Abstract
ABSTRACT Geospatial information acquired with Unmanned Aerial Vehicles (UAV) provides valuable decision-making support in many different domains, and technological advances coincide with a demand for ever more sophisticated data products. One consequence is a research and development focus on more accurately referenced images and derivatives, which has long been a weakness especially of low to medium cost UAV systems equipped with relatively inexpensive inertial measurement unit (IMU) and Global Navigation Satellite System (GNSS) receivers. This research evaluates the positional accuracy of the real-time kinematics (RTK) GNSS on the DJI Matrice 600 Pro, one of the first available and widely used UAVs with potentially surveying-grade performance. Although a very high positional accuracy of the drone itself of 2 to 3 cm is claimed by DJI, the actual accuracy of the drone RTK for positioning the images and for using it for mapping purposes without additional ground control is not known. To begin with, the actual GNSS RTK position of reference center (the physical point on the antenna) on the drone is not indicated, and uncertainty regarding this also exists among the professional user community. In this study the reference center was determined through a set of experiments using the dual frequency static Leica GNSS with RTK capability. The RTK positioning data from the drone were then used for direct georeferencing, and its results were evaluated. Test flights were carried out over a 70 x 70 m area with an altitude of 40 m above the ground, with a ground sampling distance of 1.3 cm. Evaluated against ground control points, the planimetric accuracy of direct georeferencing for the photogrammetric product ranged between 30 and 60 cm. Analysis of direct georeferencing results showed a time delay of up to 0.28 seconds between the drone GNSS RTK and camera image acquisition affecting direct georeferencing results.
Highlights
Rapid and accurate direct georeferencing has been gaining importance in many domains where unmanned aerial vehicles (UAV, referred to as drones) are seen as useful, such as in infrastructure monitoring and disaster response
The accuracy of direct georeferencing results depends on additional parameters that are untouched by the actual Global Navigation Satellite System (GNSS) real-time kinematics (RTK) performance, such as the quality of the inertial measurement unit (IMU) data and their processing, the adoption of motorized gimbals moving during the flights, the camera, the actual image acquisition in terms of image sharpness and overlap, and the photogrammetric processing, typically through Structure-from-Motion (SfM)(Sherwood et al 2018; Torres-Martínez et al 2015; Wang, Rottensteiner, CONTACT Desta Ekaso destadawit27@yahoo.com
In this study we investigate the effect of the above errors on the RTK onboard the DJI Matrice 600 Pro, and their consequences on the direct georeferencing performance
Summary
Rapid and accurate direct georeferencing has been gaining importance in many domains where unmanned aerial vehicles (UAV, referred to as drones) are seen as useful, such as in infrastructure monitoring and disaster response. UAV images are located in an Earth-fixed coordinate system by accurately measuring the position and orientation of the sensors without GCP (Mostafa and Hutton 2001) This can be achieved through high quality GNSS and IMU measurements on the UAV itself (Chiang, Tsai, and Chu 2012; Cramer et al, 2000; Mian et al 2015) to determine both the absolute positioning and the camera orientation. Given the very high cost of such sensors, a more economical solution is the use of Real-Time Kinematic (RTK) positioning, whereby the accuracy of the on-board GNSS receiver is improved via a correction signal sent by a fixed base station. Such RTK systems in theory allow the absolute position of the UAV to be determined to a few cm.
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