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[[File:Wideband Global SATCOM Satellite.jpg|thumb|250px|U.S. military [[Wideband Global SATCOM system|WGSS]] communications satellite]]
A '''communications satellite''' (sometimes abbreviated to '''COMSAT''') is an artificial [[satellite]] stationed in space for the purpose of [[telecommunications]]. Modern communications satellites use a variety of orbits including [[geostationary orbit]]s, [[Molniya orbit]]s, other [[elliptical orbit]]s and low ([[Polar orbit|polar]] and non-polar) Earth orbits.  

For fixed ([[Point-to-point (telecommunications)|point-to-point]]) services, communications satellites provide a [[microwave radio relay]] technology complementary to that of [[submarine communication cables]]. They are also used for mobile applications such as communications to ships, vehicles, planes and hand-held terminals, and for TV and radio [[broadcasting]], for which application of other technologies, such as cable, is impractical or impossible.

==History==
See: [[Geostationary Orbit]] and [[Geosynchronous orbit]] Satellites. 
===Early missions===
The first artificial satellite was the Soviet [[Sputnik 1]], launched on October 4, 1957, and equipped with an on-board [[radio]]-[[transmitter]] that worked on two frequencies, 20.005 and 40.002&nbsp;MHz. The first American satellite to relay communications was [[Project SCORE]] in 1958, which used a tape recorder to [[store and forward]] voice messages. It was used to send a Christmas greeting to the world from U.S. President [[Dwight D. Eisenhower]]. [[NASA]] launched an [[Echo satellite]] in 1960; the {{convert|100|ft|m|adj=on}} aluminized [[PET film (biaxially oriented)|PET film]] balloon served as a passive reflector for radio communications.  [[Courier 1B]], built by [[Philco]], also launched in 1960, was the world’s first active repeater satellite.

[[Telstar]] was the first active, direct relay communications satellite. Belonging to [[American Telephone & Telegraph Company|AT&T]] as part of a multi-national agreement between AT&T, [[Bell Labs|Bell Telephone Laboratories]], NASA, the British [[General Post Office (United Kingdom)|General Post Office]], and the [[France Telecom|French National PTT]] (Post Office) to develop satellite communications, it was launched by NASA from [[Cape Canaveral]] on July 10, 1962, the first privately sponsored space launch. Telstar was placed in an elliptical [[orbit]] (completed once every 2&nbsp;hours and 37&nbsp;minutes), rotating at a 45° angle above the [[equator]].

An immediate antecedent of the geostationary satellites was [[Hughes Aircraft|Hughes]]’ [[Syncom 2]], launched on July 26, 1963.  Syncom 2 revolved around the earth once per day at constant speed, but because it still had north-south motion, special equipment was needed to track it.

=== Geostationary orbits ===
{{Main|Geostationary orbit}}
[[Image:Geostat.gif|thumb|[[Geostationary orbit]]]]
A satellite in a geostationary orbit appears to be in a fixed position to an earth-based observer. A geostationary satellite revolves around the earth at a constant speed once per day over the equator.

The geostationary orbit is useful for communications applications because ground based antennas, which must be directed toward the satellite, can operate effectively without the need for expensive equipment to track the satellite’s motion. Especially for applications that require a large number of ground antennas (such as direct TV distribution), the savings in ground equipment can more than justify the extra cost and onboard complexity of lifting a satellite into the relatively high geostationary orbit.

The concept of the geostationary communications satellite was first proposed by [[Arthur C. Clarke]], building on work by [[Konstantin Tsiolkovsky]] and on the 1929 work by [[Herman Potočnik]] (writing as [[Herman Noordung]]) ''Das Problem der Befahrung des Weltraums - der Raketen-motor''.  In October 1945 Clarke published an article titled “[http://www.lsi.usp.br/~rbianchi/clarke/ACC.ETRelaysFull.html Extra-terrestrial Relays]” in the British magazine ''[[Wireless World]]''. The article described the fundamentals behind the deployment of [[artificial satellite]]s in geostationary orbits for the purpose of relaying radio signals. Thus Arthur C. Clarke is often quoted as being the [[inventor]] of the communications satellite.

