An Initial Assessment of VDE-Terrestrial Channel Performance

Abstract

The volume of marine vessels using the Automatic Identification System (AIS) has been increasing since its inception in 2000. This increase is resulting in overloading of the two channels that carry the AIS transmissions. In response to this overloading of AIS, international organizations like the International Telecommunication Union (ITU), the International Maritime Organization (IMO), and the International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA) are developing the next generation AIS – called Very High Frequency (VHF) Data Exchange System (VDES). VDES is a technological concept utilizing terrestrial and satellite radio communication links in the VHF maritime mobile band to facilitate globally interoperable digital data exchange between ships, between ships and shore, between shore and ships, between ships and satellites, and between satellites and ships. VDES consists of four components: AIS (two 25kHz channels), Application Specific Message (ASM, two 25kHz channels), VDE Terrestrial (VDE-Ter, two 100kHz channels) and VDE Satellite (VDESat, two 150kHz channels overlapped with VDE Terrestrial). The U.S. Coast Guard (USCG) Research & Development Center (RDC) is researching VDES to assess the suitability for implementation. To support VDES research, RDC developed a technology road map and designed and implemented two field tests to evaluate the system. The first field test was conducted in the New London, CT area from April – Aug 2021 and focused on the transmission of Sensitive but Unclassified Tactical Information Exchange and Display System (STEDS) messages using VDES ASM channels. STEDS is presently implemented by the USCG on AIS channels, but this study assessed the feasibility of transitioning this traffic to the ASM channels in the future. The second field test was initiated in 2022 with the goal to continue the assessment of the performance of VDES (including the larger bandwidth VDE-Ter channels) and to identify issues that need to be resolved for potential operational implementation. These issues include how to manage the bulletin boards and channel management between adjacent base stations with overlapping coverage, including those on international borders. Mobile transceivers were installed on several vessels. Small messages (position reports and weather data) were transmitted on all three channels (AIS, ASM, and VDE-Ter) in both directions (ship-to-shore and shore-to-ship). Longer messages (files of various sizes) were also transmitted on VDE-Ter from shore-to-ship and ship-to-shore. Data was collected and analyzed to examine the impact of range (signal strength) on channel performance (measured by message delivery percent and time). Interoperability between VDES devices from different manufacturers as well as international coordination with Canada was also examined. This paper provides a technical description of the VDE component of VDES as currently defined in the international standards, describes the test configuration and plan, presents the data collected, and the results of our analysis.