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A spacecraft in orbit around the earth to receive and retransmit radio signals. Communications satellites amplify and sort or route these signals. In earlier days they functioned much like ground microwave repeaters but with greatly increased coverage. Whereas a ground repeater relays signals between two fixed locations, a communications satellite interconnects may locations, fixed and mobile, over a wide area. With the advent of on-board processing, switching and rerouting of signals has been added to the functionality of some communications satellites, making them "switchboards in the sky."
The first artificial satellite was the soviet sputnik 1, launched on October 4, 1957, and equipped with an-board radio-transmitter that worked on tow frequencies, 20.005 and 40.002MHz. 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 form U.S. President Dwight D. Eisenhower. NASA launched an Echo satellite, in 1960; the 100-foot(30 m) aluminized 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.
Geostationary communications satellite
Communications satellites have a quiet, yet profound, effect on our daily lives. They link remote areas of the Earth with telephone and television. Modern financial business is conducted at high speed via satellite. The newspaper USA Today is typeset and transmitted to printing plants via satellite.
Radio signals near the microwave frequency range are best suited to carry large volumes of communications traffic. They are not deflected by the Earth's atmosphere as lower frequencies are. Basically, they travel in a straight line, known as line of sight communication. If someone in San Francisco tried to beam a microwave signal directly to Hawaii, it would never get there; it would disappear into space or dissipate into the ocean. Over short distances, we can erect microwave towers every 25 miles or so to act as "repeaters" to repeat and boost the signal. Think of a geostationary communications satellite as a repeater in the sky.
An OSCAR satellitehttp://octopus.gma.org/i_space.gif
The satellite UOSAT-11 is one of dozens of amateur satellites orbiting the earth. Sputnik, the world's first artificial Earth-orbiting satellite, transmitted a beacon on 20.005 MHz which was monitored by thousands of hams and Short Wave Listeners (SWL). Since 1957, many OSCAR (Orbiting Satellites Carrying Amateur Radio) satellites have been constructed by ordinary people interested in satellites communications. OSCAR 1, launched in December of 1961, weighed 10 pounds and transmitted a 15 milliwatt beacon for about 3 weeks. OSCAR 13, launched in the summer of 1988, provides reliable, near-global communications. Interestingly enough, the OSCAR series of satellites are actually ballast for larger primary NASA payloads. It's simpler and cheaper to ballast a rocket with dead weight rather than reduce the thrust. As a result, it is possible to add secondary payloads of homemade satellites to multi-million dollar NASA missions at minimal costs.
There are currently nineteen OSCAR satellites orbiting our planet with various communications capabilities and functions. Most are used by ordinary amateur radio operators for educational, scientific, and purely recreational purposes. Anyone interested in knowing more about the OSCAR series of satellites in encouraged to contact the Radio Amateur Satellite Corporation (AMSAT).
EXPERIMENTAL SATELLITE COMMUNICATIONS
For the purpose of test and evaluation of new technologies a number of satellites have been designed and operated for technical experiments. Various experiments have also been conducted using these satellites for demonstrating different applications of communications satellites. Prominent among these experimental satellites are:
â€¢ Applications Technology Satellite Series (ATS-1, ATS-3, ATS-5 & ATS-6) of NASA.
â€¢ Joint Canadian - US Communications Technology Satellite (CTS or Hermes)
â€¢ Advanced Communications Technology Satellite (ACTS) of NASA.
â€¢ APPLE (Ariane Passenger Payload Experiment) Satellite of India.
â€¢ Symphonie Satellite (France & Germany).
â€¢ SIRIO (Italy)
â€¢ LES (US military)
â€¢ OTS (ESA)
â€¢ JBS, CS (Japan)
MILITARY SATELLITE SYSTEM
For military communications Army, Air force and Navy use both fixed and mobile satellite systems. In addition to the normal communications, military communications are also required for tactical communications from remote and inhospitable locations.
The special requirements of military communication terminals are high reliability, ruggedness, compact, operations under hostile environment, immunity to jamming, ease of portability and transportation, etc. Examples of military satellite communications systems are:
â€¢ DSCS (US AF)
â€¢ SKYNET (UK)
â€¢ NATO (NATO)
â€¢ FLTSATCOM (US NAVY)
Because of the special frequency band used in Military satellite system and other special requirements, Military satellite Systems are always much costlier and it takes longer time to design and develop compared to commercial satellite communications systems. Realizing that not all communications are strategic in nature, there is a trend now to use commercial communications system as far as possible. US Department of Defense is one of the major users of commercial Iridium satellite system with their own gateway.
Satellites have now replaced the stars and terrestrial systems for the purpose of navigation and radiolocation. The Transit Satellite system of US Navy was the first satellite navigational system with satellites orbiting in low polar orbits. By means of triangulation, the crews could establish the location of the ship and submarine by picking up the signals transmitted by different Transit satellites. US agreed to allow civilian use of Transit Navigational System for use by merchant marine shipping industry throughout the world.
Transit system is now replaced by the Global Positioning System (GPS) of US Navistar Satellite System consisting of eighteen low earth orbiting satellites operating at L-band. GPS receiver calculates the position (latitude, longitude, height) with extremely high accuracy by receiving signals from at least three-satellite passes. Apart from its use in ships, the miniaturized GPS receiver has also found many applications related to land based fleet monitoring.
The Russian Glonass system is the other navigational satellite system. However, the system is not being maintained properly by timely replacement of the satellites.
The principle of communication by passive satellite is based on the properties of scattering of electromagnetic waves from different surface areas. Thus an electromagnetic wave incident on a passive satellite is scattered back towards the earth and a receiving station can receive the scattered wave. The passive satellites used in the early years of satellite communications were both artificial as well as natural.
In 1954, the US Naval Research Laboratory successfully transmitted the first voice message through space by using the Moon to scatter radio signal. These experiments resulted in the development of Moon-Relay System, which became operational in 1959 for communications between Washington, DC and Hawaii and remained operational till 1963.
In active satellites, which amplify and retransmit the signal from the earth have several advantages over the passive satellites. The advantages of active satellites are:
â€¢ Require lower power earth station
â€¢ Less costly
â€¢ Not open to random use
â€¢ Directly controlled by operators from ground.
Disadvantages of active satellites are:
â€¢ Disruption of service due to failure of electronics components on-board the satellites
â€¢ Requirement of on-board power supply
â€¢ Requirement of larger and powerful rockets to launch heavier satellites in orbit
These are communication satellites are very useful to the people can communicate. When we are launching the satellite our country will be get development so this system very useful to the people this was got the very good invention on the people. So we now we are using the communication satellite. Finally I am welcoming this satellite.