The first truly geostationary satellite launched in orbit was the [[Syncom 3]], launched on August 19, 1964.  It was placed in orbit at 180° east [[longitude]], over the [[International Date Line]]. It was used that same year to relay experimental television coverage of the [[1964 Summer Olympics]] in [[Tokyo, Japan]] to the [[United States]], making these Olympic games the first to be broadcast internationally. Although Syncom 3 is some times credited with the first television transmission to cross the [[Pacific Ocean]], the [[Relay 1]] satellite first broadcast from the United States to Japan on November 22, 1963.<ref name="NASA-SP-93">{{cite web|url=http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660009169_1966009169.pdf|title=Significant Achievements in Space Communications and Navigation, 1958-1964|year=1966|work=NASA-SP-93|publisher=NASA|pages=30–32|accessdate=2009-10-31}}</ref>

Shortly after Syncom 3, [[Intelsat I]], aka ''Early Bird'', was launched on April 6, 1965 and placed in orbit at 28° west longitude.  It was the first geostationary satellite for telecommunications over the [[Atlantic Ocean]].

On November 9, 1972,'s first geostationary satellite serving the continent, [[Anik A1]], was launched by [[Telesat Canada]], with the United States following suit with the launch of [[Westar 1]] by [[Western Union]] on April 13, 1974.

On December 19, 1974, the first geostationary communications satellite in the world to be [[three-axis stabilized]] was launched: the Franco-German [[Symphonie]].

After the launches of Telstar, Syncom 3, Early Bird, Anik A1, and Westar 1, RCA Americom (later GE Americom, now [[SES Americom]]) launched [[Satcom 1]] in 1975.  It was Satcom 1 that was instrumental in helping early [[cable TV]] channels such as WTBS (now [[TBS Superstation]]), [[HBO]], [[Christian Broadcasting Network|CBN]] (now [[ABC Family]]), and [[The Weather Channel (United States)|The Weather Channel]] become successful, because these channels distributed their programming to all of the local cable TV [[Cable television headend|headends]] using the satellite.  Additionally, it was the first satellite used by broadcast television networks in the United States, like [[American Broadcasting Company|ABC]], [[NBC]], and [[CBS]], to distribute programming to their local affiliate stations.  Satcom 1 was widely used because it had twice the communications capacity of the competing Westar 1 in America (24 [[Transponder (Satellite communications)|transponders]] as opposed to the 12 of Westar 1), resulting in lower transponder-usage costs. Satellites in later decades tended to have even higher transponder numbers.  

By 2000, Hughes Space and Communications (now [[Boeing Satellite Development Center]]) had built nearly 40 percent of the more than one hundred satellites in service worldwide. Other major satellite manufacturers include [[Space Systems/Loral]], [[Orbital Sciences Corporation]] with the [[STAR Bus]] series, [[Indian Space Research Organization]], [[Lockheed Martin Space Systems|Lockheed Martin]] (owns the former RCA Astro Electronics/GE Astro Space business), [[Northrop Grumman]], Alcatel Space, now [[Thales Alenia Space]], with the [[Spacebus]] series, and [[EADS Astrium]].

===Low-Earth-orbiting satellites===
{{Main|Low Earth orbit}}
[[Image:Orbits around earth scale diagram.svg|thumb|[[Low Earth orbit]] in Cyan]]
A [[Low Earth Orbit]] (LEO) typically is a circular orbit about 400 kilometres above the earth’s surface and, correspondingly, a period (time to revolve around the earth) of about 90&nbsp;minutes. Because of their low altitude, these satellites are only visible from within a radius of roughly 1000 kilometres from the sub-satellite point. In addition, satellites in low earth orbit change their position relative to the ground position quickly. So even for local applications, a large number of satellites are needed if the mission requires uninterrupted connectivity.

Low earth orbiting satellites are less expensive to launch into orbit than geostationary satellites and, due to proximity to the ground, do not require as high [[signal strength]] (Recall that signal strength falls off as the square of the distance from the source, so the effect is dramatic). Thus there is a trade off between the number of satellites and their cost. In addition, there are important differences in the onboard and ground equipment needed to support the two types of missions.

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. Another LEO satellite constellation known as [[Teledesic]], with backing from [[Microsoft]] entrepreneur [[Paul Allen]], was to have over 840 satellites. This was later scaled back to 288 and ultimately ended up only launching one test satellite. 

