R-Mode – terrestrial navigation for maritime users

Kurzfassung

Transport of passengers as well as cargo has become a key-issue for our modern economy. Especially the management of cargo traffic has become a fragile backbone for an economy that is based on “just-in-time” delivery and closely linked manufacturing processes. The International Maritime Organization (IMO) identified a gap in the provision of resilient navigation data for safe navigation of vessels sailing the world’s seas. Maritime transport has increased to an extent, where traffic regulations and restrictions have become essential to provide safe voyage on the “highways of the seas”. High traffic densities in demanding waters, like the narrow straits and vessel traffic separation schemes in shallow waters raise a demand and an increase in the requirements on reliable navigation data on-board of vessels for safe navigation. Furthermore, today’s world is facing acts of cybercrime on a daily basis and terrorism as well as growing tensions between states. All this is resulting in potential harmful threats against human life, the economy and for environmental pollution as well, when tempering with navigation systems is not detected or prevented. Current reports on Global Navigation Satellite System (GNSS) disturbances in the eastern Baltic and Skagen show the vulnerability of transport modes that rely solely on GNSS. To increase the resilience of the maritime transport and reduce the risk of using misleading information, national maritime administrations and service providers in the Baltic Sea region and worldwide are looking for solutions to identify and mitigate GNSS disruptions or deceptions. Following the IMO International Convention for the Safety of Life at Sea (SOLAS), national maritime administrations are requested to investigate and in future provide means to ensure provision of resilient Position, Navigation and Timing (PNT). An opportunity is to use alternative navigation systems in conjunction with GNSS to achieve that. The Baltic Sea is a test area for a terrestrial navigation system, known as R-Mode, which reuses maritime shore infrastructure for the synchronized transmission of ranging signals. They are implemented as two additional carrier signals to the continuous medium frequency broadcast of maritime radio beacons and as new designed R-Mode one slot message broadcast of the Very high frequency Data Exchange System (VDES) base stations which will replace base stations of the Automatic Identification System (AIS) in the future. Since 2017, two projects implemented an R-Mode test bed in the southern Baltic Sea. Eight maritime radio beacons were equipped with R-Mode ready signal modulators and accurate clocks. This enabled R-Mode signal reception in a wide area of the southern Baltic Sea. Static and dynamic position experiments showed in agreement with the theory that accuracies of 10 to 30 m can be achieved at day-time or when stations in close distance are used. The performance at night-time is especially dependent on the distance of the receiver to the transmitter. The ranging performance is significantly reduced for larger distances due to interference of the main propagation path as ground wave with the sky wave. As an indicator for sky-wave induced signal degradation a change of the probability density function from Gauss to Laplace tape for the range estimation was observed. This might be usable for the design of future positioning algorithms. Beside the permanent R-Mode radio beacon implementation also experiment with VDES were conducted in two location at the Baltic Sea and on a lake in Germany. It shows the feasibility of that approach and the usability of the standardized R-Mode message for ranging and positioning. The experiments suggest that positioning with VDES R-Mode can be achieved with accuracies of 10 m in areas with good reception conditions. The maritime infrastructure which is used for the transmission of R-Mode ranging signals was optimized to provide differential GNSS data within an average radius of 250 km to the GNSS onboard receiver or to exchange data between ship and shore over distances of about 20 nm (varies very much) using AIS/VDES. Therefore, the geometry is usually not optimized for R-Mode. But it can be compensated by an increasing number of transmitters or preferably use VDES and radio beacon R-Mode transmitters at the same time. International cooperation between the Baltic Sea states can enable terrestrial navigation for maritime users in coastal waters for manageable costs.