Types of Roofing Materials and Air navigation Signal

Background

The Kenya civil aviation act number 21 (2013) part II article 40 provides procedure for acquisition of land for the purpose of civil aviation, part V article 56 provides orders for restriction of buildings in aviation declared areas and part V article 57 provides regulations for control of structures inside and near aerodromes. This section of legislation is meant to eliminate obstacles and other threats around airports but it lacks scientific data to support its being. 
Obstacles that affect radio navigation aid systems (navaids) are structures in the vicinity of airport flight path. These obstacles include buildings, hills and mountains, masses of water, reflective ground, clouds, snow and rain. Reflections from these obstacles in the vicinity of the runway may interfere with direct radiating beam from the Instrument Landing system (ILS) and cause the courseline to deviate from a straight line. The occurrence of interference to the ILS signal is dependent on total environment around the ILS antenna array and the characteristics of the antenna.   Any large reflecting objects including vehicles or fixed objects such as structures within the radiated signal coverage have potential to cause multipath interference to the ILS signal source and path structure (Cortesi et al., 2002; Marcum, 2002).  
Construction of buildings around airports especially along flight approach path is restricted mainly due to safety and security threats arising from accidents, terrorism and other criminal activities. One major underlying factor is that buildings distort signals radiated by navaids (Cortesi et al., 2002). It is argued that by interfering with navaids radiations, such buildings provide a window for errors that contribute to air accidents. Many of these structures reflect, refract, diffract, absorb, attenuate or augment the navaids signals. Natural obstacles such as mountains, hills, water masses and reflective grounds are minimized by ensuring that airports are sited in areas free of these obstacles (ICAO, 2013). 
However, buildings around and within the airports are unavoidable especially in a country where land is a fast diminishing commodity. Physical threats are being checked using enhanced aviation security programs and safety management systems as stipulated by International Civil Aviation Organization (ICAO) standards and recommended practices for safeguarding against acts of unlawful interference (ICAO, 2011) and airport service manual (ICAO, 2010). The civil aviation act number 21 of 2013 restricts heights of buildings around airports thus leaving roofing materials as the most significant sources of interference. 
Kebabjian (2008) in his analysis (see Fig. 1.1) showed that 51% of air accidents occur during final approach and landing. It was observed that flights maximize usage of navaids in the final stretch but no data was availed to determine the contribution of navaids to the accidents and thus the effect of obstacles on propagation of navaids signals was not determined.
 

Relationship of flight sector and accidents (Kebabjian, 2008) 
 
Previous research works by Chomba et al. (2011a), Marcum (2002) and Cortesi et al. (2002) studied the effects of some of these obstacles on microwave signal transmission but very little was done to investigate and compare the effects of particular obstacles on navaids signal strength. Similarly the effects of material obstacles on wave polarization, angle of incidence and transmission distance were not considered. Unlike the ordinary microwave signal, navaids signal intelligence is contained in its significant variables such as direction, distance, orientation and strength. It was therefore necessary to consider the effect of obstacles on these significant variables. 
This research focused on the effects of roofing materials on navaids signal transmission. It aimed at investigating the behaviour of radio navigation signals when subjected to obstacles made of aluminum, iron, steel, clay, decra and plastic. The study involved conducting experiments using the provided roofing materials where vulnerable effects with high possibility of causing interference and loss of intelligence in the propagated signals were presented. . These effects included attenuation and reflection. The variables considered were angles of incidence, wave polarization and transmission distances. The measurements were done using received signal level indication via a computer system at the East African School of Aviation (EASA) aeronautical telecommunication laboratory.