24-12-2010, 03:16 PM
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Communication Satellite
A communications satellite (sometimes abbreviated to comsat) is an artificial satellite stationed in space for the purposes of telecommunications. Modern communications satellites use geostationary orbits, Molniya orbits or low polar Earth orbits.
For fixed services, communications satellites provide a technology complementary to that of fiber optic submarine communication cables. They are also used for mobile applications such as communications to ships and planes, for which application of other technologies, such as cable, are impractical or impossible.
wikipedia.com
The first satellite equipped with on-board radio-transmitters was the Soviet Sputnik 1, launched in 1957
The first American satellite to relay communications was Project SCORE in 1958, which used a tape recorder to store and forward voice messages.
NASA launched an Echo satellite in 1960; the 100-foot aluminized PET film balloon served as a passive reflector for radio communications.
Geostationary orbits
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.
Application
1. Telephony
The first and historically the most important application for communication satellites is in international telephony. Fixed-point telephones relay calls to an earth station, where they are then transmitted to a geostationary satellite. An analogous path is then followed on the downlink. In contrast, mobile telephones (to and from ships and airplanes) must be directly connected to equipment to uplink the signal to the satellite, as well as being able to ensure satellite pointing in the presence of disturbances, such as waves onboard a ship.
Improvements in Submarine communications cables caused a decline in the use of satellites for fixed telephony in the late 20th century. Hand held telephones (cellular phones) used in urban areas do not make use of satellite communications. Instead they have access to a ground based constellation of receiving and retransmitting stations.
2. Satellite Television and radio
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)
is a communications satellite that transmits to small DBS satellite dishes (usually 18 to 24 inches 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 Sky Angel in the United States, ExpressVu in Canada, and Sky Digital in the UK, Republic of Ireland and New Zealand.
Fixed Service Satellite (FSS).
Use the C band, and the lower portions of the Ku 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, 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 TV headends.
3. 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 UHF or VHF radio equipment and highly directional antennas such as Yagis or dish antennas.
4. Satellite broadband
In recent years, 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 wireline broadband or dialup connection.
