Design of a Nonlinear Autopilot for Velocity and Attitude Control Using Block Backstepping

We present a solution to the three-dimensional attitude and velocity control problem for an autopilot based on backstepping control of the full six-degrees-of-freedom equations of motion. The resulting controller has a very simple structure and requires only a limited amount of a priori information about the aerodynamic properties of the aircraft. It has global stabilizing properties in attitude and local stabilizing properties in velocity. A key element of the controller is the use of a spherical linear interpolation (slerp) to compute a geodesic on the manifold of unit norm quaternions in order to achieve the minimal rotation required to control the attitude. We give a proof of stability of the resulting controller and illustrate its behavior using a version of the ADMIRE model which is a realistic model of a small single engine fighter.