HOME > AGE 14 - 16 > MECHANICS > FORCES IN MOTION > COMMUNICATION SATELLITES

Communication satellites

Kepler's third law was confirmed by Newton's theory of gravitation and we can use it to help find the radius of the orbit for a communication satellite. These satellites are the ones used for relaying TV and radio communications round the world. The signal is transmitted from the ground, received by the satellite which then transmits it to the receiver on the ground. They are now a vital part of our lives. Almost all international communications are relayed by communication satellites.

These satellites 'hang' in the sky above one point on the Earth's surface. This means that ground based aerials do not have to be moved to pick up the transmissions from them.

The period of such a satellite is one day – it is called a synchronous, that means that the satellite moves with the same rotational speed as the Earth.

The orbit of a synchronous satellite will always be above the Earth's equator.

Example problem
Radius of Moon’s orbit = 3.8x10⁸ m (R_m), Period of Moon = 27.3 days (T_m) Period of satellite = 1 day (T_s)
Let the radius of the satellite’s orbit be R_s.
Using Kepler’s Third Law we have: R_s³/T_s² = R_m³/T_m²

Therefore: R_s = 4.2x10⁷ m = 42 000 km.
Remember that this is measured from the centre of the Earth.

A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS CD

Top of page