The development of satellite communications has been an exciting one with many challenges. Technical problems had to be solved, the science of operation of equipment in outer space understood and ways found to meet the unending demand for services. The visionaries, pioneers and all those involved in this almost unparalleled evolution are to be congratulated. Historically it is evident that the developments that have led to the world as we know it today have been driven very significantly by the role of enhanced, large-volume global communi cations of all kinds (a subject which would warrant further material in its own right). Considering the British Interplanetary Society motto From imagination to reality, it is the authors' opinion that this is most apt regarding satellite communi cations and should not to be forgotten for the future, which looks as equally exciting, if not more so, as the past!

The operation of any radio system may cause harmful interference to another. Consequently, there is a need to have some form of management of the spectrum and orbit resources in order to ensure appropriate quality of service for existing operations, at the same time permitting access to these resources for newcomers to the field. The International Telecommunications Union (ITU) was estab lished to facilitate this by means of a set of procedures detailed in the radio regulations (RR). We examine here the obligations of those wishing to establish satellite communications systems under such regulations. This is an extensive and continually evolving subject and thus it is only possible to outline the key principles involved. Further specific details can be obtained from the ITU and its radio regulations.

A number of factors resulting from changes in the atmosphere have to be taken into account when designing a satellite communications system in order to avoid impairment of the wanted signal. Generally, a margin in the required carrier-to noise ratio is incorporated to accommodate such effects.

A satellite communication system will have a number of users operating via a common satellite transponder, and this calls for sharing of the resources of power, bandwidth and time. Here we describe these techniques and examine their implications, with emphasis on principles rather than detailed structure or parameters of particular networks, which tend to be very system specific. The term used for such sharing and management of a number of different channels is multiple access. Each resource is limited, and ultimately relates to cost or revenue; their efficient use is important, as is meeting the needs of the users' traffic demands. There are four fundamental techniques of multiple access, frequency-division multiple access (FDMA), time-division multiple access (TDMA), code-division multiple access (CDMA), packet (or random) access. Each technique has its advantages and disadvantages, and in practice hybrid schemes are likely to be employed, having features of each. It should also be appreciated that a satellite transponder may be serving more than one network simultaneously, together with a variety of modulation schemes, signal powers and characteristics. An outline description of spread spectrum is also included here as a modulation technique allowing multiple access capability in the form of CDMA.

Channel coding is a signal-processing technique which makes the representation of information bits interdependent and introduces redundancy into the sequences so that noise averaging and error protection can be achieved. It originates from the development of information theory by Shannon ' in the late 1940s which showed that any channel of known characteristics has a calculable capacity for information transfer. Provided that capacity is not being exceeded, it should be possible to achieve reliable communications with an error rate which can be reduced to any desired level by increasing the length of the codes used. The passage of 50 years since Shannon's discoveries has not yielded practicable coding schemes which allow theoretical channel capacity and error rates to be achieved. There are certain recently discovered codes which appear to provide the closest approach to the Shannon performance, but mainstream practice is content to adopt more modest aims. These could include the reduction of error rates to be suitable for a particular application or to allow the extension of operations to difficult areas. A common objective would be to reduce the power requirements for acceptable error rates, since high power brings with it several expensive problems including effects induced by nonlinear power devices. In this cost-conscious age, it is unlikely that cost-effective solutions to digital communications can be achieved without coding.

This Chapter has identified the major subsystems of a satellite communications earth station, has described some of the factors leading to the selection of particular items of equipment, discussed siting matters and interference problems, and looked at testing, acceptance and ongoing maintenance aspects. Although, out of necessity, the approach has been brief, it is hoped that this Chapter has given the reader an insight into some of the likely design challenges associated with earth-station engineering.

A communication satellite payload is the system on board the satellite which provides the link for the communications signal path. Traditionally, this link was between two ground stations but present-day payloads now provide not only for this link but also can provide interconnectivity for a large number of mobile users directly to each other or via the ground stations. Payloads are providing this extra functionality to meet the needs of more mobile populations, rapidly changing traffic demands and variable operational scenarios. To do so they have become more complex and more powerful, which has become possible through advances in technology, reducing the mass and power consumption of the electronics equipment for a given level of functionality or performance.

The majority of this book concentrates on satellite communication systems from a satellite communication engineering perspective. This Chapter addresses the issue from an overall network perspective in order to clarify what the network (and its services) expects from satellites and the implications which satellites have for network design. The Chapter will be primarily concerned with main network services (such as telephony) where satellites have to compete/integrate with terrestrial facil ities, rather than independent/specialised networks (such as broadcast television using satellites) where the characteristics of satellites are built into the service offering. Analogue, PDH and SDH transmission, leased and switched analogue and ISDN services, and ATM transport will be considered. Certain network-specific equipment is also covered (for example, DCME).

This Chapter reviews low-bit-rate digital transmission of television signals on satellites and placed it in a context of existing and developing television standards and a changing regulatory environment. Some of the systems aspects have been discussed and particular details of practical satellite systems already available commercially have been given. For completeness some brief discussion of digital audio broadcasting has been included.

New challenges to mobile satellite communications have really come from the terrestrial cellular industry where small terminals have found very widespread use. The growth in the deployment of cellular telephony systems in the 1980s and 1990s has been phenomenal and this has led to massive reductions in the cost of terminals and call charges together with the offering of a wide range of services. With cellular telephony, a ground-based technology, it became apparent that industry was willing to deploy these systems only where there were sufficient numbers of subscribers, i.e. the densely populated areas - urban and suburban only. This would therefore leave out large geographical areas where the offering of cellular telephony would not be economical owing to coverage and mobility issues. This is the niche market which mobile satellite communications has come to fill. Therefore, mobile satellite communication systems tend to complement cellular systems by providing services in difficult areas. It is also possible that satellite communications may even eventually compete with cellular systems if the costs can be reduced significantly - at this stage a daunting task. The trend for the next-generation mobile systems - those referred to as the third-generation systems-is to provide seamless communication services to users wherever they are with a single multimode terminal and a single user number. This is the domain of personal communication services/ networks (PCS/PCN) and satellite PCN for the systems being planned will be an inherent component of global PCN.

VSATs, very small aperture terminals, are small earth stations capable of receiving from and sometimes transmitting to satellites. They represent an important addition to the telecommunications world because they can provide a service directly to the user at virtually any geographic location covered by a suitable satellite beam. They do not require any support from a local terrestrial communications network and can even be run from portable or alternative power supplies.

The University of Surrey embarked upon the design of its first experimental microsatcllite in 1978 and UoSAT-1 was launched by NASA in 1981-since then a further 13 low-cost yet highly sophisticated microsatellites have been built and launched into low earth orbit. The UoSAT missions have demonstrated that microsatellites can play a useful role in supporting specialised communications, earth observation, small-scale space science and in-orbit technology verification missions.

An essential part of the planning of a satellite system is the link-budget calculation. In Chapter 11 we presented the equations to be used in this exercise, but anyone entering the satellite communications field will need familiarity with the use of these equations. In this Appendix we have collected together some examples of link budgets and their use in planning satellite systems. It is recommended that the reader work through these in detail and by so doing master the art of satellite design. The detailed methodology and terminology, and the degree of accuracy employed, vary according to the application and require ment. It should be stressed that there is no one correct way to present a link budget.

This appendix contains 14 pages of acronyms and abbreviations relating to satellite sommunication systems.