Autopilot for Landing Small Fixed-Wing Unmanned Aerial Vehicles with Optical Sensors

An autopilot for landing fixed-wing mini-unmanned aerial vehicles is presented in this paper. The objective was to land at a site, which is defined by three artificial visual markers placed by the operator in a triangular shape. Therefore, the landing can be realized without any map data or previously measured landing coordinates, as well as in the absence of Global Positioning System measurements. A visual-servoing controller has been developed, which uses direct feedback from, at minimum, an inertial navigation system, an airspeed sensor, and camera-image features. The image features are used to reconstruct the approach-path deviations based on the defined triangular marker geometry. The visual-servoing controller is based on strong linearizations, while at the same time being robust, simple to implement, and sufficiently accurate. In the final stage of the landing, the base markers of the triangle leave the camera's field of view. A single optical marker and two optical-flow sensors are applied to control the touchdown phase. The proposed configuration of the flow sensors and the use of the sensor's inherent nonlinearity lead to a simple but effective controller for a coordinated roundout and touchdown relative to the true ground. Flight-test results show the feasibility of the presented method, which can be realized with relatively low additional payload and power budget constraints and moderate computational effort.