Geostationary orbit#Orbit allocation

{{Short description|Circular orbit above Earth's Equator and following the direction of Earth's rotation}}

File: Geosats compilation.jpg to the Equator are visible above this line. The satellites are pinpoint, while stars have created star trails due to Earth's rotation.]]

A geostationary orbit, also referred to as a geosynchronous equatorial orbitGeostationary orbit and Geosynchronous (equatorial) orbit are used somewhat interchangeably in sources. (GEO), is a circular geosynchronous orbit {{cvt|35,786|km|mi}} in altitude above Earth's equator, {{cvt|42,164|km|mi}} 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.

History

File:Syncom 2 side.jpg

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.{{Cite book|last=Noordung|first=Hermann|url=https://commons.wikimedia.org/w/index.php?title=File%3AHerman_Poto%C4%8Dnik_Noordung_-_Das_Problem_der_Befahrung_des_Weltraums.pdf&page=102|title=Das Problem der Befahrung des Weltraums: Der Raketen-Motor|publisher=Richard Carl Schmidt & Co.|year=1929|location=Berlin|pages=98–100|format=PDF}} The first appearance of a geostationary orbit in popular literature was in October 1942, in the first Venus Equilateral story by George O. Smith,"(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." {{cite book|last=Smith|first=George O.|author-link=George O. Smith |title=The Complete Venus Equilateral|date=1976|publisher=Ballantine Books|location=New York|isbn=978-0-345-28953-7|pages=3–4 |url=https://books.google.com/books?id=lj8H3R4J5GUC&q=squat}} 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."It is therefore quite possible that these stories influenced me subconsciously when ... I worked out the principles of synchronous communications satellites ...", {{cite book|url=https://archive.org/details/arthurcclarkeaut00mcal/page/54 |page=54 |title=Arthur C. Clarke |first=Neil |last=McAleer|year=1992 |isbn=978-0-809-24324-2|publisher=Contemporary Books}} The orbit, which Clarke first described as useful for broadcast and relay communications satellites,{{cite web| url=http://clarkeinstitute.org/wp-content/uploads/2010/04/ClarkeWirelessWorldArticle.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://clarkeinstitute.org/wp-content/uploads/2010/04/ClarkeWirelessWorldArticle.pdf |archive-date=2022-10-09 |url-status=live |author=Arthur C. Clarke|title= Extraterrestrial Relays: Can Rocket Stations Give World-wide Radio Coverage?|publisher=Arthur C. Clarke Institute for Space Education|date=October 1945|access-date=1 January 2021}} is sometimes called the Clarke orbit.{{cite web | publisher = NASA | url = http://www2.jpl.nasa.gov/basics/bsf5-1.php | title = Basics of Space Flight Section 1 Part 5, Geostationary Orbits | access-date = August 25, 2019 |editor=Phillips Davis | archive-url = https://archive.today/20121212130943/http://www2.jpl.nasa.gov/basics/bsf5-1.php | archive-date = 12 December 2012 }} Similarly, the collection of artificial satellites in this orbit is known as the Clarke Belt.{{Cite magazine|url=http://web.mit.edu/m-i-t/science_fiction/jenkins/jenkins_4.html|title=Orbit Wars: Arthur C. Clarke and the Global Communications Satellite|last= Mills|first=Mike|magazine=The Washington Post Magazine |date=August 3, 1997|pages= 12–13 |access-date=August 25, 2019}}

In technical terminology the orbit is referred to as either a geostationary or geosynchronous equatorial orbit, with the terms used somewhat interchangeably.{{cite book|chapter=Satellites and satellite remote senssing: Orbits |title=Encyclopedia of Atmospheric Sciences |edition=2 |year=2015 |pages=95–106 |last=Kidder |first=S.Q. |editor-first=Gerald |editor-last=North |editor-first2=John |editor-last2=Pyla |editor-first3=Fuqing |editor-last3=Zhang |doi=10.1016/B978-0-12-382225-3.00362-5 |publisher=Elsiver|isbn=9780123822253 }}

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.{{Cite magazine|first=Jack|last=McClintock|date=November 9, 2003|url=http://discovermagazine.com/2003/nov/communications|title=Communications: Harold Rosen – The Seer of Geostationary Satellites|website=Discover Magazine |access-date=August 25, 2019}}

