er. The usage of PMR is continuing to increase rather than decrease in the U.K. (Radiocom, 1990), indicating that public access systems Clark, T. (1988) AMSAT's Microsat/pacsat Program, 7th ARRL are complimenting rather than replacing PMR systems. The extra Computer Networking Conference, Columbia U.S.A., October. frequency spectrum needed must thus be used to the best possible AMSAT (1991) SatelLife - Healthnet satellite communications effect in congested areas, with trunked PMR becoming more pre- using transportable Earth terminals, AMSAT-U.K. Colloquium, valent. Frequency sharing between users is under control in many University of Surrey, July. countries by co-ordination work performed by national regulatory DTI (1981c) Code of practice for the transmission of digital infor- bodies, and this must be considered and complied with to ensure the mation over land mobile radio systems, Department of Trade continued usefulness of PMR. and Industry, London, MPT1317, April. 46/14 PMR and Trunked Radio Systems DTI (1986) Engineering memorandum for trunked systems in the GEC (1990) Data services for fleet managers, GEC National One. land mobile radio service, Department of Trade and Industry, HMSO (1984) Bands 1 and 111, A consultative document, May. London, MPT1318, February. Hobeche R.J. (1985) Land Mobile Radio Systems, Peter Peregrinus. DTI (1988a) Code of practice for repeater operation at communal Lee, W.C. (1982) Mobile Communications Engineering, McGraw sites, Department of Trade and Industry, London, MPT1351, Hill, New York. June. Miller, J. (1988) 9600 baud packet radio modem design, 7th Com- DTI (1978) Performance specification for continuous tone control- puter Networking Conference, Columbia U.S.A., October. led signalling (CTCSS) for use in the land mobile services, Oliver, B. (1989) U.K. national network services, IBC Trunked Department of Trade and Industry, London, MPT1306, January. Private Mobile Radio Conference, November. DTI (1981a) Code of practice, requirements for duplex operation in Orr, W.I. (1