## 2 System Description  
To ensure that the UAV system will be capable of satisfying the needs of mapping and monitoring activities, the specifications of each individual component should be investigated, i.e., whether they are commensurate with payload restrictions as well as the extent of the area to be mapped and required accuracy. The mobile mapping system used in this research includes a direct georeferencing (DG) or navigation unit based on an integrated Inertial Measurement Unit (IMU) and GNSS receiver board and one active remote sensing unit. All these components are rigidly fixed within the UAV platform as shown in Figure 1.  
For applications relying on LiDAR-based 3D point clouds, we want to maximize the flight duration and minimize the payload. Based on these requirements, in this study, the DJI M600 UAV platform is used, which is designed for professional aerial mapping applications and provides additional advantages such as safety, stability and ease of use. Its total weight with batteries is 9.6 kg and maximum take-off weight is 15.5 kg, thus allowing around 6 kg payload of sensors/equipment to be installed onboard.  
Generally, the processing of the collected LiDAR data to obtain 3D mapping frame coordinates of the scanned points requires the direct georeferencing of the mapping platform, which can be established by deriving the platform's position and orientation using an integrated GNSS/INS system. For this investigation, the Applanix APX-15 UAV is considered due to its low weight, compact size and precise positioning and orientation information. The POSPac Mobile Mapping Suite (MMS) Differential GNSS Inertial post-processing software from Applanix was used for post processing of the raw GNSS/IMU data. The accuracy attained after post-processing is 0.025\({}^{\circ}\) for pitch/roll and 0.08\({}^{\circ}\) for heading (yaw) and the position accuracy is 0.02-0.05 m [16]. The LiDAR unit used in this research is a Velodyne VLP-16 Puck HI-RES. It is a small LiDAR unit, which has 16 lasers beams that are aligned over the range of +10.00\({}^{\circ}\) to \(-\)10.00\({}^{\circ}\) that provides a total vertical field of view of 20\({}^{\circ}\) and it delivers a 360\({}^{\circ}\) horizontal field of view. It can scan up to 300,000 points per second with a range of 100 m and typical accuracy of \(\pm\)0.03 m [17]. The APX unit is fitted in a housing to accommodate a wiring harness and attached to the LiDAR unit via threaded fasteners. For storing the collected data, a Raspberry  
Figure 1: UAV-based mapping system configuration.  
Pi 3 (weighing about 50 gm) with 1.2 GHz 64-bit quad-core ARMv8 CPU is used. Its small size and light weight eases its installation on the UAV. In order to derive direct georeferencing data, the APX-15 supplies sequentially precise time pulses, known as pulse-per-second (PPS) signals, which gives the ability to generate a time-tagged point cloud. Furthermore, the APX-15 provides a navigation message, also known as GPRMC (GPS Recommended Minimum Specific) message (including information regarding position, rotation and GPS time), which is recorded over a dedicated RS-232 serial port and received by the LiDAR unit via the interface box in the form of serial data. The block diagram of the M600 UAV-based MMS, indicating triggering signals, feedback signals and communication wires/ports between sensors and power connections is shown in Figure 2.