![]() With its high electron density, Es can reflect VHF radio waves. One of the well-known radio effects caused by Es is anomalous propagation of very high-frequency (VHF) radio waves (Davis et al., 1959). This makes it difficult to predict, and mitigate, undesired effects on radio communication caused by Es. As the name “sporadic” suggests, Es appears only occasionally or not regularly and is spatially localized. Sporadic-E has been known as a medium of various radio interferences. The sporadic-E (Es) layer is an ionospheric layer which is thin in altitude, consists of high electron-density plasma and appears occasionally at an altitude of around 100 km in the E region mainly in summer (Haldoupis, 2011 Mathews, 1998 Whitehead, 1989). This study suggests that if similar conditions exist, radio interference to ILS LOC receiver can happen on any ILS LOC channels. Statistics showed that there were several outstanding EsAP events of which source was Hualien LDA. Analysis showed the causes of the strong signal from Hualien LDA: (1) the center of the narrow radio beam passed through the region very close to the receiver site (2) the elevation angle of the propagation path was very close to that of the actual beam center. During the same period, several EsAP signals were observed on other ILS LOC channels, but their strengths were less significant. The source of the signal was identified as an ILS LOC-type direction aid (LDA) at Hualien, Taiwan. A series of extremely strong signals was observed on an ILS LOC frequency at Kure, Japan, on when an ionosphere sounder observed high-density Es. In this paper, we focus on EsAP on ILS LOC channels using the data obtained in the previous study. The study showed that EsAP signals can be very strong and frequently exceeded the allowable interference limit of navigation receivers. The observed frequencies includes channels of VHF omnidirectional radio range (VOR) and instrument landing system localizer (ILS LOC). Recent study showed that VHF air navigation signals from distant stations are frequently observed when Es occurs. This effect, EsAP, may cause interference to VHF radio operation above 100 MHz. The ionospheric sporadic-E (Es) layer occasionally causes anomalous propagation (AP) of VHF radio waves to distant locations.
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