A GNSS-FREE UNMANNED AERIAL LASER SCANNING SYSTEM
In this paper, the procedure of developing and evaluating a UAV-borne mapping system is described. The system is equipped with both a LiDAR and a camera. The system mounting parameters, as well as the intrinsic parameters of the individual sensors, are calibrated rigorously. Simultaneous calibration of the LiDAR intrinsic parameters and the LiDAR-camera mounting parameters is performed in a self-calibrating bundle adjustment with additional relative orientation constraints. A visual-inertial approach is proposed to georeference the laser scans without using a GNSS receiver. This approach is motivated not only by the interest of users in low-cost systems but also by the fact that the integrity of GNSS signals might be affected under several environmental conditions, e.g., indoors, in urban canyons, under tree canopies. It is shown that a low-cost inertial measurement unit not equipped with a dual-frequency, real-time kinematic GNSS receiver is still useful for georeferencing the laser scanning data with cm-level accuracy. The scans are also textured using the images captured by the camera, which enriches the LiDAR point clouds with spectral information.