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]].

===Molniya satellites===
{{Main|Molniya orbit}}

As mentioned, geostationary satellites are constrained to operate above the equator. As a consequence, they are not always suitable for providing services at high latitudes: at high latitudes, a geostationary satellite will appear low on the horizon, affecting connectivity and causing [[multipath]] (interference caused by signals reflecting off the ground and into the ground antenna). The first satellite of the [[Molniya (satellite)|Molniya]] series was launched on April 23, 1965 and was used for experimental [[Transmission (telecommunications)|transmission]] of TV [[Signalling (telecommunication)|signal]] from a Moscow [[uplink]] station to [[downlink]] stations located in [[Siberia]] and the Russian Far East, in [[Norilsk]], [[Khabarovsk]], [[Magadan]] and [[Vladivostok]]. In November of 1967 Soviet engineers created a unique [[system]] of national TV [[television network|network]] of [[satellite television]], called [[Orbita]], that was based on Molniya satellites.

Molniya orbits can be an appealing alternative in such cases. The Molniya orbit is highly inclined, guaranteeing good elevation over selected positions during the northern portion of the orbit. (Elevation is the extent of the satellite’s position above the horizon. Thus, a satellite at the horizon has zero elevation and a satellite directly overhead has elevation of 90&nbsp;degrees). 

Furthermore, the Molniya orbit is designed so that the satellite spends the great majority of its time over the far northern latitudes, during which its ground footprint moves only slightly. Its period is one half day, so that the satellite is available for operation over the targeted region for eight hours every second revolution. In this way a constellation of three Molniya satellites (plus in-orbit spares) can provide uninterrupted coverage. 

Molniya satellites are typically used for telephony and TV services over [[Russia]]. Another application is to use them for mobile radio systems (even at lower latitudes) since cars travelling through urban areas need access to satellites at high elevation in order to secure good connectivity, e.g. in the presence of tall buildings.

== Applications ==
===Telephony===
[[Image:Iridium satellite.jpg|thumb|left|120px|An [[Iridium (satellite)|Iridium]] satellite]]
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, but they still serve remote islands such as [[Ascension Island]], [[Saint Helena]], [[Diego Garcia]], and [[Easter Island]], where no submarine cables are in service. There are also regions of some continents and countries where landline telecommunications are rare to nonexistent, 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]]. 

[[Satellite phone]]s connect directly to a constellation of either geostationary or low-earth-orbit satellites. Calls are then forwarded to a satellite [[teleport]] connected to the [[Public Switched Telephone Network]] or to another satellite phone system.

===Satellite television===
{{main|Satellite television}}
Television became the main market, its demand for simultaneous delivery of relatively few signals of large [[Bandwidth (signal processing)|bandwidth]] to many receivers being a more precise match for the capabilities of geosynchronous comsats.  Two satellite types are used for North American television and radio: Direct Broadcast Satellite (DBS), and Fixed Service Satellite (FSS)

The definitions of FSS and DBS satellites outside of North America, especially in Europe, are a bit more ambiguous. Most satellites used for direct-to-home television in Europe have the same high power output as DBS-class satellites in North America, but use the same linear polarization as FSS-class satellites. Examples of these are the [[SES Astra|Astra]], [[Eutelsat]], and [[Hotbird]] spacecraft in orbit over the European continent. Because of this, the terms FSS and DBS are more so used throughout the North American continent, and are uncommon in Europe.

====Fixed Service Satellite====
{{main|Fixed Service Satellite}}
'''Fixed Service Satellites''' use the [[C band]], and the lower portions of the K<sub>u</sub> bands. They are normally used for broadcast feeds to and from television networks and local affiliate stations (such as program feeds for network and syndicated programming, [[remote broadcast|live shot]]s, and [[backhaul (broadcasting)|backhaul]]s), as well as being used for [[distance learning]] by schools and universities, [[business television]] (BTV), [[Videoconferencing]], and general commercial telecommunications. FSS satellites are also used to distribute national cable channels to cable television headends.

[[Free-to-air]] satellite TV channels are also usually distributed on FSS satellites in the K<sub>u</sub> band. The [[Intelsat Americas 5]], [[Galaxy 10R]] and [[AMC 3]] satellites over [[North America]] provide a quite large amount of FTA channels on their K<sub>u</sub> band [[transponder]]s.

