Performance assessment of the R-Mode Baltic testbed at sea near Rostock

Abstract

Global Navigation Satellite Systems (GNSS) were used over years to get position, navigation and timing (PNT) information for ships in the maritime domain. The global coverage and medium to high achievable accuracies enable the development of technologies for navigational support that relieves bridge staff and increased the safety of shipping. Today we rely on those systems which is critical because an increasing number of signal interferences is observed which reduces the trustworthiness of GNSS-based PNT information. There is a demanding need for additional, alternative absolute position information to prove the integrity of GNSS-based PNT and to serve as a backup in times GNSS is not trustworthy or usable. One possible alternative navigation system is the terrestrial Ranging Mode, called R-Mode, which extends the service of existing maritime infrastructure with the possibility of ranging and positioning. It reuses medium frequency (MF) radio beacons and base stations of the Very high frequency Data Exchange System (VDES) for the transmission of synchronised signals in the service areas of that stations. As a contribution to the sustainable management of available resources, R-Mode is a cost-efficient way to implement a positioning backup to fulfil the urgent needs of maritime users. To prove the performance of this technology a project consortium of twelve partners from five countries of the Baltic Sea region has built up the world-wide first large-scale R-Mode test bed in the southern Baltic Sea. Eight MF radio beacons were upgraded with R-Mode-ready signal generators. These beacons can provide R-Mode signals with multiple overlaps of stations to support positioning and timing in that region with at least three available signals. First positioning experiments revealed 12 m positioning accuracy in the testbed under optimal conditions which is sufficient for support as a backup system for coastal navigation and nearly matches the requirement of a backup system of 10 m for port approaches. Further tests were necessary to prove the usability of R-Mode also in cases of non-optimal conditions. In particular, the MF signals suffer the self-interference generated by the reflection of the radio wave from the E-layer of the Ionosphere during night time. The reflected signal, called sky wave in the literature, is a form of multipath which decreases the accuracy of the system. In our work, we present the analysis of measurements that we were able to take during a measurement campaign with the Bundesamt für Seeschifffahrt und Hydrographie (BSH) ship Deneb. The achieved accuracy in optimal and non-optimal cases is given in the paper. The results are therefore fundamental for the further development of the MF R-Mode system.