Effects of Selected Roofing Materials and Angle of Incidence on Navaids Signal Strength

Interference that causes partial loss of intelligence in air navigation signal is largely dependent on the environment around radio navigation aids (navaids). Buildings around airports have been restricted partly because they pose a technical threat to flight navigation. Previous studies have shown that about 50% of air accidents occur during landing. However no data has been availed to determine the contribution of navaids to these accidents. The purpose of this paper was to determine the effects of roofing materials on air navigation signal strength. Radio transmitters, receivers and computers were used in a laboratory to measure signal level transmitted through six different roofing materials at a frequency of 9.4GHz. Decra offered the highest attenuation whereby 90% of the signal propagated was lost, out of which 60% was due to reflection. The equivalent transmission path field strength was 57dBmV/M against an International Civil Aviation Organization (ICAO) recommended minimum specification of minus 28dBmV/M. Similarly decra exhibited desired-to-undesired signal ratio of minus 16dB against ICAO recommended value of 20dB. Highest and lowest reflections occurred at angles of incidence of 90 and 135 degrees respectively. Generally roofing materials had little effect on navaids signal strength in the transmission paths but had significant effect in the reflection paths. Highly reflective roofing materials such as steel and decra are not recommended for use in aerodrome areas. Based on these findings, building industry and flight navigation authorities have been challenged to develop a compromise roofing material. Keywords: Interference, Navaids, Propagation, Signal strength, Aerodrom