 pseudo-random in satellite communication has been greatly influenced by the cost sequence, to be subtracted at the receiving earth station, to disperse of carrier power reaching the receiving antenna. This cost is tending these lines if the down-link frequency allocation is shared with to fall, in particular as satellites with higher antenna gain come into terrestrial radio services. When the G/T of receiving earth stations use, but another limitation on down-link power levels is likely to is very small, as it may be with some very small aperture terminal remain as long as most spectrum allocated for space services is (VSAT) networks, it may be preferable to disperse the spectral shared with terrestrial radio services which have equal allocation energy of the carrier by using frequency hopping spread spectrum status. Despite the low receiver noise levels obtainable with even modulation. low cost earth station receivers, modulation techniques must be suitable for operation at relatively low carrier to noise ratios. Am- 51.4.6 Multiple access methods plitude modulation is never used for analogue signals and the use of high order phase shift and hybrid modulation for digital signals is Communication satellites are designed to relay several, or more rare. However, modulation techniques which tolerate lower pre-de- usually many, signals simultaneously. In some cases there may be a modulator carrier to noise ratios (C/N) tend to need wider band- separate transponder for each carrier; this is typical of broadcasting width for a given information capacity. Thus the modulation satellites and of satellites used for distributing television signals to parameters should be optimised for each situation, to ensure that the terrestrial broadcasting stations. More usually, each transponder best use is made of transponder capacity. will relay, not one carrier, but several or many. This is called 51/14 Communication satellites and systems 'multiple access'. There are three basic techniques for achieving FDMA may be used for groups