Telephone someone on the other side of the world and you will often notice little delays – less than a second long – in the conversation. It’s not surprising. Whatever you say is being sent far off out into space, bouncing off a communication satellite before returning to Earth to reach its destination.
Many such satellites now orbit the Earth, relaying countless TV signals, telephone calls and streams of computer data from one part of the globe to another. Satellites also play a major role in navigation. Gone are the days when the stars and a compass were all you needed to cross the ocean. The increase in air traffic over the last few years means that it is absolutely vital to know the whereabouts of any aeroplane at any time. Today, satellite systems are in use that are so accurate they allow the user to fix any point on or near Earth to an accuracy of ten meters.
Most communication satellites have to remain in an exact spot over the Earth’s surface so that receivers and transmitters on the ground can remain permanently pointed at them. They can only do this by moving in an orbit above the equator, which is 35,800 km high – about one-tenth of the distance to the Moon. This is called ‘geostationary’ orbit, and once a satellite is in it, it takes exactly the same time to complete one orbit as the Earth does to spin once around on it axis. This means that it seems to hang forever above the same point on the surface.
There are three main parts to a satellite communication system: the transmitter, the receiver and the satellite itself. Information is beamed up to the satellite by large dish-shaped transmitting aerials known as ground stations. The message travels into space in the form of microwaves – short radio waves that can pass unhindered through the atmosphere. When a small dish antenna of its own, the satellite gathers up the incoming signals. Then, almost instantly, it boosts the signals and transmits then back to a different part of the Earth. By the time the satellite transmission has arrived back at the ground it is no longer in a narrow beam. Instead, it has fanned out over a wide area called the satellite’s ‘footprint’. Any suitable receiver within this footprint can then pick up the satellite’s signal.
The receiving aerial also has to be dish-shaped to increase the strength of the weak signals coming from space. The microwaves coming down from the satellite strike the inside of the dish and are reflected inwards so that they all meet at the focus point, just above the centre of the dish.
Until a few years ago, the receiving dish needed to be huge. But because satellites can now amplify signals so much, modern receiving dishes are small enough to fix to the side of a house. These umbrella – sized aerials are now a familiar sight, receiving dozens of TV channels from satellites such as Astra. Increasingly, thought, cables are being used to bring the satellite signals from a central receiving dish to many houses in an area.
How it works
Seen from any point on the Earth today there are always several communication satellites above the horizon. Each satellite beams down many different signals at different frequencies. If you want to pick up a certain TV broadcast, your receiving aerial has to be pointed precisely at the right satellite, and be tuned into the correct frequency. The aerial then converts the incoming microwaves to electrical signals the are, in tune, fed to an ordinary TV set via a special converter