10:45 AM - 12:15 PM
[PEM12-P20] Statistical study of anomalous ILS radio propagation caused by sporadic E
Keywords:Sporadic E, Instrument Landing System (ILS)
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, VHF radio waves above 100 MHz are not reflected at an altitude of 100 km. However, the extremely high electron density in Es sometimes reflects HF to VHF radio waves and causes anomalous propagation of such waves for a long distance. Therefore, Es layer has a potential to cause interference to radio systems such as aeronautical navigation systems. In this study, observations of Es using radio waves used for aeronautical navigation systems, in particular the ones for the Instrument Landing System Localizer (ILS LOC), have been conducted in Kure, Japan.
The ILS LOC, which is the focus of this study, transmits radio waves in 108-112 MHz frequency range with amplitude modulation at 90 Hz on the left side and 150 Hz on the right side of the runway as seen from the aircraft. Then, ILS provides information about the approach course to airplanes based on the difference in the intensity of these two modulations (Difference in Depth of Modulation). Since radio waves from the ILS LOC system have strong directivity, they may propagate anomalously over a long distance due to reflection by the Es layer despite its low transmission power of 10 W.
During summer seasons, we often observe a very strong signal at 110.3 MHz at Kure, which is possibly caused by anomalous long-range propagation of VHF waves due to reflection by the Es layer. Analysis of the audio Morse code obtained from our observation revealed that the VHF signal at 110.3 MHz received in Kure was transmitted from a Localizer type Directional Aid (LDA) at Hualien Airport in Taiwan. Even though the transmission power of the LDA system is only 10 W, the transmitted radio wave should have propagated over a long distance due to the Es layer from its strong directivity. It was also observed that the approach course indicated by the receiver often fluctuates very much. The statistical analysis using two years of measurement indicates that the characteristics of the fluctuation of DDM (which is identical to the angle-of-arrival of the signal) changes from case to case. The variation in the behavior of DDM values may be suggesting that the complicated spatial structure of Es has a potential to change the propagation direction of radio waves used for the ILS Localizer system.
We also check the occurrence frequency of anomalous propagation received by the ILS LOC receiver during daytime (09-21 JST) in the two summer seasons. The average occurrence rate of the signal reception of the 110.30 MHz radio from Hualien is ~0.4% of the entire daytime period, which is ~3 min per day on average. This indicates that the occurrence of anomalous Es propagation of ILS LOC radio is not rare, but should be recognized as a frequent space weather effect; thus, further detailed investigation is required for stable use of this aeronautical navigation system at VHF frequency band.
The ILS LOC, which is the focus of this study, transmits radio waves in 108-112 MHz frequency range with amplitude modulation at 90 Hz on the left side and 150 Hz on the right side of the runway as seen from the aircraft. Then, ILS provides information about the approach course to airplanes based on the difference in the intensity of these two modulations (Difference in Depth of Modulation). Since radio waves from the ILS LOC system have strong directivity, they may propagate anomalously over a long distance due to reflection by the Es layer despite its low transmission power of 10 W.
During summer seasons, we often observe a very strong signal at 110.3 MHz at Kure, which is possibly caused by anomalous long-range propagation of VHF waves due to reflection by the Es layer. Analysis of the audio Morse code obtained from our observation revealed that the VHF signal at 110.3 MHz received in Kure was transmitted from a Localizer type Directional Aid (LDA) at Hualien Airport in Taiwan. Even though the transmission power of the LDA system is only 10 W, the transmitted radio wave should have propagated over a long distance due to the Es layer from its strong directivity. It was also observed that the approach course indicated by the receiver often fluctuates very much. The statistical analysis using two years of measurement indicates that the characteristics of the fluctuation of DDM (which is identical to the angle-of-arrival of the signal) changes from case to case. The variation in the behavior of DDM values may be suggesting that the complicated spatial structure of Es has a potential to change the propagation direction of radio waves used for the ILS Localizer system.
We also check the occurrence frequency of anomalous propagation received by the ILS LOC receiver during daytime (09-21 JST) in the two summer seasons. The average occurrence rate of the signal reception of the 110.30 MHz radio from Hualien is ~0.4% of the entire daytime period, which is ~3 min per day on average. This indicates that the occurrence of anomalous Es propagation of ILS LOC radio is not rare, but should be recognized as a frequent space weather effect; thus, further detailed investigation is required for stable use of this aeronautical navigation system at VHF frequency band.