19.1 Introduction

Those concerned with maritime and aeronautical communications have, since the first experiments in satellite communications, realised the potential benefits which satellite communications can bring to their respective communities. Clearly, the prospect of near global, highly reliable communications made possible by satellite technology is very attractive to mariners and aviators alike. The arrival of cellular radio services has awakened interest in land mobile satellite services. The exploitation of this technology to satisfy these applications has largely taken place during the last two decades.

These developments have been enabled by the advances in spacecraft and communications technology which have allowed systems to be developed that permit the use of acceptably small user antennas, while providing sufficient system capacity to remain economically viable. Inevitably, these constraints have meant that, in the early 1990s, user voice communications capacity has been limited to, typically, one voice channel when user antenna gains of between 12 and 22 dBi are used. Similarly, data communication rates of approximately one kilobit per second when user antenna gains of about 1 dBi are employed. To facilitate the use of economic satellite RF powers, of the order of 1 W per voice channel, low-rate vocoder and forward-error-correction coding techniques are now extensively employed. Furthermore, to make the best use of the available satellite power, capacity is always demand assigned in some way. As is implied by the preceding sentences, most systems currently in use or under development are power limited. However, with the meagre frequency allocations...