Improved Tau-Guidance and Vision-aided Navigation for Robust Autonomous Landing of UAVs Amedeo Rodi Vetrella, Inkyu Sa, Marija Popovi´ c, Raghav Khanna, Juan Nieto, Giancarmine Fasano, Domenico Accardo and Roland Siegwart Abstract In many unmanned aerial vehicle (UAV) applications, flexible trajectory generation algorithms are required to enable high levels of autonomy for critical mission phases, such as take-off, area coverage, and landing. In this paper, we present a guidance approach which uses the improved intrinsic tau guidance the- ory to create spatio-temporal 4-D trajectories for a desired time-to-contact with a landing platform tracked by a visual sensor. This allows us to perform maneuvers with tunable trajectory profiles, while catering for static or non-static starting and terminating motion states. We validate our method in both simulations and real plat- form experiments by using rotary-wing UAVs to land on static platforms. Results show that our method achieves smooth landings within 10cm accuracy, with easily adjustable trajectory parameters. 1 Introduction Unmanned aerial vehicles (UAVs) play a significant role in providing services and enhancing safety thanks to their flexible and low-cost surveillance, monitoring, and risk assessment capabilities. In many emerging applications, including environmen- tal monitoring [3], industrial inspection [4], and emergency response [13], high lev- els of system autonomy are vital to guarantee safe and reliable operation. This is especially true when UAVs act as sentinels monitoring large areas, which take-off from a nest and return to it for recharging before performing further tasks. A typical mission can be divided into the following phases: (1) take-off, (2) mid- course, (3) area scanning [14, 15], (4) visual tracking, and (5) landing. Often, the final phase is the most critical as it involves performing delicate maneuvers; e.g., landing on a station for re-charging [2] or on a ground carrier for transportation [8]. These procedures are subject to constraints on time and space, and must be robust to changes in environmental conditions, such as visibility and wind disturbances [6]. To achieve smooth landings, precise sensing and accurate control techniques are therefore required. A. R. Vetrella ( ), G. Fasano, D. Accardo University of Naples “Frederico II” E-mail: amedeorodi.vetrella@unina.it I. Sa, M. Popovi´ c, R. Khanna, J. Nieto, R. Siegwart Autonomous Systems Lab, ETH Z ¨ urich 1