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For some years now, UAVs (unmanned aerial vehicles) are commonly used for different mobile mapping applications, such as in\udthe fields of surveying, mining or archeology. To improve the efficiency of these applications an automation of the flight as well\udas the processing of the collected data is currently aimed at. One precondition for an automated mapping with UAVs is that the\udgeoreferencing is performed directly with cm-accuracies or better. Usually, a cm-accurate direct positioning of UAVs is based on\udan onboard multi-sensor system, which consists of an RTK-capable (real-time kinematic) GPS (global positioning system) receiver\udand additional sensors (e.g. inertial sensors). In this case, the absolute positioning accuracy essentially depends on the local GPS\udmeasurement conditions. Especially during mobile mapping applications in urban areas, these conditions can be very challenging, due\udto a satellite shadowing, non-line-of sight receptions, signal diffraction or multipath effects. In this paper, two straightforward and\udeasy to implement strategies will be described and analyzed, which improve the direct positioning accuracies for UAV-based mapping\udand surveying applications under challenging GPS measurement conditions. Based on a 3D model of the surrounding buildings and\udvegetation in the area of interest, a GPS geometry map is determined, which can be integrated in the flight planning process, to avoid\udGPS challenging environments as far as possible. If these challenging environments cannot be avoided, the GPS positioning solution is\udimproved by using obstruction adaptive elevation masks, to mitigate systematic GPS errors in the RTK-GPS positioning. Simulations\udand results of field tests demonstrate the profit of both strategies

PRECISE POSITIONING OF UAVS – DEALING WITH CHALLENGING RTK-GPS MEASUREMENT CONDITIONS DURING AUTOMATED UAV FLIGHTS