# UAV-Based Airport Lights Inspection Without GNSS Positioning Support
## 4 Results
In order to reduce the impact of test flights on the operation of the airport, all tests were carried out late in the day with no daylight. This approach also reduces the effect of weather conditions such as sunny days and cloudy days. The lack of illumination imposes higher provisions on the sensitivity of the camera sensor but eases the light detection problem. Detection is not hindered by objects not being light sources. The problem of stray light sources remained, and the airport light detection area was carefully limited. Flights 01-09 were carried out with different ascent speeds due to changing winds, and flights 17-19 had variable sensitivity camera settings. All flights were carried out within the flight capability of the used UAV. The UAV had a protection rating of IP55--it cannot fly in heavy rain, withstands winds up to 12 m/s, and operates in temperatures of \(-\)20-50 \({}^{\circ}\)C.  
The ascent speed was kept lower than 2.6 m/s, and with changing wind, it varied, achieving values down to 1 m/s, resulting in a different video length being recorded during the inspection missions. This gives a varying number of frames for image processing, but the number, in all flights, satisfies the requirements for the correct calculation of the colour transition angles of the lights. Camera sensitivity changes impact the range of values of the light colour components, but the behaviour of the changes is retained. The ambient light conditions changed from sunset to night as the flight missions started when there was no air traffic. The PAPI lights were tracked during the ascent of the UAV. Camera positions relative to the PAPI positions give the changing view angle. Analysis of the speed of colour changes during the ascent pinpoints the transition angle. This happens when the speed reaches the highest value. Figures 8 and 9 present examples of graphs of colour changes in pairs with calculated speed changes, that is, the first derivative of the colour change function. Figure 7: Reference points for calculating the position of the camera. The graphs show the results of test flights carried out under different conditions. Flight 01 shows a set of PAPI lights in good condition. The transitions are clear; only a small region of view angles of lights 3 and 4 indicates some fault in illumination. The flight 18 results have much smaller values for the V component. The camera settings had a reduced sensitivity for recording images in order to diminish the influence of local illumination sources impeding filming. This may also simulate some unfavourable weather conditions such as clouds. Additionally, the graphs of changes in lights 3 and 4 have corrupted courses. In particular, light 4 is faulty in some way. Table 2 presents the comparison of transition angles measured during UAV flights with airport-certified PAPI data. The certified data are the results of flight inspections with manned aircraft carried out at the airport by the Polish Air Navigation Services Agency (PANSA). Figure 8: Ascent flight 01: colour change graph and peaks of transition speed with marked transition angles. Figure 9: Ascent flight 18: colour change graph and peaks of transition speed with marked transition angles. Observations during tests indicate that changing wind conditions causing the tilting of the UAV give rise to randomly changing errors. The errors of determining colour transition angles during flight speed changes are not related to a particular ascent speed. The measuring errors, especially during calm wind conditions, are stable. Missions were carried out using category 3 UAVs. These UAVs are sensitive to wind conditions due to their low weight of less than 25 kg. Reduction in camera light sensitivity decreases the error of determining the transition angle. The optimization of the settings must take into account the ambient light conditions at the airport.