A type of space-based radio communication based on the use of artificial earth satellites, usually specialised communications satellites, as repeaters. Satellite communications take place between earth stations, which can be either stationary or mobile.
A satellite communications and broadcasting system is not just a repeater in orbit, but also a number of ground stations. The principle of operation has not changed over the years – a signal is sent from one of the ground stations to a satellite, from where it is retransmitted to other objects within the coverage area. A satellite repeater can be either passive or active. In the first case, there is no signal correction, everything is ‘hoped’ to be within the wide coverage area of the repeater. This was the case with the American “Echo-1” system mentioned above.
Modern satellite communications systems use active repeaters that not only receive the signal from the ground station, but also amplify it. The main elements of communications satellites are therefore radio-wave power amplifiers, which make it possible to increase the distance over which radio signals are transmitted. The most effective of these are known as travelling wave lamps (TWLs) – electro-vacuum devices capable of amplifying power by hundreds of thousands of times. TWLs are also used in the ground equipment of satellite communications systems.
