Appl. Sci. 2023, 13, 1872 
20f17 
The Ranging Mode (R-Mode) is a terrestrial navigation system based on the signals 
of the opportunity approach, which is currently under development in the Baltic Sea [6], 
where a testbed is now operational. The key idea of the R-Mode is to reuse the existing 
infrastructure to diminish the cost of implementation and deployment. Maritime radio 
beacons transmitting in the 300 kHz medium frequency (MF) band and the Automatic 
Identification System (AIS), or its modernised version, known as the very high frequency 
data exchange system (VDES), working at frequencies around 162 MHz, are used, in 
particular [7,8]. Initial studies on the R-Mode have been conducted by several authors 
assessing the feasibility of the system [9-11]. 
This paper presents the results of a measurements campaign conducted in the Baltic 
5ea near Rostock. At the time of the campaign, only maritime radio beacons, referred to 
as MF stations throughout the paper, were available in the testbed. Therefore, only the 
performance of the MF R-Mode system will be demonstrated. 
MF signals are affected by different impairments, which will be briefly described 
in the paper. Among them, the sky-wave effect, a multipath reflection, which appears 
mainly at night when the solar radiation is low, is of primary concern. The self-interference 
generated by the sky-wave can degrade the system accuracy, and predicting such an effect 
is quite complex, since it depends on solar radiation effects such as day-night variation [12], 
seasonal variation, solar cycles and distance between receiver and transmitter, just to 
mnention a few [13]. The initial performance results have already been presented in [14], 
demonstrating that it is possible to achieve a positioning accuracy of 12 m (95%), but only 
during the day. In this work, the night-time results are also presented and discussed. 
The paper is organized as follows: Section 2 describes the MF R-Mode system in a 
nutshell, with some of the relevant information on the testbed and receiver specification. In 
Section 3, the impairments of the R-Mode MF signals are presented, with a particular focus 
on the description of the sky-wave multipath effect. Section 4 contains the results of the 
measurement campaign during the daytime and nighttime. Finally, Section 5 concludes 
the paper. 
2. The MF R-Mode System 
One of the main outcomes of the R-Mode Baltic project was the successful estab- 
lishment of a testbed in the Baltic Sea. At the time of writing, the testbed includes eight 
operational MF radio beacons transmitting R-Mode signals. Figure 1 shows the map of 
the testbed area with the position of the R-Mode-enabled radio beacons located in four 
countries: Germany, Sweden, Poland and Denmark. 
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Figure 1. Enabled MF R-Mode radio beacons (yellow triangles) around the southern Baltic Sea.