ILL 
AL 
applied 
sciences 
(Mmbpı 
Article 
Performance Assessment of the Medium Frequency R-Mode 
Baltic Testbed at Sea near Rostock 
Filippo Giacomo Rizzi *-*®, Lars Grundhöfer 1*®, Stefan Gewies V*@ and Tobias Ehlers ?* 
Institute of Communications and Navigation, Nautical Systems Department, German Aerospace Center 
(DLR), Kalkhorstweg 53, 17235 Neustrelitz, Germany 
German Federal Maritime and Hydrographic Agency (BSH), Bernhard-Nocht-Straße 78, 
20359 Hamburg, Germany 
Correspondence: filippo.rizzi@dlr.de 
These authors contributed equally to this work. 
Abstract: Global Navigation Satellite Systems (GNSS) are the primary source of position, navigation 
and timing (PNT) information in the maritime domain. Nevertheless, there is a pressing need for 
alternative absolute position information to serve as a backup when GNSS is not trustworthy or 
usable. One possible alternative navigation system is the terrestrial Ranging Mode, also known as 
che R-Mode. 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 those stations. A large-scale R-Mode testbed is available in the southern Baltic Sea with eight 
MF radio beacons transmitting R-Mode signals. These signals suffer the self-interference generated 
by the reflection of the radio wave from the ionosphere at night. The reflected signal, known as the 
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 obtained with the research vessel Deneb. The 
horizontal accuracy achieved in the optimal condition was 15.1 m (95%), whereas under the sky wave, 
the accuracy decreased to 55.3 m. The results are a starting point for the further development of the 
VMIF R-Mode system. 
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Citation: Rizzi, F.G.; Grundhöfer, L.; 
Gewies, S.; Ehlers, T. Performance 
Assessment of the Medium 
Frequency R-Mode Baltic Testbed at 
Sea near Rostock. Appl. Sci. 2023, 13, 
1872. https://doi.org/10.3390/ 
app13031872 
Academic Editors: Marko Perkovic, 
Lucjan Gucma, Krzysztof Naus and 
Cezary Specht 
Received: 21 December 2022 
Revised: 24 January 2023 
Accepted: 29 January 2023 
Published: 31 Tanuarv 2023 
Keywords: APNT; R-Mode; medium frequency; sky-wave; maritime navigation 
1. Introduction 
Today, position, navigation and timing (PNT) services play a fundamental role in 
different areas of our society. Their use is widespread, from the provision of information 
to mass-market devices, such as smartphones and wearables, to safety-liability critical 
devices, where a trustworthy position or time data becomes vital [1]. Global Navigation 
Satellite Systems (GNSS) are the primary sources of such information, given their global 
coverage and high reliability, and particularly for the maritime sector, where the nominal 
functionality of multiple devices on board ships and vessels entirely depends on such 
systems. Clearly, this can be a single point of failure, endangering crew members and 
passengers, goods and the environment. 
In recent years, an increase in the number of threats to GNSS receivers has been 
observed, rising the concerns of the research community in the field of navigation [2,3]. 
Different research approaches have been considered in order to address this problem, 
such as jamming and spoofing detection and countermeasures [4], improvements of the 
transmitted GNSS signals with authentication services such as the Galileo Open Service 
Navigation Message Authentication (OSNMA) [5], or integration with additional sensors 
(e.g., inertial measurement unit, inertial navigation system, Camera, Lidar, and Radar). 
A further promising approach is the establishment of A(lternative)PNT systems, which 
replace GNSS in the case of outages or interference events. 
Copyright: © 2023 by the authors. 
„icensee MDPI, Basel, Switzerland. 
This article is an open access article 
distributed under the terms and 
conditions of the Creative Commons 
Attribution (CC BY) license (https:// 
creativecommons.org/licenses /by/ 
4.0/). 
Aypol. Sei. 2023. 13.1872. https: / /doi.org /10.3390 /avp13031872 
https: / /www.mdbpi.com /iournal /avplsci