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,{{Cite book|url=https://www.caltech.edu/about/news/harold-rosen-1926-2017-53790|title=Harold Rosen, 1926–2017|publisher=Caltech|last=Perkins|first=Robert|date=January 31, 2017 |access-date=August 25, 2019}} 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.{{cite book|title=Beyond The Ionosphere: Fifty Years of Satellite Communication|year=1997|chapter-url=https://history.nasa.gov/SP-4217/ch6.htm |author=Daniel R. Glover |editor=Andrew J Butrica|publisher=NASA |chapter=Chapter 6: NASA Experimental Communications Satellites, 1958-1995|bibcode=1997bify.book.....B}} 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.{{Cite news|url=https://www.latimes.com/nation/la-na-syncom-satellite-20130726-dto-htmlstory.html|title=How a satellite called Syncom changed the world|first=Ralph|last=Vartabedian|newspaper=Los Angeles Times |date=July 26, 2013 |access-date=August 25, 2019}}

By 1961, Rosen and his team had produced a cylindrical prototype with a diameter of {{convert|76|cm|in}}, height of {{convert|38|cm|in}}, weighing {{convert|11.3|kg|lb}}, light and small enough to be placed into orbit. It was spin stabilised with a dipole antenna producing a pancake shaped beam.{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1963-031A|publisher=NASA|title=Syncom 2|editor=David R. Williams |access-date=September 29, 2019}} In August 1961, they were contracted to begin 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 in Washington D.C., to phone Nigerian prime minister Abubakar Tafawa Balewa aboard the USNS Kingsport docked in Lagos on August 23, 1963.{{Cite web|url=https://www.historychannel.com.au/this-day-in-history/worlds-first-geosynchronous-satellite-launched/|title=World's First Geosynchronous Satellite Launched|publisher=Foxtel|date=June 19, 2016|website=History Channel|access-date=August 25, 2019|archive-date=December 7, 2019|archive-url=https://web.archive.org/web/20191207144926/https://www.historychannel.com.au/this-day-in-history/worlds-first-geosynchronous-satellite-launched/|url-status=dead}}

The first satellite placed in a geostationary orbit was Syncom 3, which was launched by a Delta D rocket in 1964.{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1964-047A |title=Syncom 3|publisher=NASA |editor=David R. Williams |access-date=September 29, 2019}} 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.

Today there are hundreds of geostationary satellites providing remote sensing and communications.{{Cite web|url=https://www.space.com/29222-geosynchronous-orbit.html|title=What Is a Geosynchronous Orbit?|website=Space.com|first=Elizabeth|last= Howell|date=April 24, 2015 |access-date=August 25, 2019}}

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%,{{cite web|url=https://www.itu.int/en/mediacentre/Pages/2018-PR40.aspx |title=ITU releases 2018 global and regional ICT estimates |publisher=International Telecommunication Union |access-date=August 25, 2019 |date= December 7, 2018}} some rural and remote areas in developed countries are still reliant on satellite communications.{{cite news|publisher=ABC |access-date=August 25, 2019 |title=Australia was promised superfast broadband with the NBN. This is what we got |first=Geoff |last=Thompson |date=April 24, 2019 |url=https://www.abc.net.au/news/2019-04-23/what-happened-to-superfast-nbn/11037620}}{{cite news|publisher=CNET |url=https://www.cnet.com/news/in-rural-farm-country-forget-broadband-you-might-not-have-internet-at-all/ |title=In farm country, forget broadband. You might not have internet at all. 5G is around the corner, yet pockets of America still can't get basic internet access. |first=Shara |last=Tibken |date=October 22, 2018 |access-date=August 25, 2019}}

Uses

Most commercial communications satellites, broadcast satellites and SBAS satellites operate in geostationary orbits.{{cite web|url=https://www.esa.int/Our_Activities/Telecommunications_Integrated_Applications/Orbits |title=Orbits |publisher=ESA |access-date=October 1, 2019 |date=October 4, 2018}}{{cite web|url=https://www.sws.bom.gov.au/Educational/5/4/3 |title=Satellites, Geo-stationary orbits and Solar Eclipses |access-date=October 1, 2019 |publisher=BOM|author=Richard Thompson}}

