Visual pose stabilization of tethered small unmanned aerial system to assist drowning victim recovery

This paper proposes a method for visual pose stabilization of Fotokite, a tethered small unmanned aerial system, using a forward facing monocular camera. Conventionally, Fotokite stabilizes itself only relative to its tether and not relative to the global frame. It is, therefore, susceptible to environmental disturbances (especially wind) or motion of its ground station. Related work proposed visual stabilization for unmanned aerial systems using a downward facing camera and homography estimation. The major disadvantage of this approach is that all the features used in the homography estimation must be in the same plane. The method proposed in this paper works for features in different planes and can be used with a forward-facing camera. This paper is the part of a bigger project on saving drowning victims using lifesaving unmanned surface vehicle visually servoed by Fotokite to reach the victims. Some of the used algorithms are motion sensitive and, therefore, it is desirable for Fotokite to keep its pose relative to the world. The method presented in this paper will enable to prevent gradual drifting of Fotokite in windy conditions typical for coastal areas or when the ground station is on a boat. The quality of pose stabilization was quantitatively analyzed in 9 trials by measuring metric displacement from the initial pose. The achieved mean metric displacement was 34 cm. The results were also compared to 3 trials with no stabilization.