Twelve years before the launch of the first artificial satellite of any kind, Arthur C. Clarke foresaw the coming of communication satellites and the use of the synchronous equatorial (or geostationary) orbit. It was 20 years before INTELSAT (the International Telecommunications Satellite Organisation) used a geostationary satellite to start the first commercial communications-satellite service in 1965 and another four years before it established the world service using three geostationary satellites which Clarke had envisaged. Today, in 1988, the INTELSAT system has grown to a vast enterprise, INMARSAT (the International Maritime Satellite Organisation) has transformed communication at sea and is about to start an aeronautical service, many regional and national systems have been established, satellites relay television to millions of people via cable-TV head-ends and have started broadcasting direct to homes, and parts of the geostationary orbit have become uncomfortably crowded with satellites.

The signal-to-noise power ratio in a baseband channel of a radio transmission depends mainly on the coding and modulation methods used, and the radio-frequency (RF) carrier-to-noise power ratio at the input to the receiver (i.e. before demodulation). In this chapter, factors affecting the RF carrier-to-noise power ratio are considered. The RF carrier-to-noise power ratio (C/N) at the receiving end of a radio link depends on: (a) the power delivered to the transmitting antenna; (b) the gains of the transmitting and receiving antennas; (c) the propagation loss between the antennas; and (d) effective noise temperature of the receiving system. The RF signal may also be degraded by interference.

This chapter discusses digital satellite communications. The first digital transmission method to be used commercially was pulse code modulation (PCM); this is still the most common method of encoding voice signals and it is the progenitor of most of the more complex coding methods now in use. Nevertheless, satellite communications systems are turning to digital methods because: (i) digital methods make it possible to use satellites in new and more efficient ways, such as time-division multiple access (TDMA) and will, in the future, make it possible to carry out complex processing of signals on board satellites; (ii) digital systems can be used for the efficient transmission of both voice and data signals; (iii) digital signals can easily be encoded so that they cannot be used by unauthorised persons; (iv) digital satellite systems can be designed to be part of national and international integrated service digital networks.

The term 'Earth station' is used to mean both an antenna with its associated equipment and a site housing a number of antennas. Thus, there are many antennas at the British Telecom International (BTI) Earth Station at Goonhilly, and each of these antennas with its associated equipment is an Earth station in its own right; for example, one of the antennas at Goonhilly is part of the BTI Coast Earth Station (CES) working with the INMARSAT maritime communications-satellite system. Where there is any danger of confusion, an antenna with its associated equipment is (in this book) called a 'terminal', and the term 'Earth station' is used for a terminal (or several co-located terminals) together with any ancillary facilities.