al communication services. Finally, while both cost and limitations of radio spectrum availability seem certain to ensure that most communication with land mobile stations, such as car and hand portable cellular radiotelephones, will be served by terrestrial radio systems, it is likely that satellite communication will extend the availability of such Figure 51.12 Telephone channel capacity in 36 MHz facilities into areas which would not be economically attractive channel for the establishment of terrestrial systems. 4. Broadcasting. Three transmission media, namely terrestrial tion techniques, as a function of the C/N ratio at the earth stations. radio, cable and satellite, are competing for domestic, standard Methods for calculating transponder performance are given in Hills definition, television broadcasting at present and each seems to and Evans, 1973, and in Bargellini, 1972. be identifying markets where it can dominate. Studies are well advanced on two additional broadcasting services for the satel- lite medium, namely high definition television and high quality Applications sound broadcasting suitable for reception on motor cars. Satellites can be used for virtually any kind of telecommunication These four areas of application are considered further in the next section. system. They provide a very flexible transmission medium; once a satellite is available in orbit, many kinds of earth station, for many kinds of use, can be put in place and set into operation very quickly. Satellite networks also offer high reliability. However, the costs of Satellite systems satellite systems fall quite differently when compared with the costs of terrestrial radio and cable systems where these are capable of 51.6.1 Trunk telecommunications providing equivalent facilities. In some circumstances there are major differences between the The International Telecommunication Satellite Organisation (IN- demands which satellite and terrestrial radio systems make on TELSAT) is controlled in matters of policy by the many govern- ano