Monitoring of anomalous propagation of VHF aeronautical navigation radio waves by the sporadic-E layer

The sporadic E (Es) layer is a dense in electron density and thin in altitude layer in the ionosphere at the E region altitudes. Due to its high density, radio waves at VHF band can be reflected to cause anomalous propagations over the horizon. For ground-based aeronautical radio navigation systems, such as a VHF omnidirectional range (VOR), an instrument landing system localizer (ILS-LOC) and a VHF data broadcast (VDB) of the ground-based GNSS augmentation system (GBAS) use radio frequencies from 108 to 118 MHz are used. Usually, the frequencies for such radio navigation systems are assigned assuming that their radio waves do not propagate over the horizon. When the dense Es layer appears, however, the VHF radio waves of the aeronautical radio navigation systems can even be reflected to propagate over long distances.

We have developed a netowrk to monitor anomalous propagation of VHF radio waves of aeronautical radio navigation systems over Japan. We found a number of events where the power of the anomalous propagation radio waves exceeds the maximum level allowed for airborne receivers of the aeronautical radio navigation systems.Since the locations of radio sources are known, we can monitor occurrences of anomalous propagation of VHF radio waves of aeronautical navigation systems by the Es layer in real-time on our network.

We have also developed a system to monitor the ROTI (rate of TEC (total electron contents) index) in real-time over Japan by using the real-time data provided by the GEONET (GNSS Earth Observation Network) by the Geospatial Information Authority of Japan. It is found that ROTI mapped at altitudes of the E region height (typically at 100 km) often show good correlation with the occurrence of the Es layer.

We have developed a combined system of the VHF aeronautical navigation radio anomalous propagation and GNSS ROTI. Our system can be used for monitoring the radio propagation conditions for VHF aeronautical radio navigation systems. This system can also be used to monitor the occurrece and motion of the sporadic E layer.