# Performance Assessment of the Medium Frequency R-Mode Baltic Testbed at Sea near Rostock
## 5 Conclusions  
This paper presents a detailed description of the MF R-Mode system, as implemented in the Baltic Sea. Eight radio beacons around the southern Baltic Sea are able to provide the R-Mode as an additional service for testing and development in order to support safe navigation at sea. The system presents three challenges which are addressed in the paper. The ground wave, which is the expected propagation path for the R-Mode signal, is affected by different propagation velocities depending on type of land and water (fresh, sea). The effect can be modeled or measured. Figure 9: Normalized range error distribution for the CW1 signal. Solid blue line represents nighttime data, while orange dashed line represents daytime data. (**a**) Groß Mohrdorf. (**b**) Hammerodde. (**c**) Helgoland. (**d**) Hoburg. (**e**) Holmsjo. (**f**) Rozewie. Secondly, during the night, a reflection of the transmitted signal at the E-layer of the ionosphere comes into effect, which causes a degradation of the R-Mode performance due to the interference with the ground-wave at the receiver site. The sky-wave interference was modeled in the paper and analyzed with respect to the impact on the received signal amplitude phase and the phase error compared to the ground-wave. This model enabled the calculation of the maximum error that can be expected during sky-wave conditions. In the future, this model could also be used to calculate the expected sky-wave impact, taking into account the statistics of the expected sky-wave amplitudes and delays. The third challenge is some features of the R-Mode signals that can be observed in the transmissions from different stations, which seem to have a technical cause. They appear as jumps in the phase by several meters from one second to the next, which makes it possible to detect them. For now, the affected signals are not used. The performance of the MF R-Mode system in the testbed area close to Rostock was presented in terms of horizontal positioning and timing accuracy. We demonstrated that good accuracy can be achieved during the day, whereas the problem of the sky-wave multipath degrades system performance at night. In this contribution, we have been able to confirm that the R-Mode could be a suitable backup system for the maritime domain. In particular, the 95% horizontal accuracy achieved was 15.1 m for 6 h during the day and 55.3 m for 4 h at night. In both cases, the accuracy requirement of 100 m for coastal navigation was met over the time slots considered. Furthermore, we found that at night, no sky-wave disturbance is visible for a distance of up to 62 km. For distances of 210 km to 270 km, the composition of the propagation path is important. On a nearly complete over-sea path, the sky wave has a low impact. The R-Mode signal shows a low degradation. For propagation half over sea and half over land, the standard deviation error increased by a factor of 2.6. In the future, this knowledge could be used to design an adaptive filter for weighting the different measurements in the positioning algorithm at night to provide measurements with an expected higher probability of the sky-wave with less weight. In general, predicting the effect of the sky-wave can be very challenging due to its dependency on solar activity. Different performance levels might be experienced by the users on different days, during different seasons or at different times of the day. Therefore, the correct modelling of the sky-wave interference effect on phase estimation may be an interesting area of research, given the large number of parameters to be considered in the model itself. For the moment, a working solution is to detect the sky-wave by a stronger variation of the signal amplitude and reduce the service range at night. Finally, the study of interference mitigation strategies may be an important point of the investigation for the future in order to improve the MF R-Mode system performance at night. Conceptualization, F.G.R., L.G. and S.G.; Methodology, F.G.R., L.G. and S.G.; Software, F.G.R., L.G.