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A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. Communications satellites are used for television, telephone, radio, internet, and military applications. Many communications satellites are in geostationary orbit 22,300 miles (35,900 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. Others form satellite constellations in low Earth orbit, where antennas on the ground have to follow the position of the satellites and switch between satellites frequently.
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The high frequency radio waves used for telecommunications links travel by 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. 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.
In October 1945, Arthur C. Clarke published an article titled "Extraterrestrial Relays" in the British magazine Wireless World. The article described the fundamentals behind the deployment of artificial satellites in geostationary orbits for the purpose of relaying radio signals. Because of this, Arthur C. Clarke is often quoted as being the inventor of the concept of the communications satellite, and the term 'Clarke Belt' is employed as a description of the orbit.
There are two major classes of communications satellites, passive and active. Passive satellites only reflect the signal coming from the source, toward the direction of the receiver. With passive satellites, the reflected signal is not amplified at the satellite, and only a very small amount of the transmitted energy actually reaches the receiver. Since the satellite is so far above Earth, the radio signal is attenuated due to free-space path loss, so the signal received on Earth is very, very weak. Active satellites, on the other hand, amplify the received signal before retransmitting it to the receiver on the ground. Passive satellites were the first communications satellites, but are little used now.
Work that was begun in the field of electrical intelligence gathering at the United States Naval Research Laboratory in 1951 led to a project named Communication Moon Relay. Military planners had long shown considerable interest in secure and reliable communications lines as a tactical necessity, and the ultimate goal of this project was the creation of the longest communications circuit in human history, with the moon, Earth's natural satellite, acting as a passive relay. After achieving the first transoceanic communication between Washington, D.C., and Hawaii on 23 January 1956, this system was publicly inaugurated and put into formal production in January 1960.
The first satellite purpose-built to actively relay communications was Project SCORE, led by Advanced Research Projects Agency (ARPA) and launched on 18 December 1958, which used a tape recorder to carry a stored voice message, as well as to receive, store, and retransmit messages. It was used to send a Christmas greeting to the world from U.S. President Dwight D. Eisenhower. The satellite also executed several realtime transmissions before the non-rechargeable batteries failed on 30 December 1958 after eight hours of actual operation.
The direct successor to SCORE was another ARPA-led project called Courier. Courier 1B was launched on 4 October 1960 to explore whether it would be possible to establish a global military communications network by using "delayed repeater" satellites, which receive and store information until commanded to rebroadcast them. After 17 days, a command system failure ended communications from the satellite.
NASA's satellite applications program launched the first artificial satellite used for passive relay communications in Echo 1 on 12 August 1960. Echo 1 was an aluminized balloon satellite acting as a passive reflector of microwave signals. Communication signals were bounced off the satellite from one point on Earth to another. This experiment sought to establish the feasibility of worldwide broadcasts of telephone, radio, and television signals.
Telstar was the first active, direct relay communications commercial satellite and marked the first transatlantic transmission of television signals. Belonging to AT&T as part of a multi-national agreement between AT&T, Bell Telephone Laboratories, NASA, the British General Post Office, and the 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.
A direct extension of the passive experiments of Project West Ford was the Lincoln Experimental Satellite program, also conducted by the Lincoln Laboratory on behalf of the United States Department of Defense. The LES-1 active communications satellite was launched on 11 February 1965 to explore the feasibility of active solid-state X band long-range military communications. A total of nine satellites were launched between 1965 and 1976 as part of this series.
When Intelsat was launched, the United States was the only launch source outside of the Soviet Union, who did not participate in the Intelsat agreements. The Soviet Union launched its first communications satellite on 23 April 1965 as part of the Molniya program. This program was also unique at the time for its use of what then became known as the Molniya orbit, which describes a highly elliptical orbit, with two high apogees daily over the northern hemisphere. This orbit provides a long dwell time over Russian territory as well as over Canada at higher latitudes than geostationary orbits over the equator.
As satellites in MEO and LEO orbit the Earth faster, they do not remain visible in the sky to a fixed point on Earth continually like a geostationary satellite, but appear to a ground observer to cross the sky and "set" when they go behind the Earth beyond the visible horizon. Therefore, to provide continuous communications capability with these lower orbits requires a larger number of satellites, so that one of these satellites will always be visible in the sky for transmission of communication signals. However, due to their closer distance to the Earth, LEO or MEO satellites can communicate to ground with reduced latency and at lower power than would be required from a geosynchronous orbit.
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.
Allocating frequencies to satellite services is a complicated process which requires international coordination and planning. This is carried out under the auspices of the International Telecommunication Union (ITU).To facilitate frequency planning, the world is divided into three regions:
The first and historically most important application for communication satellites was in intercontinental long distance telephony. The fixed Public Switched Telephone Network relays telephone calls 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 cables 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 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.
Fixed Service Satellites use the C band, and the lower portions of the Ku band. 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, live shots, and backhauls), 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.
A direct broadcast satellite is a communications satellite that transmits to small DBS satellite dishes (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. DBS technology is used for DTH-oriented (Direct-To-Home) satellite TV services, such as DirecTV, DISH Network and Orby TV in the United States, Bell Satellite TV and Shaw Direct in Canada, Freesat and Sky in the UK, Ireland, and New Zealand and DSTV in South Africa.
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 remote areas, and cannot access a broadband connection, or require high availability of services.