can perhaps be taken to cover the majority of cases: their feeder links with earth stations ashore at 6GHz and 4GHz. Their transponders had bandwidths of a few Megahertz and the 1. In a number of large countries where high quality terrestrial saturated output power of their transmitters was rated at a few tens communication facilities do not link all centres of population, of watts. All satellite antennas had footprints which covered the satellite networks have been set up to provide trunk connections whole earth visible from the satellite. between provincial centres. Elsewhere, where the population is The standard INMARSAT ship earth station antenna in the receive sparsely distributed over inaccessible terrain, amongst moun- mode has a Figure of Merit (G/T) of -4dB/K, typically achieved by tains or across archipelagos, satellites may provide reliable an antenna gain of 23dBi from a 1.2 metre dish and a receiver with communications to isolated communities. The trunk network a system noise temperature of 500K. Such an antenna has a -3dB 51/18 Communication satellites and systems beamwidth of about 10° and a simple beam stabilising and control These plans have been ratified by national governments, provid- system is sufficient to compensate for the movement of the ship and ing strong protection against cross frontier interference. Several slow changes in the direction of the satellite as the ship changes its countries had experimented with satellite broadcasting at 12 GHz location. Single channels of narrow bandwidth analogue telephony before the plans were finalised (Siocos, 1978; Roscoe, 1980; Ishida signals frequency modulate voice switched carriers, and the trans- et al., 1979). Others have launched satellites more recently to make ponder power per channel requirement is further reduced by com- use of the planned assignments, including TV-SAT, TDF-1 and panding. A low speed TDMA system provides many channels of Marco Polo for France, Germany and the United Kingdom, respec- telex, for use with all equipped ships