= Communications =

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. 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.{{rp|537}} 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.{{rp|538}} This delay presents problems for latency-sensitive applications such as voice communication,{{cite web|url=http://www.isoc.org/inet96/proceedings/g1/g1_3.htm |title=The Teledesic Network: Using Low-Earth-Orbit Satellites to Provide Broadband, Wireless, Real-Time Internet Access Worldwide |first=Daniel |last=Kohn |publisher=Teledesic Corporation, USA |date=March 6, 2016}} so geostationary communication satellites are primarily used for unidirectional entertainment and applications where low latency alternatives are not available.{{cite book|chapter=Satellite Communications|first=Roger L.|last=Freeman|date=July 22, 2002|publisher=American Cancer Society|doi=10.1002/0471208051.fre018|title = Reference Manual for Telecommunications Engineering|isbn = 0471208051}}

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.{{cite journal|url=http://www.ngs.noaa.gov/CORS/Articles/SolerEisemannJSE.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.ngs.noaa.gov/CORS/Articles/SolerEisemannJSE.pdf |archive-date=2022-10-09 |url-status=live|page=123|title=Determination of Look Angles To Geostationary Communication Satellites|first1=Tomás |last1=Soler|first2= David W. |last2=Eisemann|journal=Journal of Surveying Engineering |volume=120|issue=3|date=August 1994|issn=0733-9453|access-date=April 16, 2019|doi=10.1061/(ASCE)0733-9453(1994)120:3(115)}} Because of this, some Russian communication satellites have used elliptical Molniya and Tundra orbits, which have excellent visibility at high latitudes.{{cite book|page=416|url=https://books.google.com/books?id=2ZNxDwAAQBAJ&q=molniya+orbit+OKB-1+history&pg=PA416|isbn=978-1-85109-514-8|title = Space Exploration and Humanity: A Historical Encyclopedia|volume=1|author=History Committee of the American Astronautical Society |editor-first=Stephen B. |editor-last=Johnson|publisher = Greenwood Publishing Group|date = August 23, 2010|access-date=April 17, 2019}}

= Meteorology =

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.{{cite web|url=http://www.wmo.int/pages/prog/sat/satellitestatus.php|title=Satellite Status|publisher=World Meteorological Organization|access-date=July 6, 2019}} These satellite systems include:

the United States' GOES series, operated by NOAA{{Cite web|url=https://www.nesdis.noaa.gov/content/our-satellites|title=Our Satellites|work=NOAA National Environmental Satellite, Data, and Information Service (NESDIS)}}
the Meteosat series, launched by the European Space Agency and operated by the European Weather Satellite Organization, EUMETSAT{{Cite web|url=https://www.eumetsat.int/website/home/Satellites/CurrentSatellites/Meteosat/index.html|title=Meteosat|website=EUMETSAT.int|access-date=July 1, 2019|archive-date=January 14, 2020|archive-url=https://web.archive.org/web/20200114164122/https://www.eumetsat.int/website/home/Satellites/CurrentSatellites/Meteosat/index.html|url-status=dead}}
the Republic of Korea COMS-1 and{{cite web|url=http://www.arianespace.com/images/launch-kits/launch-kit-pdf-eng/GB-ARABSAT-5A-COMS.pdf|title=Satellite Launches for the Middle East and South Korea|publisher=Arianespace|access-date=June 26, 2010|url-status=dead|archive-url=https://web.archive.org/web/20100704234145/http://www.arianespace.com/images/launch-kits/launch-kit-pdf-eng/GB-ARABSAT-5A-COMS.pdf|archive-date=July 4, 2010}} GK-2A multi mission satellites.{{Cite web|url=https://www.airbus.com/newsroom/press-releases/en/2014/09/airbus-defence-and-space-supports-south-korean-weather-satellite-programme.html|title=Airbus Defence and Space supports South Korean weather satellite programme|website=Airbus|date=September 9, 2014|first=Ralph|last=Heinrich|access-date=July 2, 2019|archive-date=December 26, 2019|archive-url=https://web.archive.org/web/20191226065525/https://www.airbus.com/newsroom/press-releases/en/2014/09/airbus-defence-and-space-supports-south-korean-weather-satellite-programme.html|url-status=dead}}
the Russian Elektro-L satellites
the Japanese Himawari series{{Cite web|url=https://www.nasaspaceflight.com/2014/10/japan-loft-himawari-8-satellite-h-iia/|title=Japan lofts Himawari 8 weather satellite via H-IIA rocket |publisher=NASASpaceFlight.com|first=William |last=Graham|date=October 6, 2014}}
Chinese Fengyun series{{cite news |title=China plans to launch additional nine Fengyun meteorological satellites by 2025 |date=November 15, 2018 |url=https://gbtimes.com/china-plans-to-launch-additional-nine-fengyun-meteorological-satellites-by-2025 |newspaper=GBTimes |access-date=July 2, 2019 |archive-date=July 2, 2019 |archive-url=https://web.archive.org/web/20190702115542/https://gbtimes.com/china-plans-to-launch-additional-nine-fengyun-meteorological-satellites-by-2025 |url-status=dead }}
India's INSAT series{{cite web|url=https://www.isro.gov.in/rapid-gateway-to-indian-weather-satellite-data|publisher=Indian Space Research Organisation|title=RAPID: Gateway to Indian Weather Satellite Data|date=July 2, 2019|access-date=July 2, 2019|archive-date=December 25, 2019|archive-url=https://web.archive.org/web/20191225031828/https://www.isro.gov.in/rapid-gateway-to-indian-weather-satellite-data|url-status=dead}}