The American [[Dish Network|DISH Network]] DBS service has also recently utilized FSS technology as well for their programming packages requiring their [[SuperDish]] antenna, due to Dish Network needing more capacity to carry local television stations per the [[Federal Communications Commission|FCC]]'s "must-carry" regulations, and for more bandwidth to carry [[HDTV]] channels.

====Direct broadcast satellite====
{{main|Direct broadcast satellite}}
A '''direct broadcast satellite''' is a communications satellite that transmits to small DBS satellite dishes (usually 18 to 24&nbsp;inches or 45 to 60&nbsp;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]] and DISH Network in the United States, [[Bell TV]] and [[Shaw Direct]] in Canada, [[Freesat]] in the UK and [[Sky Digital (UK & Ireland)|Sky Digital]] in the UK, the [[Republic of Ireland]], and [[New Zealand]].

Operating at lower frequency and lower power than DBS, FSS satellites require a much larger dish for reception (3 to 8&nbsp;feet (1 to 2.5m) in diameter for K<sub>u</sub> band, and 12&nbsp;feet (3.6m) 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.

Satellites for communication have now{{when?}} 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 as well has launched experimental satellites using the K<sub>a</sub> band recently.

===Mobile satellite technologies===
Initially available for broadcast to stationary TV receivers, by 2004 popular mobile direct broadcast applications made their appearance with that arrival of two satellite radio systems in the United States: Sirius and XM Satellite Radio Holdings.  Some manufacturers have also introduced special antennas for mobile reception of DBS television. Using [[Global Positioning System|Global Positioning System (GPS)]] technology as a reference, these antennas automatically re-aim to the satellite no matter where or how the vehicle (that the antenna is mounted on) is situated.  These mobile satellite antennas are popular with some [[recreational vehicle]] owners.  Such mobile DBS antennas are also used by [[JetBlue Airways]] for DirecTV (supplied by [[LiveTV]], a subsidiary of JetBlue), which passengers can view on-board on LCD screens mounted in the seats.

===Satellite radio===
{{main|Satellite radio}}
Satellite radio offers audio services in some countries, notably the United States. Mobile services allow listeners to roam a continent, listening to the same audio programming anywhere.

A satellite radio or subscription radio (SR) is a digital radio signal that is broadcast by a communications satellite, which covers a much wider geographical range than terrestrial radio signals.

Satellite radio offers a meaningful alternative to ground-based radio services in some countries, notably the United States. Mobile services, such as Sirius, XM, and Worldspace, allow listeners to roam across an entire continent, listening to the same audio programming anywhere they go. Other services, such as Music Choice or Muzak's satellite-delivered content, require a fixed-location receiver and a dish antenna. In all cases, the antenna must have a clear view to the satellites. In areas where tall buildings, bridges, or even parking garages obscure the signal, repeaters can be placed to make the signal available to listeners.

Radio services are usually provided by commercial ventures and are subscription-based. The various services are proprietary signals, requiring specialized hardware for decoding and playback. Providers usually carry a variety of news, weather, sports, and music channels, with the music channels generally being commercial-free.

In areas with a relatively high population density, it is easier and less expensive to reach the bulk of the population with terrestrial broadcasts. Thus in the UK and some other countries, the contemporary evolution of radio services is focused on Digital Audio Broadcasting (DAB) services or HD Radio, rather than satellite radio.

===Amateur radio===
[[Amateur radio]] operators have access to the [[OSCAR]] satellites that have been designed specifically to carry amateur radio traffic. Most such satellites operate as spaceborne [[repeaters]], and are generally accessed by amateurs equipped with [[Ultra high frequency|UHF]] or [[VHF]] radio equipment and highly directional [[antennas]] such as [[Yagi antenna|Yagi]]s or dish antennas. Due to launch costs, most current amateur satellites are launched into fairly low Earth orbits, and are designed to deal with only a limited number of brief contacts at any given time. Some satellites also provide data-forwarding services using the [[AX.25]] or similar protocols.

