11.1 Introduction

The overall design of a complete satellite communications system involves many complex trade offs in order to obtain a cost-effective solution. Factors which dominate are the size, weight and thus DC power that can be generated from the satellite, earth-station size, complexity and frequency of operation and constraints upon interference which may apply via international bodies such as the ITU. We will concentrate here on a single satellite link which is applicable to both GEO (geostationary-earth orbit) and nonGEO systems, but with modifications to allow for the dynamics of the latter. Moreover, we will be concerned with the so-called link planning or link budgets. This is illustrated in Figure 11.1 in which a single link consists of:

• uplink (earth station to satellite);

• downlink (satellite to earth station);

• a satellite path.

  
  Figure 11.1: Single Satellite Link

Such a link, from earth station to earth station, does not form the total overall connection between customers or users. In the early days of fixed-service satellite communications each country had one or more large international gateway earth stations which interconnected to the terrestrial network (and hence the users) via the international exchange. Such earth stations (e.g. INTELSAT Standard A) were large (30 m (NB now 16 m) diameter dishes) and costly ( ? US$5 10 million). The user-to-user connection, which defines the overall path over which quality of services must be planned, thus involved terrestrial-network tails (Figure 11.2a). Modern satellite communication systems, on the other hand, can go direct, from...