Time delays caused by ground wave propagation are the primary source of systematic error limiting the performance of the medium-frequency R-Mode radionavigation system. To achieve the desired ranging accuracy and compensate these delays, we have conceived a comprehensive correction scheme based on the prediction and application of the Atmospheric and Ground wave Delay Factor (AGDF). The AGDF was computed and mapped in 2D for a number of MF R-Mode transmitters in the Baltic Sea that were embedded into the receiver and evaluated during a large-scale measurement campaign. Our results show that the proposed AGDF approach is valid for the MF R-Mode system and provides accurate corrections of ground wave propagation delays within the performance requirements.

Nowadays, the maritime sector strongly relies on global navigation satellite systems (GNSS) for the provision of position, navigation, and timing (PNT) information. The standard functionality of several ships’ bridge instruments depends on such information, which becomes critical to have a safe and reliable navigation. Nevertheless, the possibility of GNSS outages combined with unintentional and intentional interference to GNSS signals, which have been increasing over the last years, can severely threaten the nominal activity of the crew onboard of vessels. For these reasons, alternative position, navigation, and timing (APNT) systems become fundamental in order to provide operational continuity. R(anging)-Mode is a terrestrial alternative system to GNSS for the maritime domain. In one of its possible implementation, it exploits synchronized medium frequency (MF) signals transmitted by maritime radio beacons. In the MF band, the radio waves are attenuated and distorted by the change of electrical properties of the ground along the propagation path. Moreover, these signals are affected by terrain elevation changes and large metallic infrastructures which induce an overall loss in the accuracy of the system. In this paper, we propose a mitigation technique based on the estimation of these additional error sources by using in-field range measurements. The theoretical details are explained in depth and the technique is validated with real data gathered on a measurement campaign. A clear and significant performance improvement is achieved on the range accuracy when the proposed approach is applied.

The ground-wave signals of terrestrial radio navigation systems, which operate in the medium and low frequency band, are sensitive to changes in the electrical parameters of the Earth's surface between the transmitter and receiver. Sea ice affects the electrical parameters of the sea and leads to an additional signal phase delay compared to the propagation over salt water. To ensure the uniform performance of the navigation receivers for these systems throughout the year, the impact of sea ice on the signal has to be known. A challenge here is the high spatial and temporal dynamic of sea ice in some regions. Earth observation data can be used to obtain information regarding the world-wide sea-ice coverage and further electrical ground parameters. In this paper, our proposed model for the ground-wave propagation and Copernicus data are used to compute the impact of varying conditions on the signal propagation. Simulation results for a real-world scenario show that the signal propagation delay caused by sea ice can lie in the order of 20 ns with respect to sea water.

The reliance on global navigation satellite systems (GNSS) for modern vessel poses a critical point of failure. GNSS is vulnerable to jamming, spoofing, and other threats that can increase the risk of accidents. In response, alternative sources of navigational information are being sought. R-Mode offers a promising solution by leveraging terrestrial infrastructure to provide PNT data independently of GNSS. A minimum of three stations in view is needed to obtain a position and timing information. While a single R-Mode station in view cannot provide independent positioning, the received data can still be used to validate a GNSS solutions and detect threats like spoofing or outages. In this study, we introduce a novel approach to validate GNSS positions using R-Mode ranging information from a single station by combining the expected accuracy of the measurements with the geometrical relationship between the GNSS solution and the known R-Mode transmitter location. Our method was tested with real measurements in post-processing, where simulated spoofing events were introduced to mimic real-world scenarios. During these events, the GNSS solution deviated by approximately 100 m from original position. Our technique successfully detected the spoofing instances and raised warnings to increase the awareness of GNSS-based navigation threats.

Assessing the quality of a received signal is of fundamental importance to predict the performance of the receiver. In general, the signal to noise ratio (SNR) or the carrier to noise density $C/N_0$ ratio is used as an indicator to describe the signal quality of most receivers. For the medium-frequency (MF) R-Mode, a terrestrial positioning navigation and timing (PNT) system, the knowledge of the SNR or $C/N_0$ is important for monitoring purposes, helping the service operator to assess the healthiness of the transmitted signals, as well as for the users to optimize the receiver algorithms and performance. In this paper, we present how the $C/N_0$ can be estimated from the output of the discrete Fourier transform (DFT). The DFT is already used in the receiver to perform the phase estimation; hence, the receiver computational load is reduced. Theory is first presented and discussed, followed by the definition of the estimator, which is tested with Monte Carlo simulation as well as with real data to validate the approach. The results show good agreement between theory, simulation, and in-field measurements, which proves that the estimated $C/N_0$ is a good indicator to measure the received signal quality under optimal propagation condition.

Terrestrial radio navigation systems are an important data source for increasing the integrity of position, navigation, and timing information and for strengthening the immunity to the spoofing and jamming of satellite-based systems. A possible solution for maritime use is the medium frequency R-Mode. Here, the first real results were presented in the Baltic Sea within the R-Mode Baltic project. For the development of receivers and future signal evolution of R-Mode, there is a great need to provide simulated signals to the receiver hardware. For our laboratory work, we developed different hardware in the loop test systems which enable the simulation of each of the three components and the entire medium frequency R-Mode signal. With this setup, we are able to conduct reproducible tests of the R-Mode receiver’s ranging and positioning performance without the necessity of field tests. Furthermore, the impact of R-Mode signal modifications can be initially analyzed without the need of an implementation in the real-world test bed. Firstly, we describe the usage of arbitrary wave generators that can be used to replay received or simulated signals. Due to their wide distribution in electronic laboratories, there are cost-efficient ways to build up test capabilities. For this work, we tested the Tektronix AFG 3022 and the Rigol DG1032. For further tests, we utilize software-defined radios that are capable of streaming continuous signals. We utilize the ETTUS N210 to directly output the simulated signals. Additionally, we test the LimeSDR with an external down-converter. To generate these signals, we utilize software packages that were created to support the development of the digital signal processing. This approach allows us to test our receiver with a continuous integration from pure software to hardware in the loop test. A comprehensive summary gives an overview of the pros and cons for the different suggested systems.