Investigation of the statistical relationship between MF R-Mode and VLF signals in relation to D- and E-layer variability

Kurzfassung

In an era of high demand for international trade and concurrent increases in GNSS signal manipulation in the Baltic Sea region, ensuring safe and reliable maritime navigation has become challenging. To address this issue, the R-Mode system, a terrestrial navigation system with medium frequency (MF) and very high frequency (VHF) signal subsystems, is being established as a backup to GNSS. Here we focus on the MF R-Mode, operating at frequencies of 283.5 - 315 kHz. The MF R-Mode signal can be described as a combination of the ground wave, which propagates near the earth's surface, and the sky wave, which is reflected off the E-layer of the lower ionosphere and directed back towards the earth. While the ground wave provides the desired positional information, the sky wave interferes with it, degrading navigation accuracy. During the day, the sky wave is damped in the D-layer by absorption and scattering, reducing its disruptive influence. At night, however, the D-layer dissipates, allowing the sky wave to return at nearly full intensity and severely impair MF R-Mode performance. The interactions between the sky wave and the E- and D-layers make MF R Mode performance intrinsically dependent on the state of the lower ionosphere. A deeper understanding of these mechanisms and their dependence on ionospheric variability is required to (a) allow for real-time MF R-Mode accuracy estimation and (b) apply well-founded corrections to the received signal to compensate for the sky wave's influence. The D- and E-region lie at altitudes of about 50-150 km, where drag is too high for satellites and buoyancy too low for balloons, making in-situ data acquisition challenging. A possible source of information about these altitudes are very low frequency (VLF, 3 - 30 kHz) radio waves, which also reflect off the lower ionosphere and can thus provide insights into the state of the D- and E-regions under varying solar and geomagnetic conditions. This work examines the statistical relationship between MF R-Mode and VLF signals to assess whether VLF data allow inferences about the MF R-Mode's positioning accuracy. Initial analyses indicate a potential anti-correlation between the two signals, suggesting that VLF observations could be used as a proxy for MF R Mode sky wave behaviour. We aim to identify seasonal and diurnal changes in MF R-Mode accuracy driven by variations in atmospheric and solar parameters. Ultimately, this work shall pave the way to establish an MF R-Mode accuracy forecast system guided by VLF measurements. At this conference, we present the first results of this ongoing research.