These satellites typically capture images in the visual and infrared spectrum with a spatial resolution between 0.5 and 4 square kilometres. The coverage is typically 70°,{{cite web|publisher=BOM|url=http://www.bom.gov.au/australia/satellite/about_satellites.shtml |title=About environmental satellites

|access-date=July 6, 2019}} and in some cases less.{{Cite web|url=http://www.planetary.org/multimedia/space-images/charts/coverage-of-a-geostationary.html|title=Coverage of a geostationary satellite at Earth|publisher=The Planetary Society}}

Geostationary satellite imagery has been used for tracking volcanic ash,{{Cite web|url=http://www.spaceref.com/news/viewpr.html?pid=15216|title=NOAA Satellites, Scientists Monitor Mt. St. Helens for Possible Eruption|website=SpaceRef|date=October 6, 2004|access-date=July 1, 2019|archive-date=September 10, 2012|archive-url=https://archive.today/20120910225555/http://www.spaceref.com/news/viewpr.html?pid=15216|url-status=dead}} measuring cloud top temperatures and water vapour, oceanography,{{cite web |publisher=NASA |title=GOCI |url=https://oceancolor.gsfc.nasa.gov/data/goci/ |access-date=August 25, 2019 |archive-date=June 24, 2021 |archive-url=https://web.archive.org/web/20210624041150/https://oceancolor.gsfc.nasa.gov/data/goci/ |url-status=dead }} measuring land temperature and vegetation coverage,{{Cite journal|last1=Miura|first1=Tomoaki|last2=Nagai|first2=Shin|last3=Takeuchi|first3=Mika|last4=Ichii|first4=Kazuhito|last5=Yoshioka|first5=Hiroki|date=2019-10-30|title=Improved Characterisation of Vegetation and Land Surface Seasonal Dynamics in Central Japan with Himawari-8 Hypertemporal Data|journal=Scientific Reports|language=en|volume=9|issue=1|page=15692|doi=10.1038/s41598-019-52076-x|issn=2045-2322|pmc=6821777|pmid=31666582|bibcode=2019NatSR...915692M}} facilitating cyclone path prediction, and providing real time cloud coverage and other tracking data.{{Cite web|url=https://sos.noaa.gov/datasets/goes-r-todays-satellite-for-tomorrows-forecast/|title=GOES-R: Today's Satellite for Tomorrow's Forecast Dataset|work=Science On a Sphere|date=November 14, 2016 |publisher=NOAA}} 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.{{Cite journal|title=Latest US weather satellite highlights forecasting challenges|first=Jeff|last=Tollefson|date=March 2, 2018|journal=Nature|volume=555|issue=7695|pages=154|doi=10.1038/d41586-018-02630-w|pmid=29517031|bibcode=2018Natur.555..154T|doi-access=free}}{{Cite web|url=https://public-old.wmo.int/en/resources/bulletin/noaa%E2%80%99s-eyes-sky-after-five-decades-of-weather-forecasting-environmental|archive-url=https://web.archive.org/web/20231218171711/https://public-old.wmo.int/en/resources/bulletin/noaa%E2%80%99s-eyes-sky-after-five-decades-of-weather-forecasting-environmental|url-status=dead|archive-date=December 18, 2023|title=NOAA's Eyes in the Sky – After Five Decades of Weather Forecasting with Environmental Satellites, What Do Future Satellites Promise for Meteorologists and Society?|date=November 12, 2015|website=World Meteorological Organization|first1=Derek|last1=Hanson|first2=James|last2=Peronto|first3=Douglas|last3=Hilderbrand|access-date=July 2, 2019}}

