Satellite communications

Communications satellites are used to send and receive radio signals for television, phone or internet communications. Large concave reflectors called satellite dishes are normally used to send and receive these signals.

Satellite dishes send and receive signals

All signals for television, telephone or internet are converted into radio signals and then sent towards the satellite using a transmitting satellite dish. Most satellite dishes are designed to only receive satellite signals, but some are used to send signals to satellites as well.

A satellite dish works in the same way as the reflector for a torch or car headlights. A signal is produced or reflected from a focal point. This signal then reflects off the large part of the satellite dish and travels towards the satellite.

At the satellite, the signal reflects off another curved reflector towards a focal point where the signal is received. The signal is then amplified and sent back towards Earth using a different frequency.

The shape of a satellite dish is called a parabola. Home satellite dishes normally only contain a small section of the side part of the parabolic shape. The signal is reflected towards the receiver, which contains a small metal bar that acts as the antenna. For television reception, the signal is then amplified and sent to the television decoder.

Time delay

Radio waves are used to carry satellite signals. These waves travel at 300,000 km/s (the speed of light). This means that a signal sent to a satellite 38,000 km away takes 0.13 s to reach the satellite and another 0.13 s for the return signal to be received back on Earth.

If several satellite links are used, the delay for a signal to be received from the other side of the world is a quarter of a second greater for each satellite link used. This time delay can be seen when a television interviewer asks a question to a person on the other side of the world. There is often a short delay between the question asked and the answer given.

Satellites and orbits for television and communication

Only one satellite is needed to transmit television signals to a whole country or other large region of the Earth. The signals sent from a satellite can be shaped to match any coverage footprint required.

For example, the Optus D1 satellite was launched in 2006 to provide television signals for the whole of Australia and New Zealand.

The Optus D1 satellite is about the size of a car and is in a geostationary orbit above the equator at an altitude of 36,000 km and a longitude of 160°.

Signal noise and rain fade

The radio signal used to carry the coded information is called the carrier signal. As well as this, there is always some signal noise received. All objects produce this signal noise because of the vibration of their atoms. Warmer objects produce more signal noise. To receive a clear signal, the carrier signal needs to be significantly greater than the noise signal.

Rain fade is a lower-quality television signal that occurs when signals from a satellite pass through a region of rain. This happens for two reasons. Firstly, some of the signal is absorbed or reflected by the rain, so the strength of the carrier signal becomes weaker. Secondly, the rain itself produces more signal noise. As the signal strength decreases and the noise increases, a less clear signal is received, and picture quality can deteriorate.

A larger satellite dish can reduce the effects of rain fade.