# Performance Assessment of the Medium Frequency R-Mode Baltic Testbed at Sea near Rostock
## 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(Iternative)PNT systems, which replace GNSS in the case of outages or interference events.  
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; 10; 11].  
This paper presents the results of a measurements campaign conducted in the Baltic Sea 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 mention 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.