The Global Positioning System (GPS) is a cheap, accurate (±100 m), satellite-based position fixing system. These qualities suggest that it might be used in the identification of marine radar targets. The first application of such a system on any scale will probably be as part of a Vessel Traffic Service (VTS) and here GPS would seem to be ideally suited. Vessels entering the VTS control zone would need to be fitted with GPS interfaced with either a dedicated VHF receiver or a ‘guard’ channel on their normal receiver A VTS radar operator needing to identify an echo would work out the echo's position from the radar range and bearing and transmit that position as the interrogating signal. Only the vessel in the control zone with that position (derived from its GPS interface) would respond with its identity. There would thus be no possibility of interference between responses or garbling that can be a problem in interrogation systems based on secondary radar. Even if the VTS had several radars, these could easily be synchronized to prevent simultaneous interrogations. However, outside VTS zones there is a clear need for a universal identification system that would allow any ship to identify any other. It would be unfortunate and wasteful if a system developed for VTS was found to be unsuitable for this general application. This note will therefore discuss the adequacy of GPS as the basis of a universal marine identification system.