Abstract
Abstract. In recent years, unmanned aerial vehicles (UAVs) have increasingly been used in various application areas, such as in the remote sensing or surveying. For these applications the UAV has to be equipped with a mapping sensor, which is mostly a camera. Furthermore, a georeferencing of the UAV platform and/or the acquired mapping data is required. The most efficient way to realize this georeferencing is the direct georeferencing, which is based on an onboard multi-sensor system. In recent decades, direct georeferencing systems have been researched and used extensively in airborne, ship and land vehicle applications. However, these systems cannot easily be adapted to UAV platforms, which is mainly due to weight and size limitations. In this paper a direct georeferencing system for micro- and mini-sized UAVs is presented, which consists of a dual-frequency geodetic grade OEM GPS board, a low-cost single-frequency GPS chip, a tactical grade IMU and a magnetometer. To allow for cm-level position and sub-degree attitude accuracies, RTK GPS (real-time kinematic) and GPS attitude (GPS compass) determination algorithms are running on this system, as well as a GPS/IMU integration. Beside the direct georeferencing, also the precise time synchronization of the camera, which acts as the main sensor for mobile mapping applications, and the calibration of the lever arm between the camera reference point and the direct georeferencing reference point are explained in this paper. Especially the high accurate time synchronization of the camera is very important, to still allow for high surveying accuracies, when the images are taken during the motion of the UAV. Results of flight tests demonstrate that the developed system, the camera synchronization and the lever arm calibration make directly georeferenced UAV based single point measurements possible, which have cm-level accuracies on the ground.
Highlights
For some years an increasing use of unmanned aerial vehicles (UAVs) is visible in many different application areas
One option to realize the georeferencing of UAV mapping data is the indirect georeferencing, based on ground control points (GCPs)
With the signal of the first shutter curtain the start of an image exposure can be assigned clearly. This signal has been lead outside the camera with a cable, which can be connected to the direct georeferencing system, where the time is logged in hard real-time, when the synchronization signal arrives
Summary
An increasing use of unmanned aerial vehicles (UAVs) is visible in many different application areas. Examples can be found in the areas of precision farming, infrastructure inspection, remote sensing or surveying. The reasons for the growing popularity of UAVs are that they simplify and improve many existing operations and that they enable new perspectives, which have sometimes even been unattainable so far. Since there exist UAVs in very different weight and size classes, we like to point out that this paper is focused on micro- and minisized aerial vehicles, having a weight limit of 5 kg and a size limit of 1.5 m (Eisenbeiss, 2009). The weight limit is important for us, since this weight limit corresponds to the requirements, which are written in the German aviation security act
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