Analyzing Interference on Aviation Instrument Landing System from Frequency Modulated Broadcasting Systems: the Case of Bole International Airport

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Addis Ababa University


An Instrument Landing System (ILS) is a ground-based instrument approach system that provides precision guidance to aircrafts approaching and landing on a runway using combinations of radio signals. There are three components of ILS; namely, localizer (LOC), Glide slope (GP) and Distance Measuring Equipment (DME) or series of markers. LOC is used by the pilot to determine the location of the airplane relative to the centerline of the runway; GP provides the slope information (inclination angle) and DME distance from the beginning of the runway. Among the three ILS subsystems (components) this thesis focuses on LOC which is operating from 108.1 MHZ to 111.975 MHZ. This frequency range is prone to adjacent channel interference from Frequency Modulation (FM) broadcasting systems that are operating from 87.5MHZ to 108MHZ. In the past few years there is an increase in number of FM broadcasting radio stations in Addis Ababa. FM broadcasting stations are likely to cause intermodulation interference to the ILS LOC, hence, posing a great risk to aircraft navigation safety. This thesis addresses interference of Addis Ababa Bole international airport RWY 25 ILS localizers and analyzes the interference mechanism. The study accomplished through the establishment and implementation of models regarding the characterization of the transmitter and its signal, as well the definition of the aircraft approach path. Model development was done from the bases of field measurements of ILS LOC power level on a certain approach rout and then translates each parameter into defining the scenario. Furthermore, the assessments of interference were performed by using two criteria. One consisted on the raw analysis of the ratio between the LOC signal power and interfering signals power, and the other is based on an International Telecommunication Union (ITU) Recommendation. The results verified the non-existence of any harmful interference generated by Addis Ababa commercial FM broadcasting networks on ILS LOCs runway (RWY) 25 heading for the time being. However, the results also indicate that there is interference RWY 07, though it is not being operational for the time being.



ILS, Localizer, broadcasting, Interference., inter-modulation