m and assemble the signals in appropriate time division 'frequency re-use' by dual linear polarisation, providing up to 1GHz multiplex streams for more economical transmission back to Earth of usable bandwidth, some accessed by narrow band (36MHz) (Evans, 1986). transponders and some by wide band (72MHz) transponders. All transponders have redundant TWT amplifiers rated at 50 watts 51.4.4 Satellite antennas and footprints single carrier saturated output. Where the demand for satellite services is high it is economically advantageous to maximise the bandwidth that is available for sig- A circular beam 17.5° wide is just sufficient to cover the whole disc nals. Many satellites, like EUTELSAT II, obtain two fold frequency of the Earth visible from a geostationary satellite. Such a beam has re-use by dual polarisation. Many satellites are equipped to operate a beam edge gain of about 16dBi and it is generally provided by a in more than one pair of frequency bands, typically having some horn antenna. transponders operating at 6GHz and 4GHz and others operating at For most purposes, coverage of the whole visible Earth is not 14GHz and 11/12GHz. In satellites serving extensive geographical necessary, in which case it is also not desirable. Antennas with areas, the transponder capacity may be enhanced by frequency higher gain which, nevertheless, cover the required service area can re-use between non-overlapping spot beams. Thus, applying all of provide much greater transmission capacity for given transponder these frequency use techniques, the INTELSAT VI satellites have power than global coverage antennas. Furthermore, frequency co- 38 transponders which use the 6 and 4 GHz bands 8 times over, plus ordination of such satellites with others operating in the same 10 transponders which use the 14 and 11/12 GHz bands twice over. frequency band is made easier, and the possibility may arise for At the sub-unit level, major changes are taking place. Solid state frequency re-use within the satellite network if beams do not ove