bining networks. In other respects the equipment of an the global network are required to have high performance, to ensure earth station accessing the INTELSAT global network is similar, in that bandwidth and satellite power can be used efficiently. Until principle, to that of a major radio relay station. recently the standard receiving Figure of Merit (G/T) was 40.7dB/K The INTELSAT system is by far the biggest provider of satellite at 4GHz and 39dB/K at 11GHz, requiring antennas with primary facilities for trunk telecommunications but the EUTELSAT system reflectors about 32 metres and 18 metres in diameter respectively. provides digital international links with Europe, closely resembling Many antennas now in service meet these requirements, but the the TDMA system operated by INTELSAT and using very similar minimum G/T requirements for new all purpose antenna has been satellite and earth station equipment (EUTELSAT, 1981). Similar reduced to 37dB/K (1GHz) and 35dB/K (11GHz). Much lower configurations also arise in some of the North American domestic performance is acceptable for some limited applications (INTEL- satellite networks. Recent satellite designs, like EUTELSAT II, tend SAT, 1977; Thompson and Buchsbaum, 1985). to have transponders with higher output power, and a single carrier The high gain antennas must use active satellite tracking tech- saturation rating of 50 watts if often found now. niques to avoid loss of performance due to satellite orbital perturba- For systems which are used in this way, the gain of transponders tions; typically earth station beam pointing errors are detected by and such earth station emission parameters as carrier power and (for observing the effect on the level of the telemetry beacon radiated by FM) carrier deviation are typically determined so that the CCIR and the satellite of small deliberate movements of the beam. CCITT channel noise and bit error ratio (BER) objectives for Satellite systems 51/17 international telephone, data and other channels will be met (CCIR, co