Statistical study of the distance of closest approach of aircraft to ground based emitters

The flight safety issues associated with the exposure of aircraft to High Intensity Radiated Fields (HIRF) are being addressed by the FAA. The current HIRF environment for aircraft certification is based on a worst case evaluation of exposure, leading to a concern that the required test levels may be excessive and unnecessarily burdensome. A clearer understanding of the interaction of HIRF with aircraft is desirable, in order to understand both the likelihood of occurrences and the intensity of encounters. This report primarily examines one of the main determinants of interaction, the closest approach of an aircraft to a HIRF emitter in the course of normal flight. Areas surrounding Denver, CO and Seattle, WA were chosen as study sites. Information on emitters above 400 MHz in these areas was obtained from the Government Master File. Aircraft flight positions were obtained from System Analysis Recordings (SAR tapes) made at the Denver and Seattle En Route Centers. For each site, over 5000 flights covering a three day period were examined for proximity to emitters. Distributions of closest approach to any emitter were generated separately for Seattle and Denver. Flight operations with different beacon codes were examined for variations in the closest approach distributions. Local flight operations showed characteristics distinct from other types of flights. Minimum values of closest approach often occurred either at or very near airports. The results for Denver and Seattle shared general features but individual differences were noticeable. Furthermore, issues of resolution, data reliability and validity of generalization to other airport sites have been discussed