# Development and Performance Assessment of a Low-Cost UAV Laser Scanner System (LasUAV)
## Materials and Methods
### Angular Static and Dynamic Setups: Laser Scanner and INS Measurement Uncertainty Analysis  
In angular static and dynamic conditions, to characterize the uncertainty in planimetric measurements, (X,Y) is difficult because the LasUAV is an aerial system geared to near-nadir flight. For this reason, only uncertainty in elevation measurements (Z) originating from the laser scanner in S1 and S2 was assessed.  
According to Equation (1), three variables originating from the laser scanner are included in the LiDAR equation: the distance from the point of emission of the laser beam (\(\overrightarrow{\rho}\)), which is dynamic, i.e., it changes in time during the scanning; the laser scan angle \(\upbeta\), which is dynamic; and the layer transversal beam divergence angle \(\upalpha\), which is fixed, i.e., it does not change. Angles \(\upbeta\) and \(\upalpha\) are assumed to be correct, since they are related to the physical design of the instrument and to the geometric alignment of the laser sensor and the scanning mirrors. Distance measured by the laser scanner may be impacted by the accuracy of the time measurement of each pulse. For these reasons, the uncertainty in Z measurements of the laser scanner was assessed keeping the LasUAV in a stationary position pointing to a flat surface located at a known distance of 317.0 cm (Figure 6), measured by the DISTO D2 laser distance meter (Leica Geosystems AG,, Heerbrugg, Switzerland).  
According to Equation (1), the INS provides values of the yaw, pitch, and roll to be imputed in the LiDAR equation. In S2, angular dynamic conditions were generated by varying the pitch and roll of the LasUAV, pointing to the same flat surface located at the same known distance, in the range of \(+3^{\circ}\) and \(-3^{\circ}\), which corresponds to the typical range in operational flight conditions on multirotor UAV platforms.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Type of Scenario/Setup** & **Component of LasUAV** & **Parameters Characterizing Uncertainty** \\  & **under Assessment** & **and Error** \\ \hline S1: Angular static & Laser scanner & \(\upsigma\), RMSE, MAE \\ S2: Angular dynamic & Laser scanner + INS & \(\upsigma\), RMSE, MAE \\ S3: In-flight & Laser scanner + INS + GNSS RTK & \(\upsigma\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Three setups to assess the accuracy of LasUAV components and parameters to characterize uncertainty. RMSE, root mean-squared error; MAE, mean absolute error.  
Figure 6: LasUAV maintained in a stationary position pointing to a flat floor located 317.0 cm from the point of emission of the laser pulses.  
For S1 and S2 setups, we started from the assumption that in both cases, with LasUAV stationary and moving, the floor is horizontal and flat. The standard deviation and MAE of Z represent the uncertainty and accuracy, respectively, of the laser scanner or the uncertainty and accuracy of the laser scanner and INS when data from both sensors are coupled.  
Two subsets were generated from the same acquisition and used to assess the uncertainty in angular static and dynamic conditions (Figure 7).