# Development and Performance Assessment of a Low-Cost UAV Laser Scanner System (LasUAV)
## Materials and Methods  
### The Sensors of LasUAV and UAV  
The following sensors, with descriptions of their specific characteristics, were used to develop the LasUAV laser scanner system. As an ALS system, the principle of measurement of UAV-borne laser scanning is based on laser-ranging measurements supported by position and orientation information derived with the use of a GNSS device and an INS (or an inertial measurement unit, IMU) mounted on a drone (Figure 1). The laser scanner is the LUX 4L (ibeo Automotive Systems GmbH, Germany), a multilayer and multiecho laser scanner, i.e., it scans and measures in four parallel layers, each with a nominal transversal beam divergence of 0.8\({}^{\circ}\), able to record up to three returns per pulse. The horizontal field of view (HFOV) is 110\({}^{\circ}\) (50\({}^{\circ}\) to \(-\)60\({}^{\circ}\)) in two layers and 85\({}^{\circ}\) (35\({}^{\circ}\) to \(-\)50\({}^{\circ}\)) in four layers. It can scan at a rate of 12.5, 25.0, or 50.0 Hz. The ibeo LUX 4L is able to detect targets at a maximum range of 200 m. The laser scanner was set to scan at a frequency of 12.5 Hz, the HFOV was set between \(-\)30\({}^{\circ}\) and 30\({}^{\circ}\), and the angular resolution was set at 0.25\({}^{\circ}\). Previous experience [13] demonstrated that large scan angles have a significant impact on the derivation of key metrics used in forestry. The VN-300 (VectorNav Technologies LLC, Dallas, TX, USA) is the dual antenna GNSS-aided INS of the LiDAR system. It has a three-axis accelerometer, a three-axis gyroscope, a three-axis magnetometer, a pressure sensor, and two GNSS receivers. The vector processing engine is characterized by an onboard extended Kalman filter, allowing a maximum output rate of 400 Hz. The L1 frequency GNSS receiver has a solution update rate of 5 Hz. It bases identification of the heading on the dual GNSS antenna data instead of the magnetometer that is used on single antenna units, which can be inaccurate due to the presence of metal and electrical materials of the UAV. The VN-300 was set to output GNSS/INS data at 50 Hz. The manufacturer-specified dynamic and static accuracies (RMS) in the yaw, pitch, and roll of the VN-300 are reported in Table 1.  
The GNSS RTK receiver of the LiDAR system is the Reach (Emlid Ltd., Saint Petersburg, Russia). It is made of two small modules, a rover mounted onboard the UAV, and a stationary base on the ground. Reach uses 72 tracking channels in the signals of GPS/QZSS L1, GLONA \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Dynamic accuracy (heading, true inertial) & 0.3\({}^{\circ}\) RMS \\ Dynamic accuracy (pitch/roll) & 0.1\({}^{\circ}\) RMS \\ Static accuracy (heading, GPS compass 1) & 2.0\({}^{\circ}\) RMS \\ Static accuracy (heading, magnetic 2) & 2.0\({}^{\circ}\) RMS \\ Static accuracy (pitch/roll) & 0.5\({}^{\circ}\) RMS \\ \hline \hline \end{tabular} \({}^{1}\) With a 1 m baseline, a clear view of Global Navigation Satellite System (GNSS) satellites, and a good multipath environment; 2 With proper magnetic declination, a suitable magnetic environment, and valid hard/soft calibration.  
\end{table} 
Table 1: Manufacturer-specified dynamic and static accuracies (RMS) in the yaw, pitch, and roll of the VN-300. GPS, Global Positioning System. Figure 1: The unmanned aerial vehicle (UAV) light detection and ranging (LiDAR) measurement principle (from Wikipedia Commons, file licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license). Galileo E1, and the Space Based Augmentation System (SBAS). Although Reach is able to calculate real-time coordinates with a centimeter accuracy and stream them in national marine electronics association (NMEA) or binary format to a data logger device over RS232, Bluetooth, or Wi-Fi, in LasUAV, data were recorded in a Reach card and corrected in postprocessing. The EVK-M8 QCAM (u-blox Ltd., Thalwil, Switzerland) concurrent GNSS antenna receiver was used to synchronize the LUX 4L, VN-300, and Reach data streams.