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{{Spaceflight sidebar}} | {{Spaceflight sidebar}} | ||
A '''communications satellite''' is an [[artificial satellite]] that relays and amplifies [[radio]] telecommunication signals via a [[Transponder (satellite communications)|transponder]]; it creates a [[communication channel]] between a source [[transmitter]] and a [[Radio receiver|receiver]] at different locations on [[Earth]]. Communications satellites are used for [[television]], [[telephone]], [[radio]], [[internet]], and [[military]] applications.<ref>{{cite encyclopedia|last=Labrador |first=Virgil |url= | A '''communications satellite''' is an [[artificial satellite]] that relays and amplifies [[radio]] telecommunication signals via a [[Transponder (satellite communications)|transponder]]; it creates a [[communication channel]] between a source [[transmitter]] and a [[Radio receiver|receiver]] at different locations on [[Earth]]. Communications satellites are used for [[television]], [[telephone]], [[radio]], [[internet]], and [[military]] applications.<ref>{{cite encyclopedia|last=Labrador |first=Virgil |url=https://www.britannica.com/EBchecked/topic/524891/satellite-communication |title=satellite communication |encyclopedia=Britannica.com |date=2015-02-19 |access-date=2016-02-10}}</ref> As of 1 January 2021, there are 2,224 communications satellites in Earth orbit.<ref>{{cite web| url= https://www.ucsusa.org/resources/satellite-database#.W7WcwpMza9Y |title= UCS Satellite Database|publisher= Union of Concerned Scientists |date=1 August 2020|access-date=2 January 2021}}</ref> Most communications satellites are in [[geostationary orbit]] {{convert|22,300|mi|km}} above the [[equator]], so that the satellite appears stationary at the same point in the sky; therefore the [[satellite dish]] antennas of ground stations can be aimed permanently at that spot and do not have to move to track the satellite. | ||
The high frequency [[radio wave]]s used for telecommunications links travel by [[Line-of-sight propagation|line of sight]] and so are obstructed by the curve of the Earth. The purpose of communications satellites is to relay the signal around the curve of the Earth allowing communication between widely separated geographical points.<ref>{{cite web|url=http://satellites.spacesim.org/english/function/communic/index.html |title=Satellites - Communication Satellites |publisher=Satellites.spacesim.org |access-date=2016-02-10}}</ref> Communications satellites use a wide range of radio and [[microwave]] [[frequencies]]. To avoid signal interference, international organizations have regulations for which frequency ranges or "bands" certain organizations are allowed to use. This allocation of bands minimizes the risk of signal interference.<ref name="aerospace.org"/> | The high frequency [[radio wave]]s used for telecommunications links travel by [[Line-of-sight propagation|line of sight]] and so are obstructed by the curve of the Earth. The purpose of communications satellites is to relay the signal around the curve of the Earth allowing communication between widely separated geographical points.<ref>{{cite web|url=http://satellites.spacesim.org/english/function/communic/index.html |title=Satellites - Communication Satellites |publisher=Satellites.spacesim.org |access-date=2016-02-10}}</ref> Communications satellites use a wide range of radio and [[microwave]] [[frequencies]]. To avoid signal interference, international organizations have regulations for which frequency ranges or "bands" certain organizations are allowed to use. This allocation of bands minimizes the risk of signal interference.<ref name="aerospace.org"/> | ||
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=== More firsts and further experiments === | === More firsts and further experiments === | ||
[[Telstar]] was the first active, direct relay communications commercial satellite and marked the first transatlantic transmission of television signals. Belonging to [[AT&T Corporation|AT&T]] as part of a multi-national agreement between AT&T, [[Bell Labs|Bell Telephone Laboratories]], NASA, the British [[ | [[Telstar]] was the first active, direct relay communications commercial satellite and marked the first transatlantic transmission of television signals. Belonging to [[AT&T Corporation|AT&T]] as part of a multi-national agreement between AT&T, [[Bell Labs|Bell Telephone Laboratories]], NASA, the British [[General Post Office]], and the [[Orange S.A.|French National PTT]] (Post Office) to develop satellite communications, it was launched by NASA from [[Cape Canaveral]] on 10 July 1962, in the first privately sponsored space launch.<ref>{{cite book|title=Communications Satellites: Telstar|url=http://www.satmagazine.com/story.php?number=511938650|publisher=AIAA|edition=5th|date = March 16, 2007|isbn=978-1884989193|last1=Martin|first1=Donald|last2=Anderson|first2=Paul|last3=Bartamian|first3=Lucy}}</ref><ref name=PR1962>{{cite web|url= https://history.nasa.gov/presrep1962.pdf |title= United States Aeronautics and Space Activities 1962 |date=28 January 1963|publisher=The White House|pages=20, 96|access-date=3 January 2021}}</ref> | ||
