COMMUNICATION SATELLITE

               A spacecraft is placed in orbit around the Earth to intercept and retransmit radio signals. The communication satellite may also amplify,sort or route these signals.The communication function is similiar to a ground repeater relays communication satellite signals which connect many locations of both fixed and mobile over a wide area.

             There are two types of communication satellites of practical interest; passive and active. There is no amplification capability in a passive satellite; it simply reflects incident signals. passive communication satellite systems fall into two types: One uses large discrete structures, such as the Echo satellite and the other uses teh west ford approach in which a large volume of space is filled with a large number of tiny passive satellite. The general disadvantage of passive system is the limitation on message capacity resulting from the extremely low level of reradiated energy.

            In all active satellite system, complete electronic equipment for processing and transmission of signal must be carried. Also, means must be provided for directing antennas and maintaining desired satellite orientation and orbital position. with on-board alectronic processing capability, an active satellite can amplify,modify, and retransmit the signals as appropriate. Active satellite can also be considered as two basic types; the store-and-forward design and the massage real-time repeater.In the store and forward type,an Earth atation transmits a message to a satellite passing overhead.The massage is stored in an on board recorder and later played back through the satellite transmitter when the satellite passes over the appropriate Earth receiving station. The only communication satellite now considered for practical  use is real-time repeater.Operation of this instantaneous repeater requires the transmitting and receiving stations to be simultaneously in view of the spacecraft.

          The first satelite with the primary purpose of communication was score, launched on december 18th,1958.It received message from earth station in the United States and recorded them on tape for later transmission  when the satellite was over the receiving station. Echo 1,launced on august 12th,1960, was the first artificial satellite whose primary purpose was to provide real-time relay of message.It was a metallic coated plastic ballon 100ft(30m) in diameter which could only reflect the signal directed toward it. The American telephone and telegraph company developed and build Telstar,which was launced by NASA on july 10th 1962.It was the first active communication satellite with real time repeater. Telstar provided the first live television between the United States,europe,Japan,South America, on July 26th 1963.