=Navigation=

Image:SBAS Service Areas.png |title= Deployment of an SBAS system demonstration in Southern Africa |date=August 6, 2016|access-date=October 1, 2019}}]]

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.{{Cite web|url=https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/waas/howitworks/|title=Satellite Navigation – WAAS – How It Works|publisher=FAA|date=June 12, 2019}} This improves position accuracy from approximately 5m to 1m or less.{{cite web|url=https://www.ga.gov.au/scientific-topics/positioning-navigation/positioning-for-the-future/satellite-based-augmentation-system |archive-url=https://web.archive.org/web/20190707040109/https://www.ga.gov.au/scientific-topics/positioning-navigation/positioning-for-the-future/satellite-based-augmentation-system |archive-date=July 7, 2019|publisher=Geoscience Australia|title=Satellite Based Augmentation System test-bed project}}

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.{{cite press release|url=http://isro.gov.in/pressrelease/scripts/pressreleasein.aspx?Jan03_2014|date=January 3, 2014|title=GAGAN System Certified for RNP0.1 Operations|archive-url=https://web.archive.org/web/20140103233538/http://isro.gov.in/pressrelease/scripts/pressreleasein.aspx?Jan03_2014|archive-date=January 3, 2014|publisher=Indian Space Research Organisation}}{{Cite news|url=https://www.thehindu.com/news/national/kerala/gagan-system-ready-for-operations/article5565700.ece|title=GAGAN system ready for operations|first=S. Anil|last=Radhakrishnan|date=January 11, 2014|work=The Hindu}}
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.{{Cite conference|title=Ten Years of Experience with A Commercial Satellite Navigation System|last= Ott |first= L. E. |conference=International Cooperation in Satellite Communications, Proceedings of the AIAA/ESA Workshop |place= ESTEC, Noordwijk, the Netherlands. |editor-first=C. |editor-last=Mattok |page=101 |bibcode=1995ESASP.372..101O}}

Implementation

=Launch=

{{multiple image|perrow = 1|total_width=

| image1 = Animation of EchoStar XVII trajectory.gif

| image2 = Animation of EchoStar XVII trajectory Equatorial view.gif

| footer = An example of a transition from temporary GTO to GSO.
{{legend2|magenta|EchoStar XVII}}{{·}}{{legend2|RoyalBlue|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 of inclination change needed later. 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.{{Cite web|url=https://www.eumetsat.int/website/home/Satellites/LaunchesandOrbits/LaunchingSatellites/index.html|title=Launching Satellites|website=Eumetsat|access-date=July 22, 2019|archive-date=December 21, 2019|archive-url=https://web.archive.org/web/20191221164243/https://www.eumetsat.int/website/home/Satellites/LaunchesandOrbits/LaunchingSatellites/index.html|url-status=dead}}

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.{{cite conference|url=https://www.researchgate.net/publication/282014319 |conference=20th International Symposium on Space Flight Dynamics|first1=Nicholas |last1=Farber |first2=Andrea |last2=Aresini |first3=Pascal |last3=Wauthier |first4=Philippe |last4=Francken|date=September 2007 |title=A general approach to the geostationary transfer orbit mission recovery |page=2}}{{cite web|url=http://www.planetary.org/blogs/jason-davis/20140116-how-to-get-a-satellite-to-gto.html |title=How to get a satellite to geostationary orbit |author=Jason Davis|date=January 17, 2014|access-date=October 2, 2019 |publisher=The Planetary Society}}

= Orbit allocation =

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.{{cite web|url=http://www.itu.int/ITU-R/conferences/seminars/mexico-2001/docs/06-procedure-mechanism.doc |archive-url=https://web.archive.org/web/20090327092830/http://www.itu.int/ITU-R/conferences/seminars/mexico-2001/docs/06-procedure-mechanism.doc|archive-date=March 27, 2009 |title=Orbit/Spectrum Allocation Procedures Registration Mechanism under the Radio Regulations |last=Henri |first=Yvon |publisher=Space Services Department}}{{cite web|url=http://www.itu.int/ITU-R/go/space/en |access-date=July 26, 2019 |publisher=ITU |title=Space Services Division}} 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.{{cite journal|last=Oduntan|first= Gbenga|s2cid= 10047170| title = The Never Ending Dispute: Legal Theories on the Spatial Demarcation Boundary Plane between Airspace and Outer Space|journal= Hertfordshire Law Journal|volume=1|issue= 2|page=75}}

