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					                         ROCKETS FOR SATELLITE COMMUNICATION

    The world needs a telecommunications system that could give constant field strength at
    all times over the whole earth. Although it is possible to provide telephone links between
    any two points on earth, the ionosphere can make long distance communication difficult.

5   A rocket flying at 8 km/sec outside the earth's atmosphere would stay in orbit. It would be
    like a second moon, going round the earth without using power. If the rocket were in orbit
    42,000 km above the earth, it would circle the earth once every 24 hours. It would stay
    above the same spot on the earth.

10 It would be possible to build a space station in such an orbit by carrying up materials in
   rockets. The station could be provided with transmitting and receiving equipment, and
   could act as a repeater to relay transmissions between any two points on the hemisphere
   beneath.

15 For world service, at least three stations would be needed, placed at equal distances around
   the earth. This is the only way to transmit around the whole world, using beams in an
   unlimited number of channels. The station would have very low power needs, and even if it
   were expensive at first the system would be much cheaper to run than present ones.

20 The first manmade satellite was launched on 4 October 1957. Five years later the Telstar,
   working in a low altitude orbit, made possible live broadcasts between North America and:
   Europe. The transmissions lasted for about 24 minutes at a time, but the satellite had the
   capacity for about sixty telephone circuits or a very limited TV channel.

25 Intelsat I had only 240 telephone channels, but now satellites like Intelsat V relay 75% of
   international telephone calls. New satellites have high capacity, long life, and can be used
   for telephone, television or computer data transmissions. Intelsat V has 12,000 circuits for
   all types of telecommunications.

30 Transmissions from a satellite can give global or spot beam coverage, but they are received
   by all earth stations in sight of the satellite. Each station selects the channels or carriers
   which are addressed to it. In the same way, all stations in sight of a satellite can use it
   simultaneously, and this is known as multiple access.

35 Earth stations must be capable of detecting very low level signals. Their parabolic
   antennas, which track the satellite automatically, feed the signal to a low noise first stage
   amplifier, which may be followed by a second stage travelling wave tube amplifier. The
   earth station transmitting equipment is very powerful, emitting signals at up to 10 kW. In
   some cases it is possible to control the satellite transponders and antennas from an earth
40 station. This process is known as telecommanding.
(Text adapted from Telecommunications Developing Reading Skills in English. by D. Oxford, Pergamon Press, 1985)
     INTELSAT was formed in 1964 to operate and maintain the global satellite
     communications system. The agency designs and builds satellites, and organizes their
     launching.

45 NASA, the U.S. space organization, is the main agency in the world for launching
   satellites. NASA’s two and three stage Titan rockets safely launched all ten Gemini
   manned spacecraft, as well as numerous satellites.

   E.S.A., the European space agency, has produced the Ariane launch vehicle as a rival to
   Titan. For a 1.2 tonne geostationary satellite an Ariane launch costs $30 million, compared
50 to $75 million by Titan. However, although it is more costly, Titan is also more reliable
   with 119 successful launches out of 122.

   The Ariane consists of three stages, each of which is a pair of fuel tanks with one or more
   rocket engines. The fuel from the two tanks burn when mixed in the engine. The large first
55 stage lifts, the vehicle through the atmosphere, and when the fuel tanks are empty the
   section falls away, igniting the second stage. The process is repeated by the second stage,
   then the third stage, which carries a computer and directional equipment in its front end,
   and maneuvers the satellite into position

60 Expendable launch vehicles such as Titan and Ariane now face strong competition from
   NASA’s new generation of re-usable "Space Shuttle" launch vehicles. Although the cost
   of a shuttle is as high as a billion dollars, the airplane like spacecraft is designed to go
   into orbit repeatedly, thus reducing the cost of launching a satellite to around $16 million.

65 The space shuttle is launched with the aid of two booster rockets and an external fuel tank.
   When the boosters are no longer needed they fall to earth by parachute, to be picked up
   from the sea and used again. Later the fuel tank is released, but this is not recovered. Once
   In orbit, the orbiter uses its own engines to maneuver. After the mission the shuttle re-
   enters the earth's atmosphere and lands on a runway in the same way as an aircraft.




(Text adapted from Telecommunications Developing Reading Skills in English. by D. Oxford, Pergamon Press, 1985)

				
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