ther resource which is in short supply, namely the frequency ments that are parties to the agreement that set it up and it is owned 51/16 Communication satellites and systems by the telecommunications undertakings that those governments Both time division and frequency division multiple access have designated for the purpose. The INTELSAT system exhibits methods are used in the INTELSAT global network for the trans- well the characteristics of a trunk telecommunications satellite mission of narrow band channels which are designed primarily for system. The satellites, all geostationary, are located in three groups, telephony but which are, of course, also used for other narrow band for the Atlantic, Pacific and Indian Ocean regions. Some of the telecommunications services. It is usual for the earth station to transponders are leased for domestic networks but the larger part of combine the telephone channels it transmits to many destinations the system provides facilities for an international network linking onto one or a few multiplexed carriers; conversely, a typical earth earth stations owned by national telecommunications operating station receives multiplex carriers from many other earth stations, organisations. It is characteristic of this global network that the total extracting from the multiplexes only those channels which are its required capacity is very large; that large numbers of national earth concern. stations need access to it; that most of those earth stations need to The INTELSAT TDMA systems operate at an information rate of be able to communicate directly with many of the other earth 120.832Mb/s, using 4-phase PSK modulation and coherent de- stations having access and that these country to country links vary modulation, with transponders of 72MHz bandwidth ((INTELSAT, greatly in scale, some requiring only one circuit but others requiring 1972). Digital speech interpolation (DSI) is used to increase the thousands of circuits (Hall and Moss, 1978; Alper and Pelton, capacity of the system for tele