Compensation of MF R-Mode signal instability with the usage of differential correction

Kurzfassung

With the increase of jamming and spoofing attacks targeting global navigation satellite systems (GNSS) in the Baltic Sea region, the demand is growing, to establish an alternative navigation system which can provide a reliable source of position, navigation and timing (PNT) information. The most promising alternative PNT system for the Baltic Sea region is known as R-Mode. R-Mode makes use of very high frequency (VHF) and medium frequency (MF) signals transmitted from maritime ground infrastructure to provide positioning and timing services in their overlapping coverage areas. Currently supported by several countries of the Baltic Sea region and under the umbrella of the project ORMOBASS (pre-Operational R-Mode Baltic Sea System), the number of available transmitters is rapidly increasing, with the system foreseen to be declared pre-operational by the end of 2026. Several measurement campaigns have shown that MF R-Mode can provide horizontal positioning accuracies approaching 10 m (95%) under optimal propagation conditions. Less favorable conditions occur in case one or several transmitters provide unstable signals with jumps within a range of several meters or the propagation of the groundwave from the MF R-Mode transmitter to the receiver is affected by changes in the environmental conditions in between. These effects can cause significantly increased positioning errors. This work presents how the authors tackled this challenge with a differential R-Mode approach. The R-Mode reference station is placed at a known location. It receives MF R-Mode signals from different transmitters. The role of the reference station is to assess the quality of the received signals and compute corrections which can then be transmitted to the users to increase the R-Mode ranging accuracy. The approach is explained in details and results from real measurement campaign are presented to demonstrate the validity of the developed technique.