DIRECT GEOREFERENCING WITH ON BOARD NAVIGATION COMPONENTS OF LIGHT WEIGHT UAV PLATFORMS

International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012 XXII ISPRS Congress, 25 August – 01 September 2012, Melbourne, Australia DIRECT GEOREFERENCING WITH ON BOARD NAVIGATION COMPONENTS OF LIGHT WEIGHT UAV PLATFORMS Norbert Pfeifera , Philipp Gliraa , Christian Briesea,b a Institute of Photogrammetry and Remote Sensing of the Vienna University of Technology, Austria - (np, pg, cb)@ipf.tuwien.ac.at b LBI for Archaeological Prospection and Virtual Archeology, Vienna, Austria KEY WORDS: direct georeferencing, UAV, orientation, GPS/INS, IMU, photogrammetry ABSTRACT: Unmanned aerial vehicles (UAV) are a promising platform for close range airborne photogrammetry. Next to the possibility of carrying certain sensor equipment, different on board navigation components may be integrated. These devices are getting, due to recent developments in the field of electronics, smaller and smaller and are easily affordable. Therefore, UAV platforms are nowadays often equipped with several navigation devices in order to support the remote control of a UAV. Furthermore, these devices allow an automated flight mode that allows to systematically sense a certain area or object of interest. However, next to their support for the UAV navigation they allow the direct georeferencing of synchronised sensor data. This paper introduces the direct georeferencing of airborne UAV images with a low cost solution based on a quadrocopter. The system is equipped with a Global Navigation Satellite System (GNSS), an Inertial Measurement Unit (IMU), an air pressure sensor, a magnetometer, and a small compact camera. A challenge using light weight consumer-grade sensors is the acquisition of high quality images with respect to brightness and sharpness. It is demonstrated that an appropriate solution for data synchronisation and data processing allows a direct georeferencing of the acquired images with a precision below 1 m in each coordinate. The precision for roll and pitch is below 1 ◦ and for the yaw it is 2.5 ◦ . The evaluation is based on image positions estimated based on the on board sensors and compared to an independent bundle block adjustment of the images. 1 • information on the assembly for full exploitation of the quality, that can be delivered by the camera, INTRODUCTION Unmanned aerial vehicles (UAVs) are promising platforms for the provision of geo-referenced Earth Observation(s) for a number of reasons: ease of deployment, costs, and close range acquisition from an elevated position, more specifically ‘airborne vertical close range photogrammetry’. However, also oblique and horizontal viewing at large scale from elevated positions are an option. The advantages materialize in comparison to manned aerial vehicles, on the one hand, and to terrestrial elevated platforms, e.g., ladders, on the other hand. These advantages, however, only apply to relative light-weight UAVs, which are therefore restricted with respect to payload and therefore sensor quality. • the methods necessary for synchronizing and processing the data streams of the POS and image sensors, • the quality obtained by the above procedures, specified as accuracy of all elements of the exterior orientation of the acquired images. Concentration is laid on direct geo-referencing, i.e. also no exploitation of tie points is performed (integrated geo-referencing), which has the advantages and drawbacks as given in (Cramer et al., 2000). The restriction of payload advocates for a careful design of the entire system, comprising, next to the platform itself and the imaging sensor also a position and orientation (POS) component and the power supply for the sensors and the aerial vehicle. For the purpose of navigation, e.g., flying along pre-defined waypoints, or taking user control input in the form of movement direction and speed, UAVs are typically equipped with position and motion sensors. Medium weight platforms, e.g., with a payload of 15 kg, can accommodate a high quality inertial navigation system (INS) and on board storage, to compute the vehicle’s trajectory by Kalman Filtering in post-processing. For small platforms the accelerations and rotation rates are typically measured with micro-electro-mechanical systems (MEMS). In this contribution we demonstrate for a light weight UAV, a quadrocopter with platform and payload together below 1 kg, that the on board components for navigation can be used for direct georeferencing of the acquired imagery. In the remainder of the introduction related work is reviewed. For general information on UAVs the reader is refered to (Eisenbeiß, 2009) and (Everaerts, 2008). In Sec. 2 the UAV and the sensors will be described. Following, in Sec. 3, the methods for the computation of the trajectory are given, including information on the synchronization. The method of evaluation and the obtained quality are given in Sec. 4. 1.1 Related Work Direct georeferencing of UAVs, if not performed with high grade INS and differential GPS, was investigated to some extent before. Direct georeferencing can be performed with GPS and INS, but alternatives are tracking by tacheometers. (Eisenbeiß et al., 2009) investigated the accuracy of the trajectory determined with low cost GPS receivers onboard a Surveycopter 1B. A 360 ◦ prism was mounted on the UAV and its 3D position was measured with a tracking total station. The differences between direct georeferencing, using a Kalman-Filter for the integration of GPS and INS, has a std.dev. in (X, Y, Z) of of 70 cm, 40 cm, and less than 20 cm, the offset reach up to 2 m. Because of the superior accuracy of the tracking total station, this This has the advantage of having only the cameras and its mounting as additional payload, and only the existing GNSS, gyroscopes, accelerometers, air pressure sensor, and magnetometer are used for direct georeferencing. Specifically, we provide 487 International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012 XXII ISPRS Congress, 25 August – 01 September 2012, Melbourne, Australia The ‘MikroKopter’ costs about 2000 $, offers an acceptable payload (250 g) for use in photogrammetry, and the on board software is open source. The availability of the programming code of the main processing unit was essential for the aim of direct georeferencing. Modifications of the source code allowed to synchronize the camera with the sensors and to save the raw data of these sensors. The sensor data was used to continually estimate position and orientation of the UAV. The position was derived by the measurements of the GNSS-receiver (‘u-blox LEA 6S’ with SBAS/EGNOS) and the air pressure sensor, the orientation by the measurement of the IMU and the magnetometer. The raw data of the IMU and the air pressure sensor can be recorded with a frequency of 20 Hz, whereas the data of the GNSS-receiver and the magnetometer is only available a (...truncated)