Effects of roofing materials on propagation of air navigation signals

Summary

The findings of this study concur with Laws of Kenya civil aviation act number 21 (2013) on restriction of structures around designated operational areas of aerodromes and flight paths. It also concurs with International Civil Aviation Organization annex 14 Vol.1 (2009) on civil aviation security regulations for protection of airports, aircrafts and navigation facilities. This concurrency means that highly reflective roofing materials are significant hazards to air transport.  According to NIST and Pauli et al (2008), metals offer higher reflectance to radio signals than non-metallic materials which offer higher absorption. This study generally concurs that metals reflect more than non-metals. The findings show that decra, steel and aluminum are the most reflective. Plastic iron and clay are least reflective.  Decra is a metal alloy coated with stone dust. A closer look shows that there are shinny metallic pigments in the coating which probably makes it more reflective than aluminum. Iron sheets used in roofing are galvanized (CGI) which probably makes them less reflective than plastic which appears polished and shiny. These two factors make a difference in reflectance trend.

The analysis found that highly reflective materials such as decra and steel have high attenuation effect. High reflection is a major source of interference in Navaids signal transmission especially with the Distance Measuring Equipment where echoes create significant errors in measured distance. Therefore the effect of steel and decra on Navaids signal strength is more significant when compared to all other roofing materials. Clay and iron had least effects on signal strength. Steel and decra are considered unsuitable for use in aerodrome areas therefore not recommended.

Considering the transmission path, interaction between signal strength, materials and angle of incidence showed that decra inclined at an angle of 135 degrees offered the worst received signal strength (29dBmV/M). But iron material inclined at 90^o offered the best signal strength (89dBmV/M). ICAO recommends minimum signal strength of minus 28dBmV/M for designated operational coverage of navaids (ICAO Doc 8071, 2010). It is therefore evident that in the transmission path, all the roofing materials in this study irrespective of the angle of inclination, showed significant effects on propagation of navaids signal. However the affected signal strength remained above the recommended minimum.

But when reflection path was considered, the highest reflected signal strength was offered by steel (87dBmV/M) inclined at 90^o and the lowest was offered by clay (29dBmV/M) inclined at 135^o. Recalling that reflection is a major cause of multipath interference, it is quite clear that the reflected signal was way above the recommended minimum. Such a strong reflected signal can find its way into the transmission path and cause significant interference on the forward signal strength (Selex Inc., 2009). International Civil Aviation Organization has specified that the minimum Desired to Undesired (D/U) signal ratio should be 20 dB for air-ground communication systems (ICAO, 2012). All the roofing materials in this study fall below this specification even though iron, clay and plastic have better values compared to aluminum, steel and decra. However aluminum exhibits unique characteristics whereby its D/U value is 0 dB. It means that aluminum propagates and reflects in equal proportions. The study also showed that the major component of the undesired signal is due to reflection. Compared with the recommended minimum D/U, it was established that roofing materials have significant effects on navaids signal strength.

The study also found that effects of roofing materials on transmission distance varied depending on selected material. Signals via plastic materials decreased with distance as those via aluminum increased with distance. However, the overall effect of roofing materials on transmission distance was not significant since the received signal was within the recommended strength. The materials had path loss exponent factors of between 3.0 and 3.7; this meant that the rate at which the signal was propagated through these structures was significantly slower than in free space where exponent factor is 2.

Further, the study found that the selected roofing materials had no significant effect on wave polarization. It meant that roofing materials had little effect on radiation patterns and therefore radiation patterns cannot be altered by the presence of roofing materials in the propagation path.