A communication satellite is an artificial satellite stationed in space for the purpose of telecommunications. Modern communication satellites use a selection of orbits including geostationary orbits, other elliptical orbits and low earth orbits. For fixed point-to-point services, communications satellites provide a microwave radio relay technology to that of submarine communication cables. They are also used for mobile applications such as communications to ships, vehicles, planes and hand-held devices, and for TV and radio broadcasting, which application of other technologies.
The first satellite was the Soviet Sputnik 1, which was lunched into orbit on October 4, 1957, and was equipped with an on-board radio-transmitter that had two working frequencies only. The first American satellite to relay communications was Project SCORE in 1958, which used tape recorders to forward and store recorded data such as voice messages. Interestingly it was used to send a Christmas greeting to the world from U.S. President Dwight D. Eisenhower. NASA also launched an Echo satellite in 1960; the 100-foot aluminized PET film balloon served as a passive reflector for radio communications.
Telstar was the first active direct relay communications satellite. Belonging to AT&T as a part of a multi-national agreement between AT&T, Bell Telephone Laboratories, NASA, the British General Post Office, and the French National Post Office to develop satellite communications, it was launched by NASA from Cape Canaveral on July 10, 1962, which was the first privately sponsored space launch. Telstar was placed in an elliptical orbit soon after. An immediate forerunner of the geostationary satellites was Hughes? Syncom 2, it was launched on July 26, 1963. Syncom 2 revolved around the earth once per day at steady speed, but because it still had north-south motion, special equipment was needed to track it because of its capabilities.
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, which could be anything such as on demand by Comcast or by direct TV, the savings in ground equipment can more than justify the extra cost and onboard difficulty of lifting a satellite into the relatively high geostationary orbit.
The perception of the geostationary communications satellite was first proposed by Arthur C. Clarke. In October 1945 Clarke published an article titled ?Extra-terrestrial 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. Thus Arthur C. Clarke is often quoted as being the inventor of the communications satellite.
The first legitimate geostationary satellite launched in orbit was the Syncom 3, launched on August 19, 1964. It was used that same year to relay experimental television coverage on the 1964 Summer Olympics in Tokyo, Japan to the United States, the first television transmission sent over the Pacific Ocean.
Shortly after Syncom 3, Intelsat I, aka Early Bird, was launched on April 6, 1965. 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, which was called the Franco-German Symphonie. After the launches of Telstar, Syncom 3, Early Bird, Anik A1, and Westar 1, RCA Americom launched what they called the Satcom 1 in 1975. It was Satcom 1 that was instrumental in helping early cable TV channels such as TBS, HBO, ABC Family, and The Weather Channel become successful and working properly, because these channels distributed their programming to all of the local cable TV stations and distribution centers using the satellite. As well, it was the first satellite used by broadcast television networks in the United States, like ABC, NBC, and CBS, to distribute programming to their local affiliate stations. Satcom 1 was widely used because it had twice the communications ability of the opposing Westar 1 in America, which had 24 transponders as opposed to the previous 12 of Westar 1, resulting in lower transponder-usage costs. Satellites in later decades tended to have even higher transponder numbers because of the competitors and the advances of technology in the future.
By 2000, Hughes Space and Communications, which is now called Boeing Satellite Development Center, had built nearly 40 percent of the more than one hundred satellites in service worldwide today. Other major satellite manufacturers include Space Systems/Loral, Orbital Sciences Corporation with the STAR Bus series, Indian Space Research Organization, and many more, with the Spacebus series capabilities, and EADS Astrium A Low Earth Orbit, or LEO, is normally a circular orbit about 400 kilometers above the earth?s surface and, in the same way, a period of about 90 minutes. Because of their low altitude, these satellites are only able to be seen from within a radius of about 1000 kilometers from the point of sub-satellite. Also, satellites in low earth orbit change their position relative to the ground position quickly and effectively. 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, due to proximity to the ground, don't require as high signal strength. Therefore 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 of the constellations, intended to provide satellite phone services, mainly to remote areas. 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 irregular 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.
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, 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 phones 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. Television became the main market, its demand for simultaneous delivery of relatively few signals of large 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 Direct Broadcast Satellite -class satellites in North America, but use the same linear polarization as Fixed Service Satellite -class satellites. Examples of these are the Astra, Eutelsat, and Hotbird spacecraft in orbit over the European continent. Because of this, the terms FSS and DBS are more so used throughout North America, and are uncommon in Europe. A direct broadcast satellite is a communications satellite that transmits to small DBS satellite dishes. Direct Broadcast Satellite technology is used for DTH-oriented (Direct-To-Home) satellite TV services, such as DirecTV and DISH Network in the United States
Operating at lower frequency and lower power than Direct Broadcast Satellite, FSS satellites require a much larger dish for reception (3 to 8 feet (1 to 2.5m) in diameter for Ku band, and 12 feet (3.6m) or larger for C band. They use linear polarization for each of the transponders' RF input and output, which is different to circular polarization used by Direct Broadcast Satellite satellites, but this is a minor technical difference that users don't notice. Fixed Service Satellite technology was also originally used for Direct-To-Home 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 Ku band form for the now-defunct Primestar satellite TV service.
Satellites for communication have now been launched that have transponders in the Ka band, such as DirecTV's SPACEWAY-1 satellite. NASA as well has launched experimental satellites using the Ka band recently. 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 Direct Broadcast Satellite television. Using GPS technology as a reference, these antennas automatically re-aim to the satellite no matter where or how the vehicle, which is where the antenna is mounted on, is situated. These mobile satellite antennas are popular with some recreational vehicle owners. Such mobile Direct Broadcast Satellite antennas are also used by Jet Blue Airways for DirecTV, which passengers can view on-board on LCD screens mounted in the seats.
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 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 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 and very popular for some. In areas with a relatively high population density, it is easier and less expensive to reach the bulk of the population. 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. Communications satellites are used for military communications applications, such as Global Command and Control Systems. 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.
Need an essay? You can buy essay help from us today!
Please rate the quality of this essay:
Thanks for your rating :)
Struggling with your essay?
You can get your essay custom written by an expert in your subject area. Fully researched and referenced, the perfect model answer...
Get a quote here
Request the removal of this essay.
evan longoria john edwards conocophillips capitals ryan braun dan savage new world trade center
No comments:
Post a Comment
Note: Only a member of this blog may post a comment.