Spacecraft High power satellites with sensitive onboard receivers are central to the concept of a mobile satellite service, as these requirements are a prerequisite for support ing services to small mobile terminals. Therefore the advent of mobile satellite conimunications had to wait until satellite and Earth station technology had developed to an extent that satellite transmitters could provide the requisite power and Earth stations had shrunk to a size supportable on mobiles. Over the past two decades, both components of technology have progressed to an extent that it is possible to communicate with hand held satellite telephones. In this chapter we will outline trends in satellite technology.
The main communication unit of a satellite (or payload) comprises an antenna system and a repeater (or transponder). When an intersatellite link (ISL) is used, the payload also supports an ISL payload. A payload requires support from a number of subsystems collectively known as bus, which comprise a structural subsystem for housing components, a thermal subsystem for temperature maintenance, a propulsion system for orbit raising and orbital maintenance and an electrical power subsystem. Spacecraft status monitoring, tracking and commanding are performed by the telemetry, tracking and command system the spacecraft's link to the ground control centre.
Keeping to the theme of the book, the majority of the chapter will address the payload, as it has a direct bearing on the capabilities and architecture of an MSS system. At present, transparent (or 'bent pipe') satellite transponders arc the norm, but regenerative transponders with onboard computers are expected gradually to supersede them because of several advantages offered by the latter, The chapter includes: a brief overview of transparent transponders but with the emphasis on regenerative transponders, as the technology of the former is well documented in several textbooks; a look at the influence of orbital altitude on spacecraft design; system aspects of inter satellite links; and a summary of the current state of technology. To begin with, some enabling spacecraft technologies are explored, followed by issues related specifically to MSS. Regenerative transponders have been addressed in greater detail because of their increasing importance to MSS needs.
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