Introduction

In Chapter 5–11 we covered the classical spacecraft platform subsystems. The present chapter introduces the payload section of this book. The payloads of interest in geostationary satellite applications perform the functions of communication (this Chapter) and meteorological imagery (Chapter 13).

All geostationary applications satellites communicate information, be it language, pictures or abstract numbers. When we talk about communications satellites, however, we limit the definition to spacecraft which relay voice, data, telex, facsimile, or television pictures from one part of the earth to another, as opposed to telemetry and imagery which originate in the satellites themselves.

Satellite communications is a new, vast and fascinating engineering science. Along with orbital mechanics and attitude and orbit control, it is also one of the most “difficult” spacecraft subjects to understand, mainly because the student is faced with a wealth of mathematical abstractions from the outset. In the present chapter we will provide an outline of basic communications theory and how it influences the architecture of a communications payload. The interested reader is advised to consult the books listed in the Bibliography for a more indepth coverage of the subject. The list is far from exhaustive, and suggestions for further reading are made in the Reference sections of those books.

Transmission Capacity Versus Power and Bandwidth

We will begin this chapter by examining how various transmission techniques allow a designer to optimize the communications capacity of a satellite by trading off two precious commodities, namely spacecraft power and frequency bandwidth.