Introduction This report deals with the general principles of a public mobile telephone service to aircraft and@ in particular@ with operational and technical characteristics which are important for satisfactory operation. Public mobile telephone systems considered in this report are defined as aircraft systems for public correspondence which are connected to the international public switched telephone network (PSTN). Several systems are in operation or are under consideration by different administrations to provide this service. Terrestrial systems offer some advantages over satellite-based systems. Satellite systems will have greatest application over large bodies of water or land masses that cannot be served by land-based systems. Frequency reuse in a terrestrial system@ using cellular-like geometry@ can yield far more channels than satellite coverage@ even assuming multiple satellite spot beams. In heavily populated areas where a high volume of local aircraft movement requires high channel capacity@ terrestrial systems will be the most practical. Features of the terrestrial system developed in Japan are presented in Annex I. Features of the terrestrial experimental system currently operated in the United States and Canada and as proposed for implementation in Australia are given in Annex II. Outline features of the terrestrial system selected for interim use in the United Kingdom@ and forming one of the approaches under review by the European Telecommunications Standards Institute (ETSI) for use in Europe@ are given in Annex III.