Technical facts
A telecommunications satellite consists of a platform and a payload. The payload comprises a number of transponders (radio receivers and transmitters) which receive and re-transmit information. There are two types of satellite platforms, three-axis-stabilised and spin-stabilised. The satellites that are used for television and broadband services are positioned 36,000 kilometres above the equator. For comparison, the distance to the Moon is 370,000 kilometres.
Three-axis-stabilised satellite
At that altitude, the satellite orbits the Earth in 24 hours. This means that it is stationary relative to the surface of the Earth. This is why they are referred to as geostationary satellites. Satellites are launched from sites as close as possible to the equator, to take advantage of the rotation of the Earth. This saves fuel and extends the life of the satellite. The life of a geostationary satellite is 12-15 years. It depends primarily on the amount of fuel available for station-keeping.
The distance from the zero (Greenwich) meridian is used to describe the position of a satellite. The position is expressed in degrees. SIRIUS 3 and SIRIUS 4 are positioned 5 degrees east. This is the position allocated to Sweden by the ITU (International Telecommunication Union) for the use of frequencies for direct-broadcast television satellites.
Spin-stabilised satellite
Television programs, for example, are sent to a satellite via a teleport (uplink station). A signal is sent to the satellite, where it is amplified and broadcast to all subscribers.
A parabolic antenna or "dish" is needed to receive signals from a satellite. The size of the dish depends on the strength of the signal. If the signal is strong, only small dish is needed. The SIRIUS satellites transmit at high power, so small dishes are enough to receive the signals across Europe.