=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. 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.{{cite patent |country = US |number = 5183225 |status = patent |title = STATITE: SPACECRAFT THAT UTILIZES SIGHT PRESSURE AND METHOD OF USE |pubdate = February 2, 1993 |pridate = 1989-01-09 |inventor-surname = Forward |inventor-given = Robert}}{{cite magazine|magazine=New Scientist |date=March 9, 1991 |title=Science: Polar 'satellite' could revolutionise communications|issue=1759|url=https://www.newscientist.com/article/mg12917594-000-science-polar-satellite-could-revolutionisecommunications/ |access-date=October 2, 2019}}

Retired satellites

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,{{cite web | url=http://www.satsig.net/satellite/inclined-orbit-operation.htm | title = Inclined orbit operation | work=SatSig.net}} 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.{{Cite web|url=https://phys.org/news/2017-04-satellites-die.html|title=Where old satellites go to die|website=phys.org|date=April 3, 2017|author=EUMETSAT}}

= Space debris =

File:Debris-GEO1280.jpg with two areas of concentration: geostationary orbit and low Earth orbit.]]

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 {{cvt|4|km/s|km/h mph|-2}}. Although a collision is comparatively unlikely, GEO satellites have a limited ability to avoid any debris.{{Cite web|url=https://physicsworld.com/a/space-debris-threat-to-geosynchronous-satellites-has-been-drastically-underestimated/|title=Space debris threat to geosynchronous satellites has been drastically underestimated|date=December 12, 2017|website=Physics World |author=Marric Stephens}}

At geosynchronous altitude, objects less than 10 cm in diameter cannot be seen from the Earth, making it difficult to assess their prevalence.{{Cite web|url=https://spacenews.com/exoanalytic-video-shows-telkom-1-satellite-erupting-debris/|title=ExoAnalytic video shows Telkom-1 satellite erupting debris|date=August 30, 2017|website=SpaceNews.com |author=Caleb Henry}}

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,[http://www.selkirkshire.demon.co.uk/analoguesat/olympuspr.html "The Olympus failure"] ESA press release, August 26, 1993. {{webarchive |url=https://web.archive.org/web/20070911181644/http://www.selkirkshire.demon.co.uk/analoguesat/olympuspr.html |date=September 11, 2007 }} and in 2006 the Russian Express-AM11 communications satellite was struck by an unknown object and rendered inoperable,[https://archive.today/20130104185122/http://www.spaceref.com/news/viewsr.html?pid=20320 "Notification for Express-AM11 satellite users in connection with the spacecraft failure"] Russian Satellite Communications Company, April 19, 2006. 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.{{Cite web|url=https://spacenews.com/op-ed-do-we-care-about-orbital-debris-at-all/|title=Do we care about orbital debris at all?|first=James E.|last=Dunstan|date=January 30, 2018|website=SpaceNews.com}}{{Cite web|url=http://spaceflight101.com/amc-9-satellite-anomaly-orbit-change/|title=AMC 9 Satellite Anomaly associated with Energetic Event & sudden Orbit Change – Spaceflight101|date=June 20, 2017|website=spaceflight101.com}}

Properties

A typical geostationary orbit has the following properties:

Inclination: 0°
Period: 1436 minutes (one sidereal day){{cite book|title=Space Mission Analysis and Design|publisher=Microcosm Press and Kluwer Academic Publishers |editor1-first=Wiley J. |editor1-last=Larson |editor2-first=James R. |editor2-last=Wertz |bibcode=1999smad.book.....W |last1=Wertz |first1=James Richard |last2=Larson |first2=Wiley J. |year=1999|isbn=1-881883-10-8}}{{rp|121}}
Eccentricity: 0
Argument of perigee: undefined
Semi-major axis: 42,164 km

=Inclination=

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).{{rp|122}}

=Period=

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.{{rp|121}} This orbital period, T, is directly related to the semi-major axis of the orbit through Kepler's Third Law:

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

where:

