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A communications satellite is essentially a communications relay high in the sky. A satellite works by receiving radio signals sent from the Earth and resending the radio signals back down to the Earth. In a simple system, a signal is reflected, or "bounced," off the satellite. For example, it is possible to bounce a signal off the surface of the Moon back down to Earth. Because the Moon is very far away, for this to work the signal from the Earth must be very strong and the receiver receiving the signal must be sensitive enough to detect the very weak signal receive back from the Moon.

Satellite uses solar energy collected from large arrays of solar cells, which supply the satellite with all the electrical power it needs to function and transmit to earth. In theory the satellite could therefore function forever however a small amount of fuel is used to maintain it in the right orbit (at the right position in space), which allows it to function for a given period of time, usually around 15 years for geostationary satellites.

Today most of the commercial communications satellite are flying at 36,000km (so-called ‘geostationary orbit’) above the Earth’s surface. This means the satellite appears fixed in the sky to an observer on Earth. This allows signals to be transmitted over distances that are very much greater than the eye can see.

How does it work?

People communicate to a satellite using an antenna on the ground, which called an "earth station" in technical terms. The earth station sends up radio signals to the satellite. These signals are called "uplinks". The satellite receives these signals, makes them stronger, and then re-transmits them back down to the Earth. These signals back to the Earth are called "downlinks". Sometimes the uplink and downlink earth stations perform various specialized functions. For example, some uplink stations deliver - or "feed" - video or audio programming to the satellite, which is then retransmitted to users all over world. These links are called "feederlinks". Other uplink stations are used to control the satellite. Such uplinks are called "control" links. Downlink stations can used to allow the satellite to connect with the telephone network or the Internet. These stations are often called "hub" stations or "gateway" stations. Other earth stations receive information from the satellite on how it is performing and what it is doing. This information is called "telemetry." Users also directly send information up to satellites and receive information directly from the satellite. The links that connect users to the satellite are called "service links."

The area that can be served by a satellite is determined by the "footprint" of the antennas on the satellite. The "footprint" of a satellite is the area of the Earth that is covered by a satellite's signal.

Did you know?

It takes from 2 to even 4 years to manufacture one communications satellite with 100% upfront investment by satellite operator.

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