Satellite Communication

The modulated carrier waves are beamed by a transmitter directly towards the satellite. The satellite receiver amplifies the received signal and re-transmits it to earth at a different frequency to avoid interference.

These stages are called up-linking and down-linking.

The capacity of a communication channel can be increased by increasing the frequency of communication. Ionosphere does not reflect waves of frequencies above 10 MHz, and for such high frequencies space wave propagation is used with direct transmission from tall towers.

But this line-of-sight transmission also has a limited range or reach. Hence for long-range wireless communication with frequencies above 30 MHz, such as for TV transmission in the range of 50-1000 MHz, communication through a satellite is used.

The gravitational force between the earth and the satellite serves as the centripetal force needed to make the satellite circle the earth in a free fall motion at a height of about 36,000 km. An orbit in which the time of one revolution about the equator exactly matches the earth’s rotation time of one day is called a geostationary orbit, i.e., the satellite appears to be stationary relative to the earth.

Ground stations transmit to orbiting satellites that amplify the signal and re-transmit it back to the earth. If the satellites were not in geostationary orbits, their motion across the sky would have required us to adjust receiver antenna continually.

Two other orbits are also used for communication satellites:

  • (i) polar circular orbit at a height of about 1000 km almost passing over the poles (i.e., with an inclination of 90°).
  • (ii) highly elliptical inclined orbit (with an inclination of 63°) for communications in regions of high altitudes.