ceed one in 106 programme signals to terrestrial radio broadcasting stations and and one in 10³ respectively. the head ends of cable broadcasting networks. For a video signal the objective is for a weighted signal to noise 4. In recent years extensive use has developed, in particular in the ratio of not worse than 53dB, for 99% of any month, measured USA, of very low cost earth stations for data networks. These in a baseband 0.01MHz to 5.0MHz. earth stations use very small antennas, typically less than 2.5 metres in diameter; they are the so called Very Small Aperture 51.6.2 Thin route telecommunications Terminals (VSATs). Their use may become more widespread in the future. INTELSAT and EUTELSAT transponders which are not required for trunk telecommunications may be leased to telecommunications 51.6.3 Satellite communication for mobile operating organisations for national networks and for onward leas- stations ing to users for their private networks. A considerable number of other organisations have brought satellites into operation primarily An important advantage of satellite communication over all other to provide for national networks and private networks, and many media for long distance communication is its ability to operate with more satellites are currently in the planning stage or under construc- mobile terminals. Indeed, satellites provide the only feasible way of tion, their frequency assignments and orbital locations being co-or- providing reliable communication with ships and aircraft which are dinated with those of other satellites which are already in operation. far out of sight of land. The microwave frequencies which are used These national satellites are broadly similar to those of INTELSAT for communication between satellites and fixed earth stations are, and EUTELSAT. They use the same frequency bands, although however, unsuitable for use with mobile stations. Lower frequencies there is a growing tendency to concentrate on the frequency bands provide a better trade off between the satellite c