Trajectory tracking controller for vision-based probe and drogue autonomous aerial refueling

Practical autonomous aerial refueling between unmanned tanker and receiver aircraft using the probe-and-drogue method requires a reliable relative position sensor integrated with a robust relative navigation and control algorithm. The system must provide successful docking in the presence of disturbances caused by both the tanker wake vortex and atmospheric turbulence. This paper develops a reference-observer-based tracking controller and integrates it with an existing vision-based relative navigation sensor. Relative drogue position measured by the sensor is transformed into a smooth reference trajectory generated onboard the receiver aircraft in real time, and an output injection observer estimates the feedforward control and the reference states to be tracked. Accurate tracking is provided by a state feedback controller with good disturbance rejection properties. A frequency-domain stability analysis for the combined reference observer and controller shows robustness to sensor noise, atmospheric turbulence, and high-frequency unmodeled dynamics. Feasibility and performance are demonstrated by simulated docking maneuvers with a nonstationary drogue in the tanker flowfield, subjected to various levels of atmospheric turbulence. Results presented in the paper show that the integrated sensor and controller enable precise aerial refueling, including consideration of realistic sensor measurements errors, plant modeling errors, and disturbances.