Robust Trajectory Tracking Controller for Vision Based Probe and Drogue Autonomous Aerial Refueling Monish D. Tandale * , Roshawn Bowers † and John Valasek ‡ Texas A&M University, College Station, TX 77843-3141 This paper addresses autonomous aerial refueling between an unmanned tanker aircraft and an unmanned receiver aircraft using the probe-and-drogue method. An important consideration is the ability to achieve successful docking in the presence of exogenous inputs such as atmospheric turbulence. Practical probe and drogue autonomous aerial refueling requires a reliable sensor capable of providing accurate relative position measurements of sufficient bandwidth, integrated with a robust relative navigation and control algorithm. This paper develops a Reference Observer Based Tracking Controller that does not require a model of the drogue or presumed knowledge of its position, and integrates it with an existing vision based relative navigation sensor. A trajectory generation module is used to translate the relative drogue position measured by the sensor into a smooth reference trajectory, and an output injection observer is used to estimate the states to be tracked by the receiver aircraft. 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 that the system is robust to sensor noise, atmospheric turbulence, and high frequency unmodeled dynamics. Feasibility and performance of the total system is demonstrated by simulated docking maneuvers of an unmanned receiver aircraft docking with the non-stationary drogue of an unmanned tanker, in the presence of atmospheric turbulence. Performance characteristics of the vision based relative navigation sensor are also investigated, and the total system is compared to an earlier version. Results presented in the paper indicate that the integrated sensor and controller enable precise aerial refueling, including consideration of realistic measurements errors, plant modeling errors, and disturbances. I. Introduction Boom-and-receptacle and probe-and-drogue are the hardware configurations and methods commonly used for aerial refueling. 1 In the former, a refueling boom on the rear of the tanker aircraft is steered into the refueling port on the receiver aircraft. In this method, the job of the receiver aircraft is to maintain proper position with respect to the tanker, and research has been done for applying this method to unmanned air vehicles. 2–4 With the probe and drogue method, the tanker aircraft trails a hose with an aerodynamically stabilized flexible “basket” or drogue. The receiver aircraft has a probe which must be placed or docked into the drogue. This is the preferred method for small, agile aircraft because the equipment is small and lightweight, and a human operator is not required on the tanker aircraft. This is the refueling method considered in the present research, and studies have been conducted using this method for the refueling of unmanned air vehicles using a vision-based sensor. 1, 5–7 The controllers developed in these studies were a Non-Zero Set Point Controller with Control Rate Weighting (NZSP-CRW) that is most suitable for tracking a relatively stationary drogue, and a Proportional Integral Filter - Command Generator Tracker (PIF-CGT) that generates a reference trajectory is generated by a model of the drogue with known inputs, and is suitable * Graduate Research Assistant and Doctoral Candidate, Flight Simulation Laboratory, Aerospace Engineering Department. Student Member, AIAA. monish@neo.tamu.edu † Graduate Research Assistant, Flight Simulation Laboratory, Aerospace Engineering Department. Student Member, AIAA. roshawn-bowers@tamu.edu Currently Flight Control Engineer I with Northrop-Grumman Corporation, El Segundo, CA. ‡ Associate Professor & Director, Flight Simulation Laboratory, Aerospace Engineering Department. Associate Fellow, AIAA. valasek@tamu.edu, http://jungfrau.tamu.edu/valasek/ 1 of 16 American Institute of Aeronautics and Astronautics AIAA Guidance, Navigation, and Control Conference and Exhibit 15 - 18 August 2005, San Francisco, California AIAA 2005-5868 Copyright © 2005 by Monish D. Tandale, Roshawn Bowers, and John Valasek. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.