“Satellite communications” means “communications through the use of satellites”. The process of sending messages like radiotelephone, radio broadcasts, television broadcasts, Telex and Fax with the help of satellites stationed in outer space is called satellite communication. Satellite communication is mainly done through geostationary satellites whose period of revolution is equal to the period of rotation of earth, that is, 24 hours. The communication satellites have some devices called “transponders” which can receive signals from an earth station and transmit them again in different directions. The audio (sound) or video (picture) signals transmitted by an earth station like a TV. station in the form of electromagnetic waves are received by the transponder of a communication satellite stationed in outer space.
The transponders of the satellite retransmit these signals received from the earth station in different directions either at the same frequency or a different frequency. These signals sent by the satellite are then received by a large number of ground stations located in various regions of the earth. The signals received from the satellite are very weak, therefore, the ground stations amplify these signals before transmitting them again. It is these signals which are received by the TV. sets located far away from the programme transmitting station or TV. station.
The other types of telecommunications like long distance telephone calls; Telex (printed messages) and FAX (photos of printed matter or pictures) also use a similar mechanism in sending messages through communication satellites. And for intercontinental telecommunications, usually two or more satellites are linked together.
The natural satellite of earth called moon cannot be used for communication purposes because of the following reasons :
(i) The distance between earth and moon is very, very large,
(ii) The time of revolution of moon around the earth is not equal to the time of rotation of earth on its axis (which is 24 hours).
It should be noted that the nature of electromagnetic waves carrying audio-visual messages is similar to those of light waves. But the frequency of electromagnetic waves carrying audio-visual messages is much lower than those of light waves.