The VHF Data Exchange System, A New Communications System for Maritime

The VHF Data Exchange System ( VDES) is a new communications system the development of which is being coordinated by the International Association of Marine Aids-to-navigation and Lighthouse Authorities ( IALA) in consultation with the International Telecommunication Union ( ITU). It is seen as a key supporting element of the International Maritime Organisation ( IMO) e-Navigation initiative. The General Lighthouse Authorities of the United Kingdom and Ireland ( GLA) recognise the potential of VDES to serve as an important component of e-Navigation communications and support its development through a range of research projects. After giving an overview of the VDES concept, this paper reports on progress to date, including the results of a VDES channel sounding campaign conducted by the GLA. The campaign examined radio propagation conditions for all channels intended for use in ship-to-shore and shore-to-ship VDES communications. A comprehensive series of sea trials were conducted over five days, near Harwich in the United Kingdom. Five operational scenarios were examined, spanning four of the six IMO Maritime Service Portfolio ( MSP) area categories. System components and deployment were consistent with real world maritime use. Key findings from the sounding campaign have been published by the ITU ( Rep. ITU-R M.2317). The current paper expands on earlier work by presenting the results of a detailed statistical analysis of the sounding data. The channels are characterised both in terms of their frequency-selectivity and time-variant behaviour. A set of tapped-delay line channel models are derived which can serve as a basis for waveform-level computer simulations and evaluation of VDES physical layer performance. The integration of realistic channel models in such simulations makes it possible to assess the communication performance in different operational scenarios and under a range of propagation, weather and sea state conditions, replacing the need for repeated and costly sea trials during the initial system design phase. Finally, the paper gives an update on the work of the IALA e-Navigation Committee to develop a comprehensive VDES channel model, including the ship-air and ship-satellite channels.