{{mvar|a}} is the length of the orbit's semi-major axis
{{mvar|μ}} is the standard gravitational parameter of the central body{{rp|137}}

=Eccentricity=

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.{{rp|122}}

Stability

A geostationary orbit can be achieved only at an altitude very close to {{convert|35786|km|mi|0|abbr=off}} and directly above the equator. This equates to an orbital speed of {{convert|3.07|km/s|mi/s|2| abbr=off}} 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.{{Cite conference|url=http://hanspeterschaub.info/Papers/Anderson2015c.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://hanspeterschaub.info/Papers/Anderson2015c.pdf |archive-date=2022-10-09 |url-status=live|title=Operational Considerations of GEO Debris Synchronization Dynamics|first1=Paul|last1=Anderson|display-authors=etal|conference=66th International Astronautical Congress|location=Jerusalem, Israel|date=2015|id=IAC-15,A6,7,3,x27478}}{{rp|156}} To correct for this perturbation, regular orbital stationkeeping maneuvers are necessary, amounting to a delta-v of approximately 50 m/s per year.{{cite conference|doi-access=free|title=Plasma propulsion for geostationary satellites fortelecommunication and interplanetary missions |conference=IOP Conference Series: Materials Science and Engineering |first1=M |last1=Dundeck |first2 = F |last2=Doveil |first3=N |last3=Arcis |first4=S |last4=Zurbach |year=2012 |number=29 |doi=10.1088/1757-899X/29/1/012010}}

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).{{rp|156}} There are two stable equilibrium points sometimes called "gravitational wells"{{cite book |last1=Jefferson Barker |title=U.S. Military Space Reference Text |date=Mar 2006 |publisher=National Security Space Institute |page=56 |url=https://edocs.nps.edu/2012/December/U.S.%20Military%20Space%20Reference%20Text%20-%202006.pdf}} (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. 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.

Solar wind and radiation pressure also exert small forces on satellites: over time, these cause them to slowly drift away from their prescribed orbits.{{cite conference|url=http://www.riccardobevilacqua.com/SOLAR%20RADIATION%20PRESSURE%20APPLICATIONS%20ON%20GEOSTATIONARY%20SATELLITES_SUBMISSION.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.riccardobevilacqua.com/SOLAR%20RADIATION%20PRESSURE%20APPLICATIONS%20ON%20GEOSTATIONARY%20SATELLITES_SUBMISSION.pdf |archive-date=2022-10-09 |url-status=live |first1=Patrick |last1=Kelly |first2=Richard S.|last2= Erwin|first3=Riccardo |last3=Bevilacqua|first4=Leonel |last4=Mazal |title=Solar radiation pressure applications on geostationary satellites |conference=Proceedings of the 2016 AAS GP & C Conference |publisher=American Astronautical Society |year=2016}}

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.

Derivation

Image:Comparison satellite navigation orbits.svg, GLONASS, Galileo and Compass (medium Earth orbit) satellite navigation system orbits with the International Space Station, Hubble Space Telescope and Iridium constellation orbits, and the nominal size of the Earth.Orbital periods and speeds are calculated using the relations 4π²R³ = T²GM and V²R = GM, where R is the radius of orbit in metres; T, the orbital period in seconds; V, the orbital speed in m/s; G, the gravitational constant ≈ {{val|6.673e-11|u=Nm²/kg²}}; M, the mass of Earth ≈ {{val|5.98e24|u=kg}}. The Moon's orbit is around 9 times larger (in radius and length) than geostationary orbit.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).]]

For circular orbits around a body, the centripetal force required to maintain the orbit (F_c) is equal to the gravitational force acting on the satellite (F_g):

: $F_\text{c} = F_\text{g}$

From Isaac Newton's universal law of gravitation,

: $F_\text{g} = G \frac{M_\text{E} m_\text{s}}{r^2}$ ,

where F_g is the gravitational force acting between two objects, M_E is the mass of the Earth, {{val|5.9736e24|u=kg}}, m_s is the mass of the satellite, r is the distance between the centers of their masses, and G is the gravitational constant, {{val|6.67428e−11|0.00067|u=m³ kg⁻¹ s⁻²}}.{{cite book |last=Pople |first=Stephen |date=2001 |title=Advanced Physics Through Diagrams |url=https://books.google.com/books?id=pjIW6QRlugsC&q=physics+of+geostationary+orbits&pg=PA72 |publisher=Oxford University Press |page=72 |isbn= 0-19-914199-1}}

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

: $a = \frac{v^2}{r}$

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 F_c is given by:

: $F_\text{c} = m_\text{s}\frac{v^2}{r}$ .

