[P-EM12] Study of coupling processes in solar-terrestrial system

[PEM12-P11] Observation of Es Layer Substructure
Using ILS Far-Field Propagation Wave for Aeronautical Navigation

*Shumpei Tabuchi¹, Keisuke Hosokawa¹, Susumu Saito², Jun Sakai³, Ichiro Tomizawa³, Toru Takahashi², Hiroyuki Nakata⁴ (1.Department of Communication Engineering and Informatics, University of Electro-Communications, 2.Electronic Navigation Research Institude, National Institute of Maritime, Port, and Aviation Technology, 3.Center for Space Science and Radio Engineering, University of Electro communications, 4.Graduate School of Engineering, Chiba University)

Keywords:sporadic E layer

The sporadic E (Es) layer is a phenomenon in which the electron density increases locally at an altitude of about 100 km in the ionosphere. Normally, radio waves in the VHF band above 100 MHz are not reflected in the ionosphere, but these waves may be reflected by Es layer if it is sufficiently dense. The radio waves reflected by Es layer can propagate anomalously over long distances; thus, Es layer has a potential to cause interference to radio systems such as aeronautical navigation systems. The Es layer has been studied for a long time, and its vertical structure has been well understood. In recent years, anomalous propagation, which may be caused by the spatial structure of the Es layer, has been observed, and there is a need to understand not only the vertical structure but also the horizontal structure and its effect on radio waves. The purpose of this study is to estimate the horizontal structure of the Es layer using the ROTI map at the time of the anomaly propagation event, and to clarify the influence of the spatial structure of the Es layer on the propagation of radio waves for air navigation.
The aeronautical navigation uses the frequency band of 108-118 MHz to provide navigation information to aircrafts. The Instrument Landing System Localizer (ILS LOC), which is the focus of this study, transmits radio waves at 108-112 MHz frequency with amplitude modulation at 90 Hz on the left side and 150 Hz on the right side as seen from the aircraft. Then, it provides information about the approach course based on the difference in the intensity of these two modulations (Difference in Depth of Modulation: DDM). Since the output radio wave has strong directivity, it may cause anomalous propagation over a long distance due to reflection by Es layer. Recently, it has been reported that a 110.3 MHz radio wave, which seems to have been transmitted from the instrument landing system at the Hualien Airport in Taiwan, was received in Kure, Hiroshima, Japan, using a software receiver. In this study, in addition to the software receiver, an ILS LOC receiver, which was actually used in the aircraft, was installed in Kure, and the direction of arrival of the radio wave was measured continuously.
During an anomalous propagation event on May 17, 2021, the ILS LOC receiver also received the 110.3 MHz radio signal. From the analysis of the received Morse code, it was confirmed that the radio signal actually arrived from Hualien. However, the most frequent direction of arrival of the radio wave in four months from May to August 2021 was slightly different from the direction of Hualien. This may be because the beam direction of the ILS LOC at the Hualien Airport changes due to the propagation direction and the spatial structure of Es layer.
We have also investigated the deviation index (ROTI) of total electron content from GPS receivers during the intervals of anomalous propagation. The midpoint between Kure and Hualien, which is considered to be the reflection point of Es layer, is located at sea. Therefore, in the case of weak Es layer, it may not be possible to confirm the occurrence and propagation direction of Es layer only by using ROTI. However, in the three cases of May 27, June 8, and June 30, 2021, Es layer signatures in the ROTI maps were observed to be moving southwestward from the mainland of Japan to the southern part of Kyushu. The variation of the angle-of-arrival calculated from the DDM values shows that the angles are not always constant during the anomalous propagation, and there is sometimes a systematic decreasing trend. In particular, on June 30, 2021, there was a characteristic change in the angle, which continuously decreased by about 1.5 degrees in about 30 minutes. Based on the direction of motion of Es layer inferred from the ROTI mapping and the change in the angle-of-arrival on June 30, 2021, it can be suggested that Es layer has a wedge-like shape, which may explain the peak of the difference in the angle-of-arrival. This means that the motion and spatial of Es layer can be inferred from the change in the angle calculated from DDM of the ILS LOC receiver.