# Development and Performance Assessment of a Low-Cost UAV Laser Scanner System (LasUAV)
## Materials and Methods
### The Sensors of LasUAV and UAV
It is able to send a pulse per second (PPS) signal and a serial NMEA data stream, with the current absolute time information as the recommended minimum navigation information (RMC) to the internal timer of the laser scanner through the extended interface connector of the LUX 4L. The Zbox Pi321 pico mini PC (Zotac International Ltd., Taipei, Taiwan) is the miniaturized PC of the system, allowing for the maximum storage of 64 GB. The system components were chosen after analyzing the options available in the market among sensors in the medium-low price range, with cost containment as the main driver of the selection. Among those with the same declared accuracy, the cheapest sensor was selected. The choice of the GNSS RTK receiver was the most awkward and concerned the possibility of selecting a single or dual frequency. Considering that no long baseline (approximately more than 5000 m) between the rover and base receivers would be planned in a UAV survey, which is the condition that determines the degradation of single-frequency receivers during periods of high solar activity, an L1 GNSS RTK receiver was chosen. Reach was chosen considering that its price is 1/10 th the price of a high-end L2 GNSS RTK receiver. The resulting architecture of the system is shown in Figure 2. The GNSS time from the EVK-M8 QCAM is sent to the LUX 4L, VN-300 data are recorded in the mini PC by means of a Python [17] script, and Reach receiver data are directly registered in the record card of the rover and base modules. All integrated components of the LiDAR system are protected inside a polycarbonate box, which is installed on a rigid frame. To reduce transmitted shock and vibration from UAV rotors, the frame is connected to the UAV body using silicone rubber vibro-insulators selected according to the weight and vibration frequency of the UAV propellers. Figure 2: The architecture of the LiDAR system. The GNSS receiver sends a PPS signal in NMEA-RMC format to the laser scanner; laser scanner and GNSS/INS data are recorded in a mini PC, while GNSS RTK data are directly registered in the cards of the receivers. The weight of all sensors, including antennas, reached 1.5 kg, meeting the requirements of the maximum take-off weight of the UAV used to carry the laser scanner system, which is 5.5 kg (batteries included). The UAV, which was designed and developed by engineers involved in the project, has 12 brushless motors, a hexa-coaxial configuration, and a mono frame in polyamide powder sinterized by means of selective laser sintering (SLS). It has a stand-alone control system and an onboard navigation system, so it is able to autonomously fly via uploaded waypoints. From the hardware side, there are two electronic circuit boards. One, the flight control (FC) board (Mikrokopter Flight Ctrl V2.5), is responsible for flight dynamics and mode setup by the remote pilot. The other, the navigation control (NC) board (Mikrokopter Navi Ctrl V2.1), is responsible for managing the UAV position when the GPS mode is set up. The operating mode by the GPS allows the UAV to follow the predefined position. Whether or not the UAV is not unsettled by the pilot, the NC orders the FC to maintain the current position. Whether or not a way point and information about the route distance covered is uploaded before the flight, in automatic mode, the UAV follows the consecutive points of the waypoint list. In this case, information about height and speed are also uploaded in the NC. Flight time of the UAV is around 10 min, with a payload of 5 kg. Data acquisition can be controlled from the ground by means of Wi-Fi remote control. The LiDAR system mounted in the UAV in operating conditions is illustrated in Figure 3, while Figure 4 shows each sensor integrated in the system. For the acquisition of laser scanner LUX 4L and GNSS/INS VN-300 data, a script in Python was developed. The script runs beneath a simple graphical user interface (GUI) that allows the user to start and stop the acquisition process.