===Satellite Internet===
{{main | Satellite Internet access}}
After the 1990s, satellite communication technology has been used as a means to connect to the [[Internet]] via broadband data connections. This can be very useful for users who are located in very remote areas, and cannot access a [[broadband]] connection.

===Military uses===
Communications satellites are used for [[military communications]] applications, such as [[Global Command and Control System]]s. Examples of military systems that use communication satellites are the [[MILSTAR]], the [[DSCS]], and the [[FLTSATCOM]] of the United States, [[NATO]] satellites, [[United Kingdom]] satellites, and satellites of the former [[Soviet Union]]. Many military satellites operate in the [[X-band]], and some also use [[UHF]] radio links, while MILSTAR also utilizes [[Ka band]].

===Navigation===
One of the fascinating applications of satellites is GPS (Global Positioning System). Its primary application is navigation. There is a network composed of 24 to 32 satellites in Medium Earth Orbit spaced equally around the world in overlapping pattern for this purpose. They use low microwave frequencies such as 1.57542Ghz and 1.2276&nbsp;GHz for transmission. Receivers on the earth pick up transmissions from four satellites simultaneously. The receiver uses the microprocessor to compute and display the exact position, in terms of [[latitude]] and [[longitude]].

==See also==

* [[Broadcasting Satellite (Japanese)]]
* [[Commercialization of space]]
* [[COMSAT]] Corporation
* [[Data Transmission Network]]
* [[Direct broadcast satellite|DBS]] satellite
* [[Digital Video Broadcasting|DVB]]
* [[DigiCipher 2]]
* [[DSCS]] satellite
* [[free-space optical communication]]s
* [[FLTSATCOM]] satellite
* [[Intelsat]] organization
* [[Integral Satcom Initiative]] European Technology Platform
* [[Intersputnik]] organization
* [[List of communications satellite firsts]]
* [[List of communication satellite companies]]
* [[List of orbits]]
* [[MILSTAR]] - the [[Military Strategic and Tactical Relay satellite]] system
* [[EME (communications)|Moonbounce]]
* [[Reconnaissance satellite]]
* [[PanAmSat]] Corp.
* [[Project West Ford]]
* [[Satellite dish]]
* [[Satmodem]]
* [[Satellite radio]]
* [[Satellite space segment]]
* [[Spacebus]]
* [[Symphonie]]
* [[Submarine communications cable]]
* [[Telesat]]
* [[Telstar]] satellite
* [[VSAT]] satellite terminal

{{Satcomm}}

==References==
{{nofootnotes|date=August 2009}}
{{Reflist}}

==External links==
* [http://www.sia.org/ Satellite Industry Association.]
* [http://www.esoa.net/ European Satellite Operators Association.]
* [http://www.prmt.com Online Satellite Glossary & Resource for Satcoms.]
* [http://www.satmagazine.com SatMagazine an on-line magazine on communications satellites.]
* [http://www.satnews.com SatNews an on-line directory of communications satellites.]
* [http://www.lyngsat.com LyngSat, an on-line directory of FSS & DBS communications satellites, and their transponder information]
* [http://news.bbc.co.uk/2/hi/science/nature/3721312.stm The future of communication satellite business]
* [http://www.hq.nasa.gov/office/pao/History/satcomhistory.html Communications satellites short history] by David J. Whalen
* [http://history.nasa.gov/SP-4217/sp4217.htm ''Beyond The Ionosphere: Fifty Years of Satellite Communication'' (NASA SP-4217, 1997)] – an entire book online—scroll down for “contents” link.
* [http://roland.lerc.nasa.gov/~dglover/sat/satcom2.html NASA experimental communications satellites]
* [http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1963-031A Syncom 2 satellite description]
* [http://www.ee.surrey.ac.uk/Personal/L.Wood/constellations/index.html Lloyd’s Satellite Constellations]
* [http://www.satcom.co.uk/ Satcom Online – A Resource for Satcom Engineers]
* [http://www.canadaconnects.ca/broadband/main/1113/ An Overview of Satellite Operating Frequencies and their Applications.]
* [http://www.2e1x1.com Unofficial USAF Satellite, Wideband and Telemetry Communications Career Field Page]

{{DEFAULTSORT:Communications Satellite}}
[[Category:Telecommunications equipment]]
[[Category:Communications satellites| ]]
[[Category:Telecommunications history]]

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