Another passive relay experiment primarily intended for military communications purposes was [[Project West Ford]], which was led by [[Massachusetts Institute of Technology]]'s [[Lincoln Laboratory]].<ref name=BTI-8>{{cite book|last1=Ward|first1=William W.|last2=Floyd|first2=Franklin W.|url=https://history.nasa.gov/SP-4217/ch8.htm|chapter=Chapter 8: Thirty Years of Space Communications Research and Development at Lincoln Laboratory|title= Beyond The Ionosphere: Fifty Years of Satellite Communication|editor-last=Butrica|editor-first=Andrew J|publisher=NASA History Office|date=1997|bibcode=1997bify.book.....B}}</ref> After an initial failure in 1961, a launch on 9 May 1963 dispersed 350 million copper needle dipoles to create a passive reflecting belt. Even though only about half of the dipoles properly separated from each other,<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1963-014A-01|title=Project West Ford|publisher=NASA|access-date=4 January 2021}}</ref> the project was able to successfully experiment and communicate using frequencies in the [[Super high frequency|SHF]] [[X band]] spectrum.<ref name=NASAComp5>{{cite web|url= https://ntrs.nasa.gov/api/citations/19760014165/downloads/19760014165.pdf |title= NASA Compendium Of Satellite Communications Programs |date=December 1975|publisher=NASA|pages=5-1 to 5-16|access-date=4 January 2021}}</ref> | Another passive relay experiment primarily intended for military communications purposes was [[Project West Ford]], which was led by [[Massachusetts Institute of Technology]]'s [[Lincoln Laboratory]].<ref name=BTI-8>{{cite book|last1=Ward|first1=William W.|last2=Floyd|first2=Franklin W.|url=https://history.nasa.gov/SP-4217/ch8.htm|chapter=Chapter 8: Thirty Years of Space Communications Research and Development at Lincoln Laboratory|title= Beyond The Ionosphere: Fifty Years of Satellite Communication|editor-last=Butrica|editor-first=Andrew J|publisher=NASA History Office|date=1997|bibcode=1997bify.book.....B}}</ref> After an initial failure in 1961, a launch on 9 May 1963 dispersed 350 million copper needle dipoles to create a passive reflecting belt. Even though only about half of the dipoles properly separated from each other,<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1963-014A-01|title=Project West Ford|publisher=NASA|access-date=4 January 2021}}</ref> the project was able to successfully experiment and communicate using frequencies in the [[Super high frequency|SHF]] [[X band]] spectrum.<ref name=NASAComp5>{{cite web|url= https://ntrs.nasa.gov/api/citations/19760014165/downloads/19760014165.pdf |title= NASA Compendium Of Satellite Communications Programs |date=December 1975|publisher=NASA|pages=5-1 to 5-16|access-date=4 January 2021}}</ref> | ||
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{{Main|Satellite constellation}} | {{Main|Satellite constellation}} | ||
A group of satellites working in concert is known as a [[satellite constellation]]. Two such constellations, intended to provide [[satellite phone]] services, primarily to remote areas, are the [[Iridium (satellite)|Iridium]] and [[Globalstar]] systems. The Iridium system has 66 satellites. | A group of satellites working in concert is known as a [[satellite constellation]]. Two such constellations, intended to provide [[satellite phone]] and low-speed data services, primarily to remote areas, are the [[Iridium (satellite)|Iridium]] and [[Globalstar]] systems. The Iridium system has 66 satellites, which [[orbital inclination]] of 86.4° and inter-satellite links provide service availability over the entire surface of Earth. [[Starlink]] is a [[satellite internet constellation]] operated by [[SpaceX]], that aims for global [[satellite Internet access]] coverage. | ||
