On the Positioning Performance of VDES R-Mode

Abstract

Ships nowadays greatly rely on Global Navigation Satellite Systems (GNSSs) in order to deter- mine their position. Since GNSS outages or jamming events do occur, there are efforts to reduce the dependency on GNSS for maritime navigation. One such effort is called R-Mode (Ranging Mode), and focuses on complementing maritime communication systems by a ranging compo- nent to enable a vessel to determine its position. One of the systems to be extended by R-Mode is the VHF Data Exchange System (VDES). The VDES communication system is currently in standardization and offers 100 kHz of bandwidth in the maritime VHF band. It utilizes sin- gle carrier modulation with pi/4-QPSK. The proposed R-Mode extension works by sending a precisely timed known data sequence, so that time of arrival estimation allows determination of the range. Using software defined radios (SDR), we implemented a test setup for VDES R-Mode with three base stations on land and one receiver located on a vessel. Using this setup, we performed the first VDES R-Mode positioning trials on the Lake Ammer in Germany. By determining the time of the arrival as well as the Doppler shift of the received signals we tracked the vessels position with an Unscented Kalman Filter. The positioning accuracy performance ranged to up to 22 m under favourable conditions. Crucial was the consideration of the Doppler measurements to enhance tracking performance considerably.