## 3 Influence Factors of the UAV-LDV System
### Testing Distance  
Remote testing allows for a wider field of view and more comprehensive coverage, which enables effective detection of a wide range of injuries. However, proximity testing is also indispensable for specific applications, such as damage detection based on mobile platforms such as UAVs or wall-climbing robots. Therefore, it is necessary to investigate the effect of testing distance on the system. Comparative experiments were conducted for typical conditions of intact, unilateral, bilateral, and multilateral debonding damage, as illustrated in Figure 6. The diagram shows the glass panel (dotted shadow), the border of the specimen (blue area), and the length of the structural adhesive where damage occurred (yellow area). The four types of structural adhesive damage conditions are as follows: intact condition, complete damage on one side, half of the damage on each of the two sides, and complete damage on all three sides. The distances tested were 1 m and 10 m from the specimen, respectively. The first-order natural frequencies of samples under each condition were measured using the nine-point method identified in Section 3.2. Photographs of the laboratory tests are shown in Figure 7. Manual tapping is employed to create an incentive for the curtain wall. As the natural frequency is an inherent property of the structure which is independent of the magnitude of the excitation force, there is no need to deliberately ensure the same force's magnitude. Table 4 compares the average first-order natural frequencies obtained from the numerical simulations with the experimental results of 1 m and 10 m ranging. The four identical  
Figure 6: Testing conditions. (**a**) Condition 1. (**b**) Condition 2. (**c**) Condition 3. (**d**) Condition 4. Figure 7: Testing photographs. (**a**) 1 m ranging fixed test. (**b**) 10 m ranging fixed test. (**c**) 1 m ranging mobile test. (**d**) 10 m ranging mobile test. The hammering excitation is labeled as an example in (**c**). panels were subjected to the same experimental procedure. The findings demonstrate that the fixed test conducted at a closer range exhibits a smaller error than the one conducted at a longer range under greater damage conditions. This is attributed to the shorter laser path, which results in a reduction in the power loss of the laser. Meanwhile, under all conditions, the mobile test consistently has a greater error than the fixed test. However, the maximum measuring error for the mobile tests was 12.91% (10 m ranging test under Condition 4). Since most curtain wall boundaries have failed, the sample is easily disrupted by external factors, resulting in unstable results. The remaining error is no more than 10%. This indicates the reliability of the UAV-LDV system for first-order natural frequency testing. The system has acceptable testing errors at various distances, but tests should be performed as closely as possible for the stability of results. The effectiveness of the UAV-LDV system in identifying the debonding damage on HFSCGCW also requires verification. The acceleration time history under four conditions is tested and analyzed using the nine-point method identified in Section 3.2. In Figure 8, the results obtained by the UAV-LDV system for detecting debonding damage on HFSCW closely align with the results predicted by numerical simulation and those obtained through fixed LDV testing. This alignment mainly refers to the intuitive indication of where the damage occurred and the clear reflection of the extent of the damage. It demonstrates the system's capability to effectively identify the occurrence and location of debonding damage. The testing distance did not significantly affect the system when using the SSD indicator to detect debonding damage.