In a relatively short time, satellites have become an essential part of global communication. In 1960, the first TV satellite, named Echo, was launched. It was basically not much more than a reflector, which reflected the TV signals it received from earth. Two years later Telstar followed, which was the first so-called active TV satellite. Instead of only reflecting the incoming signals, it also converted the signals in order to avoid interference between the incoming and outgoing signals.

Telstar had a rotational speed which was different from the rotational velocity of the earth, so it had to be followed very accurately by both transmission and reception stations. In 1964, this problem was solved, when the first earth-synchronous satellite, Syncom, was launched. Many others have followed since. The most well known is probably Intelsat I, which was launched in 1965. By 1969 the satellite net had expanded to a worldwide communication and TV network.

In December 1982, the Astra I satellite was launched, which generated new interest in satellites from the general public in Europe. With its coming it has become possible for people in Europe to receive TV and radio transmissions with a small dish antenna.

Communication Satellites

All current communication satellites are earth-synchronous or geo-stationary. This means they circle the earth in a specified orbit, at the same speed as the earth itself. As a result, they appear to stand still. All geo-stationary satellites revolve around the earth at a height of 36, 000 km, precisely over the equator. Here, the centrifugal and gravitational forces of the earth are in equilibrium, ensuring that the satellites stay in their position and do not fall back to earth. Their speed is approximately 11, 000 km per hour and the distance to Central Europe is approximately 41, 000 km. As neither the distance nor the position over the equator changes, transmission and receiving stations can remain fixed, maintaining their aim at the satellite. The geo-stationary orbit where the satellites are in is also called the Clarke Belt, named after Arthur C. Clarke. He was a British writer and scientist who first proposed the idea of the geo-stationary orbit used by today's satellites.

The Clarke Belt used by geo-stationary satellites.

Non-geo-stationary satellites are used for applications such as weather observations, military surveillance and experiments. Most of them orbit the earth at a lower altitude than the geo-stationary satellites. Their orbital speed must therefore be faster, or else the earth's gravity would pull them down.

Fixed Service Satellites

Fixed Service Satellites (FSS) are satellites designed to transport telephone calls, data transmission and TV signals for broadcasting and cable organizations. Because these satellites have a relatively low power output of 10-20 watts per transmitted channel, it means that a large dish antenna is required for good reception. (Less power means a weaker signal which is harder to pick up, therefore requiring a larger antenna.) However, the advantage of low power satellites is that more programs can be broadcast.

Consumer Satellites - DBS and MPS

A DBS, or Direct Broadcasting Satellite, is a satellite with high transmission powers, especially designed to transmit radio and TV programs. Because of its high power (up to ten times the power of a FSS satellite), its signals can be received with smaller dish antennae of 25-40 cm in central receiving areas.

Another kind of satellite is the Medium Powered Satellite (MPS), which is a satellite with a transmission power of 50 watts. The advantage of this type of satellite is that it has more power than a FSS and its signals can therefore be received much easier. Although it has less power than a DBS, its advantage over a DBS is that it allows the satellite to broadcast more programs. The ASTRA satellite is an example of a MPS. MPS and DBS satellites are also referred to as consumer satellites.

Satellite Positions

All the consumer satellites are located in the same geo-stationary orbit 36, 000 km above the equator. Their positions vary from east to west in accordance with international agreements. These agreements about orbital positions allow several satellites to be placed in the same location, so that TV viewers can receive a greater choice of programs with a fixed dish antenna. Also, when a satellite needs to be replaced (the average lifetime of a satellite is about 15 years) the replacement satellite can be put in the same position, so that when the first one 'dies' and falls back to earth, the next one is already in place and continues to broadcast the same stations.