Communication satellites are now becoming very popular for data communication between computers. These satellites are launched either by rockets or by space shuttles and parked in geostationary orbit at 36000km above the equator. The speed of the satellite in this orbit equals the speed of rotation of earth and thus the satellite is stationary relative to earth. One of the Indian satellite is INSAT-2B.It is a microwave relay station in the sky. The signals are sent at 6GHz to the satellite from a transmitter on the earth. It is received, amplified and retransmitted to the earth at 4 GHz by a system called a transponder mounted on the satellite.

The main advantage of the satellite is that it is a single microwave relay station visible from any where in a country.This transmission can be between any two randomly chosen places.The bandwidth of signals which can be handled by a transponder is about 36MHz which would give 1200 voice channels, each supporting 4800 bps data rate or 400 digital channels of 64 kbps each.A satellite has many transponders thus providing enormous communication capability at costs which favorably compute with microwave links on earth.

Features:

• The transmission and reception costs are independent of the distance between computers.
• It has got a very bandwidth width.
• A signal sent to a satellite is broadcast to all receivers within the satellite range thus special precautions would be necessary.
• There is a 270M sec propagation delay between the sending of data from one terminal or computer and its reception in another computer or terminal.
• A transmitting station can receive back its own transmission and check whether the transponder has transmitted the information correctly. If an error is detected the data would be retransmitted.
• When the no. of users of a satellite system service are more, it is desirable to locate earth station at each users site. This reduces cost and enhances clarity.

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