a. A main base station was used to communicate with a number of mobile stations mounted in vehicles. This limited the coverage of the system to that of the base station. Systems were often fairly elementary by today's standards and transmissions used simplex mode, requiring press to talk on the transmitters. For many applications such as taxi services this was perfectly satisfactory as they would operate within a given service area and there was no need for a sophisticated system, especially as this would have added extra cost. However, some operators did require much greater ranges and to accommodate this type of user, trunked PMR was developed. This allowed calls to be routed to transmitters in different areas, thereby significantly increasing the range. Systems have been considerably developed in recent years with the introduction of a system known as TETRA. Originally the letters stood for Trans European Trunked RAdio, but as the system is now being used beyond Europe the abbreviation now stands for TErrestrial Trunked RAdio. Private mobile radio 239 Local private mobile radio This is the simplest of the PMR systems and involves the use of a single antenna site for the location of the base station. In view of the fact that the antenna may be mounted on a high tower, coverage may extend up to distances of 50 kilometres, although ranges somewhat less than this are more usual. Licences are allocated for operation on a particular channel or channels. The user can then have use of these channels to contact the mobile stations in his fleet. The base station may be run by the user himself or it may be run by an operating company who will hire out channels to individual users. In this way a single base station with a number of different channels can be run by one operator for a number of different users and this makes efficient use of the base station equipment. The base station site can also be located at a position that will give optimum radio coverage, and private lines can be provided to connect the users' contro