You must be familiar with the names of satellites like INSAT 3B or Kalpana Chawla. INSAT 3Bis an example of man-made or artificial satellite. They too revolve around the Sun like the natural satellites but are closer to the earth than the moon. The launch vehicles developed by the scientists carry the satellites into the space and launch them as artificial satellites. Only six countries in the world have the technology for developing artificial satellites and India is one of them.
Artificial satellites help in long distance transmission of television programmes, communication through telephones and internet. They also help in research, defence and remote sensing. It is also used to collect information about weather, agriculture, land and ocean features including movement of fish in oceans. The speed of artificial satellites to be used for long distance communication is so adjusted, that they complete one revolution around the earth in 24 hours. The picture and sound to be transmitted is first converted into electrical signals with a video camera. These are then converted into a special type of waves and transmitted in air from a transmitting antenna so as to reach the artificial satellites. They have special instruments installed in them, which receive the signals transmitted by the earth station.
The signals received are then amplified and retransmitted by the instruments fitted on the satellite. The satellite being at a great height can transmit signals over a wide range of area on the earth. The antennas fixed in a number of stations on the earth including those of cable operators receive the signals transmitted from the satellite and retransmit them. The television sets receive the signals and convert them in the form of images and sound.
Satellites that are being put to the maximum use are the geostationary satellites. A geostationary satellite appears to remain fixed with respect to a particular point on the earth. For such satellites, the time period of their revolution, around the earth, equals the time period of rotation of the earth around its own axis (i.e. nearly 24 hours). It is these satellites that have made global audio-visual communication a reality. A satellite receives a microwave signal from a ground station on the earth (the uplink). Its amplifiers first amplify it and its transmitters retransmit the (amplified) signal back to a receiving station on earth at a different frequency (the down link).