# Development and Performance Assessment of a Low-Cost UAV Laser Scanner System (LasUAV)
## 1 Introduction  
Laser scanning (LS), also referred to as light detection and ranging (LiDAR), is a well-established and consolidated technology used extensively for environmental sciences. LS for aerial platforms, known as airborne laser scanning (ALS), has been used since 1994, when commercial systems became available [1]. The elevation accuracy of state-of-the-art ALS systems is down to 10 cm [2, 3], while planimetric accuracy ranges between 3 and 35 cm at a 1000 m flying height, depending on the accuracy of the laser scanner sensor and inertial navigation system (INS) [3]. Nowadays, ALS is mostly used for mapping and monitoring extensive areas for topographic, urban, and hydrogeological purposes. ALS also provides data for research and operational applications related to the management of forest ecosystems, although data for forestry applications are often not acquired by dedicated flights, but are generated as a byproduct of the above-mentioned campaigns(i.e., land planning and monitoring). Therefore, they are frequently characterized by a low point density and, above all, are usually acquired in winter to minimize the disturbance of vegetation ([PERSON] et al., 2016), leading to a systematic, significant tree height underestimation ([PERSON] et al., 2016). Being expensive, ALS in forestry is only affordable for large-scale continuous forest cover (e.g., Canada, USA, Scandinavian countries) and is less convenient where the forest cover is fragmented, as in most European countries. Unmanned aerial vehicles (UAVs) are new platforms that have been used increasingly over the last decade to collect data for forest research, thanks to the miniaturization and cost reduction of Global Navigation Satellite System (GNSS) receivers, INS, computers, and remote sensing sensors ([PERSON] et al., 2016). Extremely compact laser scanners based on larger aerial versions have been developed in the last 10 years. On the basis of these developments, it became possible to operate LiDAR sensors integrated into UAVs ([PERSON] et al., 2016; [PERSON] et al., 2016). Manufacturers such as Velodyne LiDAR Inc. (San Jose, CA, USA), Routescene Inc. (Edinburgh, UK), LeddarTech Inc. (Quebec City, QC, Canada), RIEGL Laser Measurement System GmbH (Horn, Austria), and Geodetics Inc. (San Diego, CA, USA) have developed small and fully integrated LiDAR sensors for UAVs. In those cases, a number of independent elements are integrated into a single device that is able to emit a laser pulse, record the back-scattered signal, measure the distance, retrieve the plane position and attitude, and compute the echo position. However, the high cost (in the range of 60-80 k\(\xi\)) of such integrated LiDAR systems makes their regular use only possible for very large projects, and the cost per unit of mapped surface is still higher than airborne systems. For these reasons, the use of such systems in environmental monitoring and forestry is still greatly limited. Airborne laser scanning acquisitions still remain profitable in economic terms, but UAV-borne laser scanning acquisitions allow greater flexibility with respect to the time period of acquisition and surface to be surveyed. In fact, flight repetitions over the same area over time are easier with UAVs and make unmanned LS more adaptable than ALS to monitoring variables with a higher frequency. In contrast to the aerial platforms, the automotive markets have recently developed laser scanner devices, used for scene mapping, obstacle detection and avoidance, and autonomous driving, that have a lower cost due to the larger dimensions of the underlying market with respect to the aerial one, e.g., ibeo Automotive Systems GmbH (Hamburg, Germany), Lasea Inc. (Liege, Belgium), and Valeo Inc. (Paris, France). To be used in a laser scanner system, such laser sensors have to be integrated with other sensors, and procedures for data acquisition, synchronization, and processing have to be developed, which is quite challenging. There are few worldwide cases of LiDAR system development in research institutes.