# UAV-Based Airport Lights Inspection Without GNSS Positioning Support
###### Abstract  
The inspection of PAPI (precision approach path indicator), ALS (approach light system), and runway lights is the basis of maintenance actions that safeguard the safety of airport operations. These actions consist of measurements requiring the accurate positioning of reference viewing points. Commonly, GNSS-based methods are employed and augmentation procedures are applied to improve the positioning precision. Augmentation involves the use of additional sources of information such as reference stations and complicates the process of position calculations. An image processing-based method is proposed for triangulating the positions of reference viewing points during inspections of airfield lights using a UAV. The Structure-from-Motion (SfM) photogrammetric range imaging technique and airport geodesic data are the basis of the method. The measurement of PAPI transition angles is presented as an application example. The results are compared to GNSS-based measurements and prove the viability of the method. U  
A 2025  
condition of the lights. A corrupted transition zone indicates problems, e.g., incorrectly installed baffles in the light unit. The inspection of PAPI lights requires the determination of positions of observation points and calculations of view angles related to these points. The mapping of the positions is a demanding measuring operation involving the use of survey equipment or GNSS-based positioning. Basic GNSS data are inadequate for accurately assessing the positions, and augmentation is applied to enhance the accuracy [3]. Real-Time Kinematics (RTK) is a widely used technique. It is based on measurements of the phase of the signal's carrier wave in addition to the information content of the signal. It requires a reference station or interpolated virtual station to provide real-time corrections. As a result, up to centimetre-level accuracy is obtained [4]. The use of reference stations complicates the procedure for calculating the positions and sets higher processing demands for satisfying short response times, which ensure efficient inspections. The hardware setup includes a GNSS receiver that supports RTK functionality and a high-quality antenna to capture satellite signals with minimal interference and multipath errors. RTK is based on measuring distances to the satellites with carrier phase. Time is needed to lock to satellites and resolve ambiguities with a set confidence level. In the case of a 99.9% confidence level, the time required can surpass 100 s. The multipath reception of the satellite signals is the main source of error in determining carrier phase; additionally, interference from other communication systems and high levels of ionospheric activities need to be accounted for. Reference stations are used to assess the parameters of the signal paths and provide these as correction data for calculating positions. After locking the receiver, these correction data can be used to improve the calculations of positions, but any disruption of reception requires a re-lock. There are companies maintaining subscription services based on networks of reference stations providing real-time correction data, which are transmitted using the radio or Internet. The subscription fees can significantly influence the cost of operation of a measuring team. A solution set of linear equations comprising satellite data and correction coefficients is needed to obtain the position coordinates. The number of equations is related to the number of observed satellites and may reach a few hundred when a dozen or so satellites are observed [5]. Certified airport light inspections are carried out with specially equipped aircrafts that employ augmented GNSS-based systems for surveying and assessing light conditions. RTK is the first choice for GNSS augmentation. Precise point positioning (PPP)-based augmentation may gain popularity in the near future as the European Galileo GNSS proposed in 2023 introduced satellite-based High-Accuracy Service (HAS) supporting PPP [6].