ium, may therefore inadvertently transmit overlapping bursts, causing the costs of which are little affected by distance, may be lower both messages to be mutilated. If this happens, they are both retrans- than those of terrestrial media. mitted automatically. 2. Thin-route telecommunications. The economies of scale which CDMA systems do not structure their use of transponders either make terrestrial transmission media SO competitive for high in frequency or time. Earth stations transmit spread spectrum sig- capacity, short and medium distance links between pairs of nals which can be identified, after re-transmission by the satellite, fixed points do not necessarily apply in some other circum- by the coding which the signal elements carry. stances. Public telecommunications facilities may be provided These various multiple access systems differ in the effectiveness economically by satellite over relatively short distances when with which they use the facilities provided by a transponder. Figure the level of demand is low, especially where the intervening 51.12 provides a measure of the capacity of a transponder having a terrain is underdeveloped and rugged. Private telecommunica- bandwidth of 36MHz, using various multiple access and modula- tions networks covering extensive geographical areas and re- quiring flexibility or non-standard facilities (such as high speed SCPC/CFM data and point to multipoint communication) are another area of application which is already important in some countries. TDMA 8-phase PSK Communication with mobile stations. There is no terrestrial SCPC/FM FM/FDMA (8 carriers) transmission medium for communication with ships and air- TDMA 4-phase PSK craft when far out of sight of land, which can be regarded nowadays as satisfactory. Satellite communication for ships is TDMA 2-phase PSK well established as the medium of choice. The use of satellites for communication with air liners on ocean crossings has begun, and it is likely to be extended soon to air traffic control and other aeronautic