hich offer access for satellite networks are a key factor here. If spectrum that is shared each user to whichever of many channels is free at the moment when between these two services is to be used efficiently by both, the a call is to start can combine convenience to the user with very receivers of one service must be protected from the transmitters of efficient spectrum utilisation. Multi channel trunked systems might the other by geographical separation or by obstacles in the direct be suitable if the vehicles remain for most of the time within range propagation path between them. Now it is necessary for the an- of a single base station. Cellular systems are necessary if the mobile tennas of most terrestrial stations to be mounted high on the skyline, station roams widely and are preferable where access to the public on towers, the roofs of high buildings and hill tops; only in this way telephone network is required. can line of sight propagation for terrestrial links of substantial length Given sufficient need for efficient spectrum utilisation, the spec- be obtained. Also, many terrestrial stations are located in city trum manager must direct the users' choice of system towards centres. There is no corresponding need for earth station antennas appropriate system architecture. to be mounted high, and interference is reduced and co-ordination simplified if they are low. In considering how this basic factor affects practical spectrum management, three typical situations can 26.6.4.3 Frequency co-ordination around earth stations be identified. The first situation arises where the earth stations are used to In shared frequency bands allocated to both the fixed and fixed provide high capacity network links, typically for international or satellite services, a frequency co-ordination procedure is used to trunk telecommunication connections or for television programme limit interference between earth stations and nearby terrestrial sta- feeds, upwards to broadcasting satellites or downwards to terrestrial tions. Th