at ac- Channels would then be assigned to links as the need arises, the ceptable cost. transmitter power being determined in each case to ensure that A busy vehicle fleet operator, passing perhaps hundreds of short the noise plus interference performance realised did not too messages per hour to taxicabs, police cars etc. operating within an greatly exceed the objective. area some tens of kilometres in radius, may well make efficient use of spectrum as the sole assignee of a channel, using a simple The key operation in this procedure is the assigning of channels transmitter receiver network. to links. When a substantial number of assignments has been made, An operator serving a small fleet of vehicles could seldom justify each frequency may be re-used several or many times over, with the exclusive use of a channel and a private user needing access geographical separation. The separation may be quite small if good from one vehicle would never do SO. The level of use of the channel use can be made of the directional properties of the antennas. would usually be raised by assigning the same channel in the same Computer software which is available does this task well, but digital area to several users. Simple control systems are available to stop maps giving very precise topographical data are necessary if the one assignee from starting to transmit at a time when another highest efficiency in the use of spectrum is to be attained. assignee is using the channel. This provides a low cost system, Efficient spectrum utilisation 26/17 GEOSTATIONARY SATELLITE ORBIT SOLID ANGLE COVERED BY RECOMMENDATION Figure 26.10 The solid angle to which the design objective indicated in Figure 26.9 relates acceptable in spectrum utilisation efficiency in locations where the disproportionate to the value of the telecommunication facilities demand for spectrum is not high, although users find shared chan- made available. nels irksome. Differences in the configuration of microwave terrestrial and In locations where demand is high, systems w