## 4 Experimental Results and Analysis
### Influence of \(F\) on the Correction Effect of FT  
The original displacement signal collected by the UAV has a serious drift caused by the ego-motion of the UAV during the measurement, as shown in Figure 6. The displacement time-history curve measured by the UAV is converted into the spectrum by FT (Figure 7). The figure shows that there is a peak at about 3 Hz, which is caused by structural vibration and is also the part to be studied. Moreover, there are zero drift and many burrs near 0 Hz. The main cause of the burrs is the motion of the UAV during measurement, which is a kind of low-frequency irregular motion. In order to better study the structural vibration, the signal close to 0 Hz is removed, and then, the obtained new frequency domain signal is processed by IFT to obtain the time domain signal (Figure 8), that is, the structural vibration signal without false displacement caused by the ego-motion of the UAV.  
In order to study the influence of \(F\) on correction effect of FT, firstly, the value of \(F\) is taken as 0.5, 1, 2 and 3 Hz, respectively. The corrected signals are shown in Figure 9 which shows that when \(F=0.5\) Hz, the corrected signal has many burrs and is unstable. When \(F=3\) Hz, even if the curve is smooth and there are few burrs, the characteristic of free vibration attenuation of displacement signal is not obvious. When \(F=1\) Hz, although there are burrs in the curve, it tends to be stable. The main reasons are as follows: when \(F\) is low, the low-frequency signals caused by the UAV ego-motion cannot be filtered out cleanly, resulting in poor correction effect; and when \(F\) is high enough to approach the natural  
Figure 6: Displacement signal measured by the UAV.  
frequency of the structure of 3.281 Hz, some signals caused by the structural vibration may be filtered out, resulting in poor correction effect. Therefore, the value of \(F\) is explored in the range of 1 Hz to 3 Hz to determine the value of \(F\) with the best correction effect.  
Although the burr degree of the curve decreases with the increase of \(F\) between 1 Hz and 3 Hz, the free vibration attenuation characteristics of the obtained displacement curve gradually decrease, and therefore, the index for judging correction effect in this part is the degree of consistency with the displacement signal measured by the fixed camera. The degree of consistency between two displacement signals can be expressed by the Time Response Assurance Criterion (TRAC) [40]. Therefore, a high TRAC indicates a higher degree of similarity, that is, a better correction effect. The corrected signal of FT (\(F=2\) Hz) and the measurement result of the fixed camera are shown in Figure 10. The relationship between \(F\) and TRAC is shown in Figure 11, the ordinate of which is the average TRAC of the six nodes of the model with larger amplitude (Nodes 14, 15, and 16 and 42, 43, and 44) corresponding to \(F\) (the subsequent TRACs mentioned in this paper are the average TRAC values of the six nodes with larger amplitude to ensure the reliability of the results). The figure shows that in the range of 1 Hz to 3 Hz, with increase of \(F\), the TRAC first increases and then decreases. When the TRAC is the largest, \(F\) is 1.7 Hz. Therefore, 1.7 Hz is regarded as the ideal \(F\) value.  
Figure 8: Processed signal of IFT.  
Figure 7: Processed signal of FT.  
Figure 11: The relationship between \(F\) and TRAC.  
Figure 10: Displacement signal measured by the fixed camera and corrected signal by FT (\(F=2\) Hz).  
Figure 9: Corrected signals of 4 cases: (**a**) \(F=0.5\) Hz; (**b**) \(F=1\) Hz; (**c**) \(F=2\) Hz; (**d**) \(F=3\) Hz.