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A communications satellite is an artificial satellite that relays and amplifies through the use of a transponder, radio telecommunications signals, between a source transmitter and a receiver at different locations on Earth. Communications satellites are used for television, telephone, radio, internet, and military applications. There are over 2,000 communications satellites in Earth’s orbit, used by both private and government organizations.〔()〕 Wireless communication uses electromagnetic waves to carry signals. These waves require line-of-sight, and are thus obstructed by the curvature of the Earth. The purpose of communications satellites is to relay the signal around the curve of the Earth allowing communication between widely separated points.〔http://satellites.spacesim.org/english/function/communic/index.html〕 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 reflects toward the receiver. Since the satellite is so far above Earth the radio signal is attenuated on its long journey due to free-space path loss, so the signal received on Earth is very weak. Active satellites, on the other hand, amplify the received signal before re-transmitting it to the receiver on the ground〔http://www.aerospace.org/2013/12/12/military-satellite-communications-fundamentals/〕 Passive satellites were the first communications satellites, but are little used now. Communications satellites usually have one of three types of orbit (although several other types of orbits are sometimes used). Geostationary satellites have a ''geostationary orbit'' (GEO), which is from Earth’s surface. This orbit has the special characteristic that the apparent position of the satellite in the sky when viewed by a ground observer does not change, the satellite appears to "stand still" in the sky. This is because the satellite's orbital period is the same as the rotation rate of the Earth. The advantage of this orbit is that ground antennas do not have to track the satellite across the sky, they can be fixed to point at the location in the sky the satellite appears. Closer to the Earth is the ''medium Earth orbit'' (MEO). It ranges from above Earth. Below MEO is ''low Earth orbit'' (LEO) and is about above Earth. As satellites in MEO and LEO orbit the Earth faster, they cannot stay visible in the sky continually like a geostationary satellite, but appear to a ground observer to cross the sky and "set" while they go behind the Earth. Therefore, to provide continuous communications capability with these lower orbits requires more satellites, so one will always be in the sky when needed. However, due to their relatively small distance to the Earth their signals are stronger. The electromagnetic signals that communication satellites work with, have a large spectrum of wavelengths and frequencies. To keep these waves from interfering with one another, international organizations have certain rules and regulations describing which wavelength a certain company or group can use. By separating out wavelengths, communication satellites will have minimal interference and be able to communicate effectively.〔 == History == Today's satellite communications can trace their origins all the way back to February 1945 and Arthur C.Clarke's letter to the editor of Wireless World magazine, Clarke further fleshed-out this theory in a paper titled ''Extra-Terrestrial Relays – Can Rocket Stations Give Worldwide Radio Coverage?'', published in Wireless World in October 1945. Decades later a project named Communication Moon Relay was a telecommunication project carried out by the United States Navy. Its objective was to develop a secure and reliable method of wireless communication by using the Moon as a natural communications satellite. The first artificial Earth satellite was Sputnik 1. Put into orbit by the Soviet Union on October 4, 1957, it was equipped with an on-board radio-transmitter that worked on two frequencies: 20.005 and 40.002 MHz. Sputnik 1 was launched as a step in the exploration of space and rocket development. While incredibly important it was not placed in orbit for the purpose of sending data from one point on earth to another. And it was the first artificial satellite in the steps leading to today's satellite communications. The first artificial satellite used solely to further advances in global communications was a balloon named Echo 1.〔(ECHO 1 space.com )〕 Echo 1 was the world's first artificial communications satellite capable of relaying signals to other points on Earth. It soared above the planet after its Aug. 12, 1960 launch, yet relied on humanity's oldest flight technology — ballooning. Launched by NASA, Echo 1 was a giant metallic balloon across. The world's first inflatable satellite — or "satelloon", as they were informally known — helped lay the foundation of today's satellite communications. The idea behind a communications satellite is simple: Send data up into space and beam it back down to another spot on the globe. Echo 1 accomplished this by essentially serving as an enormous mirror, 10 stories tall, that could be used to reflect communications signals. 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 the Echo satellite in 1960; the aluminised 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 second active, direct relay communications satellite. 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 July 10, 1962, the first privately sponsored space launch. Relay 1 was launched on December 13, 1962, and became the first satellite to broadcast across the Pacific on November 22, 1963. An immediate antecedent of the geostationary satellites was Hughes' Syncom 2, launched on July 26, 1963. Syncom 2 was the first communications satellite in a geosynchronous orbit. It 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. It's successor, Syncom 3 was the first geostationary communications satellite. Syncom 3 obtained a geosynchronous orbit, without a north-south motion, making it appear from the ground as a stationary object in the sky. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Communications satellite」の詳細全文を読む スポンサード リンク
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