Satellite, Cable TV
Satellite television is television delivered by way of communications satellites, as compared to conventional terrestrial television and cable television. more...
In many areas of the world satellite television services supplement older terrestrial signals, providing a wider range of channels and services, including subscription-only services.
The first satellite television signal was relayed from Europe to the Telstar satellite over North America in 1962. The first geosynchronous communication satellite, Syncom 2 was launched in 1963.The world's first commercial communication satellite, called Early Bird, was launched into synchronous orbit on April 6, 1965. The first national network of satellite television, called Orbita, was created in Soviet Union in 1967, and was based on the principle of using the highly-elliptical Molniya satellite for re-broadcasting and delivering of TV signal to ground downlink stations. The first domestic North American satellite to carry television was Canada’s geostationary Anik 1, which was launched in 1973. ATS-6, the world's first experimental educational and Direct Broadcast Satellite, was launched in 1974. The first Soviet geostationary satellite to carry Direct-To-Home television, called Ekran, was launched in 1976.
Satellites used for television signals are generally in either highly-elliptical (with inclination of +/-63.4 degrees and orbital period of about 12 hours) or geostationary orbit 37,000 km (22,300 miles) above the earth’s equator.
Satellite television, like other communications relayed by satellite, starts with a transmitting antenna located at an uplink facility. Uplink satellite dishes are very large, as much as 9 to 12 meters (30 to 40 feet) in diameter. The increased diameter results in more accurate aiming and increased signal strength at the satellite. The uplink dish is pointed toward a specific satellite and the uplinked signals are transmitted within a specific frequency range, so as to be received by one of the transponders tuned to that frequency range aboard that satellite. The transponder 'retransmits' the signals back to Earth but at a different frequency band (to avoid interference with the uplink signal), typically in the C-band and/or Ku-band. The leg of the signal path from the satellite to the receiving Earth station is called the downlink.
A typical satellite has up to 24 transponders for a C-band only satellite and up to 32 for Ku-band, or more for hybrid satellites. Typical transponders each have a bandwidth of about 36 to 50 Mbit/s. Each geo-stationary C-band satellite needs to be spaced 2 degrees from the next satellite (to avoid interference). For Ku the spacing can be 1 degree. This means that there is an upper limit of 360/2 = 180 geostationary C-band satellites and 360/1 = 360 geostationary Ku-band satellites. C-band transmission is susceptible to terrestrial interference while Ku-band transmission is affected by rain (as water is an excellent absorber of microwaves).
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