[S104]Satellite Uplink And Downlink

Satellites are small object orbiting around bigger objects. The moon is actually a satellite orbiting around earth. The moon is a natural satellite but today when we talk about satellites we mean human made machines which orbit around earth for a variety of purposes.

Satellite services - What are satellites used for?

There is a huge variety of satellite services. Satellites can be used for military purposes, weather forecast, navigation (GPS), communication (data transmission), internet and much more. One of the most frequent uses of satellites is to transmit TV channels around the world.

3 kinds of Satellites

Satellites orbit around the earth. There are 3 kinds of satellites classified by the orbit distance from earth.

Low earth orbit (LEO) - this satellite orbits earth at a distance of 160-480 km. because it is so close to earth it needs to orbit at a fast speed in order to prevent from the earth's gravity to pull it down. The LEO satellites move at a speed approx. 28,000 km per hour and can finish one orbit around the earth in about an hour and a half.

Medium earth orbit (MEO) - this satellite orbits earth at a distance of 9,000-18,000 km. these are mainly communication satellites above the north/south poles. Their orbit patter is oval. Because these satellites are further than earth from the first group there is a need for tracking systems down here on earth in order to keep track of these satellites.

Geostationary earth Orbit (GEO) - this satellite orbits earth at a distance of 35,60 km from earth and orbits earth at the same speed our planet revolves around itself. This means that the satellite is stationary above the same position all the time. Usually there is a need for 3 different GEO satellites to cover the whole earth (not taking into account the polls). These satellites are used mainly for constant communication or weather satellite services.

How do satellites transmit TV channels?

Television programs origin from a broadcasting studio, it could be a live broadcast or a taped TV program. The program is translated into a signal. The signal is sent up to the satellite with a process called - uplink. When it is received at the satellite, the signal is transmitted to another satellite using a process called turnaround, or it is transmitted back down to earth using a process called downlink. When a signal is down linked back to earth it is received by a satellite dish which we all know.

Because of the whole process of up linking and down linking, there is a transmission delay of a few seconds. This is why viewers of a cable channel will see it a few seconds before the viewers of a satellite channel.