UAS Based Methodology for Measuring Glide Slope Angles of Airport Precision Approach Path Indicators (PAPI)

The Precision Approach Path Indicator (PAPI) is a Visual Glide Slope Indicator that uses a two-color light projector system to produce a visual glidepath for pilots approaching a runway. This paper reports on a methodology for using an Unmanned Aircraft System (UAS) to measure angles where PAPI lights transition from white to red to assess compliance with the US Federal Aviation Administration (FAA) Order JO6850.2B. The UAS captured images from a series of elevations at varying distances from the PAPI lights on runway 10 at the Purdue University Airport on June 27, 2019. Precise location of each image was obtained by mounting a survey prism on the UAS and using ground-based surveying total stations to record the x, y, z location of the UAS. Two 15-minute UAS missions collected 70 images that were synchronized with total station readings. UAS images were classified by the number of white and red PAPI lights visible. A logit model was used to estimate the glidepath transition angle of the four PAPI Light Housing Assemblies, which were compared with the angle and tolerance of transition angles defined in FAA Order JO6850.2B. A second independent robotic total station measured the location of the UAS when images were taken. Comparing the two independent total station measurements, the root mean squared error of the UAS position on the glide slope was 1.3 arc-min. The paper concludes that low cost UAS with a total station can quickly and accurately evaluate PAPI lights to determine if they are aimed within the prescribed FAA tolerance.