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Satellite Communication

A satellite is a physical object that orbits or revolves around some celestial body. Communication satellites are relay stations used for transmitting the information which are having line-of-sight restrictions.

The transmitting stations sends information to the satellite which inturn retransmit it to the receiving stations. Thus the satellite in this application is called as Repeater.

The satellite contains a receiver, which does the following activities :

a) fix up a transmitted signal
b) Amplifies it
c) Translates it to another frequency
d) This new frequency is then retransmitted to the receiving station back on earth.

The signals (original) being transmitted from the earth station to the satellite is called “uplink” and the retransmitted signal from the satellite to the receiving station is called the “down link”.

Transponder is a term used for transmitter – receiver combination in the satellite and its basic functions are amplification and frequency transmission.

A transponder consists of following three item :

a) Low-Noise Amplifier ( LNA) that receiver and amplifies the uplink signals.
b) A mixer that converts the signal to another frequency
c) A high power amplifier that retransmits the signals on its new downlink frequency.

Subsystem of Satellite

a) Power subsystem
b) Communication subsystem
c) Antenna subsystem
d) Telemetry tracking and control subsystem
e) altitude control subsystem
f) Propulusion subsystem.

Five Earth Subsystem are

A communication satellite can be thought of a microwave repeater in the sky. It contains several transponders each of listens to some portion of the spectrum, amplifies the incoming signal and then retransmits it at another frequency to avoid interference with the incoming signal.

A number of communication satellites, owned by both governments and private organizations, have been placed in a stationery orbits about 22300 miles above the earth’s surface. A communication satellite assures most of the long distance national and international communications. The main advantage of communication station visible from any where in a country. Thus transmission and reception can be between any two randomly chosen places.

A satellite has many transponders thus providing enormous communication capability at costs which favourably compete with microwave links on earth. The following figure illustrates a communication satellite link.

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