An Introduction to Satellite Communications
The aim of this book is to give a clear and concise exposition of the principles and practice of satellite communications by describing the development of communications-satellite services. It will be useful both to engineers who have worked in other fields of telecommunication and to students.
Inspec keywords: land mobile radio; digital communication; video signals; data communication; quality of service; voice communication; marine communication; artificial satellites; satellite communication
Other keywords: digital data transmission; land mobile communication; data quality; very-small aperture terminals; aeronautical communication; digital speech transmission; satellite communication; geostationary orbit; launch vehicle; speech quality; earth station; maritime communication; analogue television transmission; communications-satellite services; television signal quality; VSAT
Subjects: Radio and television broadcasting; Mobile radio systems; Television and video equipment, systems and applications; Acoustic and other telecommunication systems and equipment; Satellite communication systems; Telecommunication applications
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.
A satellite is a body which moves in an orbit around another body (of greater mass) under the influence of the gravitational force between them. The force (in newtons) required to keep a satellite in a circular orbit is given by: mω2r. The basic principle of any launcher is that of the rocket; i.e. the vehicle is propelled by reaction to the momentum of hot gas ejected through exhaust nozzles. Engines use the oxygen in the atmosphere to burn their fuel but the rocket motors used for launchers must work in space where there is no atmospheric oxygen. The propellant for a launcher must therefore comprise both a fuel and an oxidising agent. Propellants may be either solid or liquid. Solid propellants are a more or less homogenous mixture of fuel and oxidiser whereas the fuel and oxidiser comprising a liquid propellant are carried in separate tanks on the launch vehicle.
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 frequency modulation (FM). FM was used for all the early satellite communications transmissions, firstly because it made it possible to achieve acceptable performance with the very limited power available from the satellites of the 1960s and secondly because much of the technology required had already been developed for terrestrial radio-relay systems.
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.
Radio was first used for commercial maritime communications in 1899 when a message (in Morse code) was transmitted by a Marconi station on the Isle of Wight (UK) to the US liner St. Paul. This was only two years after Marconi first demonstrated radio communication to the British General Post Office. During the first decade of the twentieth century the use of radio telegraphy to communicate with ships became well established. The value of radio in emergencies at sea was quickly recognised and, in 1906, the signal SOS was adopted as the international distress signal. Two-way voice communication between a ship and a shore station was first established in 1922 and 'Mayday' was adopted as the distress call for use with radiotelephony in 1927. However, radiotelephony using the high-frequency (HF) band has serious disadvantages; the vagaries of propagation via the ionosphere and the limited bandwidth available can result in serious congestion, and delays of hours or even days in establishing voice communication with a ship are by no means uncommon when using the HF service. HF (Morse) telegraphy transmissions require only very narrow bandwidth and are much less liable to meet with congestion, but telegraphy has the disadvantage of slow speed of information transfer. For ships within 30 to 40 miles of the coast, a reliable service of good quality can be provided by the use of VHF radio, but there was no really satisfactory means of communicating with ships over long distances until the advent of the first maritime communication-satellite system (MARISAT) in 1976.
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.