It is also possible to offer discontinuous coverage using a low-Earth-orbit satellite capable of storing data received while passing over one part of Earth and transmitting it later while passing over another part. This will be the case with the CASCADE system of [[Canada]]'s [[CASSIOPE]] communications satellite. Another system using this store and forward method is [[Orbcomm]]. | It is also possible to offer discontinuous coverage using a low-Earth-orbit satellite capable of storing data received while passing over one part of Earth and transmitting it later while passing over another part. This will be the case with the CASCADE system of [[Canada]]'s [[CASSIOPE]] communications satellite. Another system using this store and forward method is [[Orbcomm]]. | ||
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The first and historically most important application for communication satellites was in intercontinental [[long distance telephony]]. The fixed [[Public Switched Telephone Network]] relays [[telephone call]]s from [[land line]] telephones to an [[earth station]], where they are then transmitted to a geostationary satellite. The downlink follows an analogous path. Improvements in [[submarine communications cable]]s through the use of [[fiber-optics]] caused some decline in the use of satellites for fixed telephony in the late 20th century. | The first and historically most important application for communication satellites was in intercontinental [[long distance telephony]]. The fixed [[Public Switched Telephone Network]] relays [[telephone call]]s from [[land line]] telephones to an [[earth station]], where they are then transmitted to a geostationary satellite. The downlink follows an analogous path. Improvements in [[submarine communications cable]]s through the use of [[fiber-optics]] caused some decline in the use of satellites for fixed telephony in the late 20th century. | ||
Satellite communications are still used in many applications today. Remote islands such as [[Ascension Island]], [[Saint Helena]], [[Diego Garcia]], and [[Easter Island]], where no submarine cables are in service, need satellite telephones. There are also regions of some continents and countries where landline telecommunications are rare to | Satellite communications are still used in many applications today. Remote islands such as [[Ascension Island]], [[Saint Helena]], [[Diego Garcia]], and [[Easter Island]], where no submarine cables are in service, need satellite telephones. There are also regions of some continents and countries where landline telecommunications are rare to non existent, for example large regions of South America, Africa, Canada, China, Russia, and Australia. Satellite communications also provide connection to the edges of [[Antarctica]] and [[Greenland]]. Other land use for satellite phones are rigs at sea, a backup for hospitals, military, and recreation. Ships at sea, as well as planes, often use satellite phones.<ref>{{cite web|url=http://www.iridium.com/IridiumConnected/IridiumAtWork/Maritime.aspx|title=Connected:Maritime|archive-url=https://web.archive.org/web/20130815171540/http://iridium.com/IridiumConnected/IridiumAtWork/Maritime.aspx|archive-date=2013-08-15|website=Iridium|access-date=2013-09-19}}</ref> | ||
Satellite phone systems can be accomplished by a number of means. On a large scale, often there will be a local telephone system in an isolated area with a link to the telephone system in a main land area. There are also services that will patch a radio signal to a telephone system. In this example, almost any type of satellite can be used. Satellite phones connect directly to a constellation of either geostationary or low-Earth-orbit satellites. Calls are then forwarded to a satellite [[Earth station#Telecommunications port|teleport]] connected to the Public Switched Telephone Network . | Satellite phone systems can be accomplished by a number of means. On a large scale, often there will be a local telephone system in an isolated area with a link to the telephone system in a main land area. There are also services that will patch a radio signal to a telephone system. In this example, almost any type of satellite can be used. Satellite phones connect directly to a constellation of either geostationary or low-Earth-orbit satellites. Calls are then forwarded to a satellite [[Earth station#Telecommunications port|teleport]] connected to the Public Switched Telephone Network . | ||
