingclock,ephemerisandionosphericerror corrections (calculated from ground stations of a known position) and providing an additional reference signal.[43]This improves position accuracy from approximately 5m to 1m or less.[44]

Past and current navigation systems that use geostationary satellites include:

Implementation

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Launch

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See also:Geostationary transfer orbit
An example of a transition from temporaryGTOto GSO.
EchoStar XVII·Earth.

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 ofinclination changeneeded later.[48]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.[49]

Mostlaunch vehiclesplace geostationary satellites directly into ageostationary transfer orbit(GTO), an elliptical orbit with anapogeeat GEO height and a lowperigee.On-board satellite propulsion is then used to raise the perigee, circularise and reach GEO.[48][50]

Orbit allocation

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See also:Bogota Declaration

Satellites in geostationary orbit must all occupy a single ring above theequator.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 samelongitudebut differinglatitudes) andradio frequencies.These disputes are addressed through theInternational Telecommunication Union's allocation mechanism under theRadio Regulations.[51][52]In the 1976Bogota 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.[53]

Statite proposal

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Astatiteis a hypothetical satellite that usesradiation pressurefrom the sun against asolar sailto 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.[54][55]

Retired satellites

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Geostationary satellitesrequire somestation keepingto keep their position, and once they run out of thruster fuel they are generally retired. Thetranspondersand 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,[56]or else be elevated to agraveyard 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.[57]

Space debris

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Main article:Space debris § Characterization
A computer-generated image from 2005 showing the distribution of mostly space debris ingeocentric orbitwith two areas of concentration: geostationary orbit and low Earth orbit.

Space debris at geostationary orbits typically has a lower collision speed than atlow Earth orbit (LEO)since all GEO satellites orbit in the same plane, altitude and speed; however, the presence of satellites ineccentric orbitsallows for collisions at up to 4 km/s. Although a collision is comparatively unlikely, GEO satellites have a limited ability to avoid any debris.[58]

At geosynchronous altitude, objects less than 10 cm in diameter cannot be seen from the Earth, making it difficult to assess their prevalence.[59]

Despite efforts to reduce risk, spacecraft collisions have occurred. TheEuropean Space Agencytelecom satelliteOlympus-1was struck by ameteoroidon August 11, 1993, and eventually moved to agraveyard orbit,[60]and in 2006 the RussianExpress-AM11communications satellite was struck by an unknown object and rendered inoperable,[61]although its engineers had enough contact time with the satellite to send it into a graveyard orbit. In 2017, bothAMC-9andTelkom-1broke apart from an unknown cause.[62][59][63]

Properties

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A typical geostationary orbit has the following properties:

Inclination

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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 theEarth-centered Earth-fixedreference frame).[23]: 122 

Period

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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.[23]: 121 This orbital period,T,is directly related to the semi-major axis of the orbit through the formula:

where:

Eccentricity

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Theeccentricityis zero, which produces acircular 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.[23]: 122 

Stability

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A geostationary orbit can be achieved only at an altitude very close to 35,786 kilometres (22,236 miles) and directly above the equator. This equates to an orbital speed of 3.07 kilometres per second (1.91 miles per second) and an orbital period of 1,436 minutes, onesidereal day.This ensures that the satellite will match the Earth's rotational period and has a stationaryfootprinton the ground. All geostationary satellites have to be located on this ring.

A combination oflunargravity,solargravity, and theflattening of the Earthat its poles causes aprecessionmotion of the orbital plane of any geostationary object, with anorbital periodof about 53 years and an initial inclination gradient of about 0.85° per year, achieving a maximal inclination of 15° after 26.5 years.[64][23]: 156 To correct for thisperturbation,regularorbital stationkeepingmaneuvers are necessary, amounting to adelta-vof approximately 50 m/s per year.[65]

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).[23]: 156 There are two stable equilibrium points sometimes called "gravitational wells"[66](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.[64]The correction of this effect requiresstation-keeping maneuverswith a maximal delta-v of about 2 m/s per year, depending on the desired longitude.[65]

Solar windandradiation pressurealso exert small forces on satellites: over time, these cause them to slowly drift away from their prescribed orbits.[67]

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-efficiencyelectric propulsion.[65]

Derivation

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Comparison of geostationary Earth orbit withGPS,GLONASS,GalileoandCompass (medium Earth orbit)satellite navigation systemorbits with theInternational Space Station,Hubble Space TelescopeandIridium constellationorbits, and the nominal size of theEarth.[b]TheMoon's orbit is around 9 times larger (in radius and length) than geostationary orbit.[c]

For circular orbits around a body, thecentripetal forcerequired to maintain the orbit (Fc) is equal to the gravitational force acting on the satellite (Fg):[68]

FromIsaac Newton'suniversal law of gravitation,

,

whereFgis the gravitational force acting between two objects,MEis the mass of the Earth,5.9736×1024kg,msis the mass of the satellite,ris the distance between thecenters of their masses,andGis thegravitational constant,(6.67428±0.00067)×10−11m3kg−1s−2.[68]

The magnitude of the acceleration,a,of a body moving in a circle is given by:

wherevis the magnitude of thevelocity(i.e. the speed) of the satellite. FromNewton's second law of motion,the centripetal forceFcis given by:

.[68]

AsFc=Fg,

,

so that

Replacingvwith the equation for thespeed of an object moving around a circleproduces:

whereTis the orbital period (i.e. one sidereal day), and is equal to86164.09054s.[69]This gives an equation forr:[70]

The productGMEis known with much greater precision than either factor alone; it is known as thegeocentric gravitational constantμ=398600.4418±0.0008 km3s−2.Hence

The resulting orbital radius is 42,164 kilometres (26,199 miles). Subtracting theEarth's equatorial radius,6,378 kilometres (3,963 miles), gives the altitude of 35,786 kilometres (22,236 miles).[71]

The orbital speed is calculated by multiplying the angular speed by the orbital radius:

In other planets

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By the same method, we can determine the orbital altitude for any similar pair of bodies, including theareostationary orbitof an object in relation toMars,if it is assumed that it is spherical (which it is not entirely).[72]Thegravitational constantGM(μ) for Mars has the value of42830km3s−2,its equatorial radius is3389.50 kmand the knownrotational period(T) of the planet is1.02595676Earth days(88642.66 s). Using these values, Mars' orbital altitude is equal to17039km.[73]

See also

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Explanatory notes

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  1. ^Geostationary orbitandGeosynchronous (equatorial) orbitare used somewhat interchangeably in sources.
  2. ^Orbital periods and speeds are calculated using the relations 4π2R3=T2GMandV2R=GM,whereRis the radius of orbit in metres;T,the orbital period in seconds;V,the orbital speed in m/s;G,the gravitational constant ≈6.673×10−11Nm2/kg2;M,the mass of Earth ≈5.98×1024kg.
  3. ^The Moon's orbit is not perfectly circular, and is approximately 8.6 times further away from the Earth than the geostationary ring when the Moon is at perigee (363 104 km ÷ 42 164 km) and 9.6 times further away when the Moon is at apogee (405,696 km ÷ 42,164 km).

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