# UAV-Based Airport Lights Inspection Without GNSS Positioning Support
## 4 Results
The mean error of determining the transition angles during the tests does not exceed 0.1 degrees in comparison with certified measurements--see Table 3. Reference values for the airport differ slightly, and in this case, the discrepancy rises to 0.16 degrees. PAPI units 3 and 4 have distorted colour transitions, and this, in a way, introduces extra errors. Extended statistical analysis is not justified due to the small number of tests carried out. The tests prove that changes in ascent speed and light conditions do not significantly change the error rates and all results fall within ICAO regulations. The weather and lightning conditions during the tests are common for airport maintenance actions. No tests were carried out in adverse weather conditions as UAV flight regulations do not permit missions in such weather due to operator safety concerns. The introduction of the designed PAPI light colour transition angle measurement procedure for measurement practice will require a UAV mission consisting of several ascents in order to collect a statistically significant set of results. The error of evaluating the vertical view angle, colour transition angle Equation (2), using this procedure satisfies the requirements of the ICAO regulations. The procedure can be applied to the inspection of ALS and runway lights; in this case, the UAV mission was a horizontal flight along the centre line of the runway starting a few hundred metres from the first crossbar. The light-tracking function is crucial for the inspection. Correct tracking requires a low stable flight speed, obtaining a large number of images for enhancing the robustness of light position calculations. The goal of ALS and runway light inspection is to identify the positions of broken-down lights or lights with significantly lower illumination values. The accuracy of determining positions can be lower than in the case of PAPI inspection. Figure 10 illustrates the results of determining light positions in two example missions, run in different weather con \begin{table}
\begin{tabular}{c c c c} \hline
**Flight** & **Certified Values**\([^{\circ}]\) & **Measured Values**\([^{\circ}]\) & **Errors**\([^{\circ}]\) \\ \hline
01 & & 2.61; 2.88; 3.36; 3.64 & 0.19; 0.18; 0.13; 0.11 \\
06 & & 2.46; 2.72; 3.27; 3.53 & 0.04; 0.02; 0.04; 0.00 \\
08 & 2.42; 2.70; & 2.32; 2.56; 3.46; 3.67 & \(-\)0.10; \(-\)0.14; 0.23; 0.14 \\
09 & 3.23; 3.53 & 2.49; 2.63; 3.41; 3.71 & 0.07; \(-\)0.07; 0.18; 0.18 \\
17 & & 2.43; 2.71; 3.28; 3.54 & 0.01; 0.01; 0.05; 0.01 \\
18 & & 2.43; 2.76; 3.25; 3.51 & 0.01; 0.06; 0.02; \(-\)0.02 \\
19 & & 2.44; 2.68; 3.25; 3.52 & 0.04; \(-\)0.02; 0.02; \(-\)0.01 \\ \hline \end{tabular}
\end{table} 
Table 2: PAPI light colour transition angles. \begin{table}
\begin{tabular}{c c c} \hline
**Certified Values**\([^{\circ}]\) & **Mean Values**\([^{\circ}]\) & **Mean Errors**\([^{\circ}]\) \\ \hline
2.42; 2.70; 3.23; 3.53 & & 0.03; 0.01; 0.10; 0.06 \\
**Reference Values**\([^{\circ}]\) & 2.45; 2.71; 3.33; 3.59 & \\
2.50; 2.83; 3.17; 3.50 & & \(-\)0.05; \(-\)0.12; 0.16; 0.09 \\ \hline \end{tabular}
\end{table} 
Table 3: Mean errors of PAPI light colour transition angles. ditions. The light line up deviates from the reference positions but keeps an unambiguous direction differing by a small angle from the reference line of positions. Table 4 gives a summary of the detection results. The example missions, as in the case of PAPI, were carried out at night. All operating lights were successfully detected. The mission results include the registration of illumination changes as the observation angles of the light change during the UAV flight. Additional analysis of the light illumination changes during the observations can be performed to assess the angles of illumination.