Evaluation of ILS Radio Anomalies by Helicopter Flight Test and Numerical Analysis

The Instrument Landing System (ILS) is prone to signal errors due to such factors as ground surface characteristics and obstacles near the transmitting antennas. For evaluating such radio anomalies, this paper proposes a flight test method that exploits the particular capabilities of helicopters, such as hover, vertical climb and sideways flight, to map the ILS signal distribution around the approach path. This flight test method, named ‘ILS sweep’, was used to evaluate an ILS situated on the Mega‐Float, a large floating marine structure, which was expected to cause radio anomalies. ILS signal errors were determined using the National Aerospace Laboratory of Japan's MuPAL‐ε research helicopter, by comparing signal measurements with highly accurate position data obtained from a hybrid DGPS/INS system. Some characteristic ILS signal errors were observed such as periodic fluctuation of the glide slope signal and asymmetric variation of the localizer signal. Numerical analysis conducted by the Electronic Navigation Research Institute of Japan revealed that these anomalies were due mainly to the diffraction of radio waves by the edges of the Mega‐Float structure and by the control tower and ‘dolphin’ structures located beside its runway. The proposed ILS evaluation method was thus demonstrated to be effective for directly measuring the spatial distribution of ILS signal errors and clarifying their causes.