As F_c = F_g,

: $m_\text{s}\frac{v^2}{r} = G \frac{M_\text{E} m_\text{s}}{r^2}$ ,

so that

: $v^2 = G \frac{M_\text{E}}{r}$

Replacing v with the equation for the speed of an object moving around a circle produces:

: $\left(\frac{2\pi r}{T}\right)^2 = G \frac{M_\text{E}}{r}$

where T is the orbital period (i.e. one sidereal day), and is equal to {{val|86164.09054|u=seconds}}.Edited by P. Kenneth Seidelmann, "Explanatory Supplement to the Astronomical Almanac", University Science Books,1992, p. 700. This gives an equation for r:{{cite book |last= Mohindroo|first= K. K.|title=Basic Principles of Physics |volume=1|url= https://books.google.com/books?id=xsffXbNqpOsC&q=physics+of+geostationary+orbits&pg=RA6-PA19 |location= New Delhi|publisher= Pitambar Publishing Company|pages=6–8.19 |isbn= 81-209-0199-1|year= 1997}}

: $r = \sqrt[3]{\frac{GM_\text{E} T^2}{4\pi^2}}$

The product GM_E is known with much greater precision than either factor alone; it is known as the geocentric gravitational constant μ = {{val|398,600.4418|0.0008|u=km³ s⁻²}}. Hence

: $r = \sqrt[3]{\frac{\mu T^2}{4\pi^2}}$

The resulting orbital radius is {{convert|42164|km|mi|abbr=off}}. Subtracting the Earth's equatorial radius, {{convert|6378|km|mi|abbr=off}}, gives the altitude of {{convert|35786|km|mi|abbr=off}}.{{cite web |url= https://physics.info/orbital-mechanics-1/practice.shtml|title= Orbital Mechanics I |last= Elert |first= Glenn |date= 2019|website=The Physics Hypertextbook |access-date= September 30, 2019 }}

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

: $v = \omega r \quad \approx 3074.6~\text{m/s}$

In other planets

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).{{cite web |url= http://www.planetary.org/blogs/emily-lakdawalla/2013/stationkeeping-in-mars-orbit.html|title= Stationkeeping in Mars orbit

|last= Lakdawalla|first= Emily |date= 2013|website= The Planetary Society|access-date=September 30, 2019 }} The gravitational constant GM (μ) for Mars has the value of {{val|42830|u=km³ s⁻²}}, its equatorial radius is {{val|3389.50|u=km}} and the known rotational period (T) of the planet is {{val|1.02595676|u=Earth days}} ({{val|88642.66|u=seconds}}). Using these values, Mars' orbital altitude is equal to {{val|17039|u=km}}.{{cite web |url= https://ssd.jpl.nasa.gov/?planet_phys_par|title=Solar System Dynamics |date=2017 |publisher=NASA |access-date= September 30, 2019 }}

Explanatory notes

References

External links

[http://www.planetary.org/blogs/jason-davis/20140116-how-to-get-a-satellite-to-gto.html How to get a satellite to geostationary orbit]
[https://web.archive.org/web/20120204054322/http://www.braeunig.us/space/orbmech.htm Orbital Mechanics] (Rocket and Space Technology)
[http://www.satsig.net/sslist.htm List of satellites in geostationary orbit]
[https://web.archive.org/web/20060626002953/http://www.golombek.com/sat/ Clarke Belt Snapshot Calculator]
[https://web.archive.org/web/20130904234835/http://science1.nasa.gov/realtime/jtrack/3d/JTrack3D.html/ 3D Real Time Satellite Tracking]
[https://web.archive.org/web/20140404194939/http://www.radio-electronics.com/info/satellite/satellite-orbits/geostationary-earth-orbit.php Geostationary satellite orbit overview]
[https://web.archive.org/web/20141218004537/http://p-l-a.net/ Daily animation of the Earth, made by geostationary satellite 'Electro L' photos] Satellite shoots 48 images of the planet every day.
[https://books.google.com/books?id=rzw4wOHDpjQC&q=physics+of+geostationary+orbits Orbital Mechanics for Engineering Students]

Category:Astrodynamics

Category:Earth orbits