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A [[direct broadcast satellite]] is a communications satellite that transmits to small DBS [[satellite dish]]es (usually 18 to 24 inches or 45 to 60 cm in diameter). Direct broadcast satellites generally operate in the upper portion of the microwave [[Ku band|K<sub>u</sub> band]]. DBS technology is used for DTH-oriented ([[Direct-To-Home]]) satellite TV services, such as [[DirecTV]], DISH Network and Orby TV<ref>{{cite web |title=Orby TV (United States)|url=https://www.satlaunch.org/packages-orby-tv-117w.htm|access-date=9 April 2020}}</ref> in the United States, [[Bell Satellite TV]] and [[Shaw Direct]] in Canada, [[Freesat]] and [[Sky (UK and Ireland)|Sky]] in the UK, [[Republic of Ireland|Ireland]], and [[New Zealand]] and [[DSTV]] in South Africa. | A [[direct broadcast satellite]] is a communications satellite that transmits to small DBS [[satellite dish]]es (usually 18 to 24 inches or 45 to 60 cm in diameter). Direct broadcast satellites generally operate in the upper portion of the microwave [[Ku band|K<sub>u</sub> band]]. DBS technology is used for DTH-oriented ([[Direct-To-Home]]) satellite TV services, such as [[DirecTV]], DISH Network and Orby TV<ref>{{cite web |title=Orby TV (United States)|url=https://www.satlaunch.org/packages-orby-tv-117w.htm|access-date=9 April 2020}}</ref> in the United States, [[Bell Satellite TV]] and [[Shaw Direct]] in Canada, [[Freesat]] and [[Sky (UK and Ireland)|Sky]] in the UK, [[Republic of Ireland|Ireland]], and [[New Zealand]] and [[DSTV]] in South Africa. | ||
Operating at lower frequency and lower power than DBS, FSS satellites require a much larger dish for reception (3 to 8 feet (1 to 2.5 m) in diameter for K<sub>u</sub> band, and 12 feet (3.6 m) or larger for C band). They use [[linear polarization]] for each of the transponders' RF input and output (as opposed to [[circular polarization]] used by DBS satellites), but this is a minor technical difference that users do not notice. FSS satellite technology was also originally used for DTH satellite TV from the late 1970s to the early 1990s in the United States in the form of [[TVRO]] ( | Operating at lower frequency and lower power than DBS, FSS satellites require a much larger dish for reception (3 to 8 feet (1 to 2.5 m) in diameter for K<sub>u</sub> band, and 12 feet (3.6 m) or larger for C band). They use [[linear polarization]] for each of the transponders' RF input and output (as opposed to [[circular polarization]] used by DBS satellites), but this is a minor technical difference that users do not notice. FSS satellite technology was also originally used for DTH satellite TV from the late 1970s to the early 1990s in the United States in the form of [[TVRO]] (Television Receive Only) receivers and dishes. It was also used in its K<sub>u</sub> band form for the now-defunct [[Primestar]] satellite TV service. | ||
Some satellites have been launched that have transponders in the [[Ka band|K<sub>a</sub> band]], such as DirecTV's [[SPACEWAY-1]] satellite, and [[Anik (satellite)|Anik F2]]. NASA and [[ISRO]]<ref>{{cite web|title=GSAT-14|url=http://www.isro.org/satellites/gsat-14.aspx|publisher=ISRO|access-date=16 January 2014|url-status=dead|archive-url=https://web.archive.org/web/20140108052813/http://www.isro.org/satellites/gsat-14.aspx|archive-date=8 January 2014}}</ref><ref>{{cite news|title=Indian GSLV successfully lofts GSAT-14 satellite|url=http://www.nasaspaceflight.com/2014/01/indian-gslv-launch-gsat-14-communications-satellite/|access-date=16 January 2014|newspaper=NASA Space Flight|date=4 January 2014}}</ref> have also launched experimental satellites carrying K<sub>a</sub> band beacons recently.<ref>{{cite web|url=http://www.spaceref.com/news/viewpr.html?pid=16748|title=DIRECTV's Spaceway F1 Satellite Launches New Era in High-Definition Programming; Next Generation Satellite Will Initiate Historic Expansion of DIRECTV|publisher=SpaceRef|access-date=2012-05-11}}</ref> | Some satellites have been launched that have transponders in the [[Ka band|K<sub>a</sub> band]], such as DirecTV's [[SPACEWAY-1]] satellite, and [[Anik (satellite)|Anik F2]]. NASA and [[ISRO]]<ref>{{cite web|title=GSAT-14|url=http://www.isro.org/satellites/gsat-14.aspx|publisher=ISRO|access-date=16 January 2014|url-status=dead|archive-url=https://web.archive.org/web/20140108052813/http://www.isro.org/satellites/gsat-14.aspx|archive-date=8 January 2014}}</ref><ref>{{cite news|title=Indian GSLV successfully lofts GSAT-14 satellite|url=http://www.nasaspaceflight.com/2014/01/indian-gslv-launch-gsat-14-communications-satellite/|access-date=16 January 2014|newspaper=NASA Space Flight|date=4 January 2014}}</ref> have also launched experimental satellites carrying K<sub>a</sub> band beacons recently.<ref>{{cite web|url=http://www.spaceref.com/news/viewpr.html?pid=16748|title=DIRECTV's Spaceway F1 Satellite Launches New Era in High-Definition Programming; Next Generation Satellite Will Initiate Historic Expansion of DIRECTV|publisher=SpaceRef|access-date=2012-05-11}}</ref> | ||