arrier power re- which are allocated exclusively to the fixed satellite service (see quired and the cost of mobile earth station antennas. As shown in Table 51.1) in order to avoid constraints arising from sharing spec- Table 51.2, frequency bands have been allocated for satellite mobile trum with terrestrial radio systems. Their transponders are typically services around 1.6GHz and, in some parts of the world, at 800MHz. 36MHz in bandwidth and with a single carrier saturated power However, the 1.6GHz bands are narrow and the 800MHz bands are rating between 6 and 20 watts. They differ mainly in their use of crowded by terrestrial services. antennas with higher gain and footprints which are limited, more or The first commercial satellite communication service for ships less, to national boundaries. was provided in 1976 by the MARISAT satellites, owned by Comsat With few exceptions the earth stations which use these satellite Corporation (Lipke et al., 1987). These satellites covered most facilities are simple and relatively low in cost. Some have antennas significant sea areas, although with some gaps in arctic and antarctic of substantial size, up to 10 metres in diameter and requiring active waters. In 1982 the International Maritime Satellite Organisation, a satellite tracking but most antennas are less than 5 metres in global consortium similar to INTELSAT, took over and expanded diameter, making them less environmentally objectionable, and this service. The INMARSAT system initially used the MARISAT their relatively broad beams do not need to track actively a satellite satellites leased from COMSAT, some transponders which had been which is in an accurately maintained geostationary orbit. added for the purpose to three INTELSAT V satellites and two These points of similarity apart, these earth stations and the MARECS satellites on loan from the European Space Agency. All facilities that they provide show considerable variety. The following of these satellites operated their ship satellite links at 1.6GHz and 