R-Mode based Positioning at the Baltic Sea

Abstract

Global Navigation Satellite Systems (GNSS), today the primary source for position, navigation and time (PNT) information in the maritime domain, are vulnerable to unintentional and intentional interference, e.g. jamming and spoofing. A backup system is needed to overcome this threat and to enable new applications with an increased need for resilient PNT information. Diversity of signals is a promising approach to overcome the dependence on GNSS signals which have in common to be very weak at sea level and located in only few frequency bands. Terrestrial radionavigation systems, which typically operate in lower frequency bands and most often with higher transmitting power, are differently affected by signal distortions or interferences. As a complement to GNSS they can support continuous availability of PNT information even if the accuracy is lower compared to GNSS. While they were state of the art for decades, terrestrial radionavigation systems lost importance with the increasing availability of GNSS receivers. As a consequence, these services were discontinued in parts of the world. Today no world-wide available terrestrial radionavigation system exists. In 2008 Oltmann and Hoppe proposed an efficient way to get back a terrestrial radionavigation system as a backup to GNSS by using already existing maritime radio infrastructure for the transmission of ranging signals in addition to the legacy signal. The system is called ranging mode (R-Mode). Promising candidates were maritime radio beacons and land-based stations of the Automatic Identification System (AIS). Both have in common their good distribution of stations along the coastline near the main maritime traffic routes. First feasibility studies towards R-Mode were conducted by Johnson and Swaszek within the framework of the European ACCSEAS project in 2014. Based on these studies, ranging measurements with modified radio beacons were performed in the Netherlands in 2015 and in Germany in the following years. In 2015 Qing et.al reported about successful tests using AIS base stations for positioning in the Xinghai sea region of Dalian. In 2020 Johnson et.al demonstrated that R-Mode, implemented on three medium frequency DGNSS transmitter sites, can be used for positioning in a limited area on land. Missing so far are R-Mode trials of maritime long-range signals from maritime radio beacons and the R-Mode implementation that follows the new Very High Frequency (VHF) Data Exchange System (VDES) Guideline of the IALA for transmissions in the maritime VHF band. The presentation gives an overview about the achievements of the EU co-financed project R-Mode Baltic which has been conducted by 12 partners from national maritime administrations, industry and research institutions under the leadership of the German Aerospace Center in the time from 2017 to 2021. It reports about the R-Mode requirements on a terrestrial navigation system as backup to GNSS for port approaches and coastal navigation. It shows that the maritime radio infrastructure of the Southern Baltic Sea fulfils the precondition for a successful implementation of a combined MF and VDES R-Mode system. This is based on a coverage prediction and accuracy estimation study of the GRAD Department of Trinity House. During the project eight MF maritime radio beacons and four VDES base stations with R-Mode capabilities were installed in the R-Mode Baltic testbed. The results of first maritime positioning tests in the testbed show that R-Mode is a good candidate for a backup system for maritime applications. A cost-benefit analysis based on the experiences and developments of the project shows that the costs for an R-Mode system depend on the level of time synchronization accuracy. Each maritime administration has to decide if they want to support the mariner for a certain time until he reaches a safe state after detection of GNSS problems, this is called contingency system, or that R-Mode is always available as a true backup system.