ENTRY TRAJECTORY TRACKING LAW VIA FEEDBACK LINEARIZATION

As a step toward extending the two-dimensional (longitudinal) entry pr edictive/tracking guidance scheme used by the U.S. Space Shuttle Orbit er to three dimensions, a control law for tracking a three dimensional entry trajectory is designed. The tracking law commands the angles of attack and of bank that are required to follow a ground track specifi ed as a function of energy. Feedback linearization is used to design t he tracking law. Some extensions to the existing theory are required t o accommodate features of the entry tracking problem. Downrange and cr ossrange angles serve as output variables for the feedback linearizati on and lead to state and control transformations that convert the entr y dynamics to an equivalent linear system in an approximate sense, whi ch is defined. A feedback tracking law is then designed, taking advant age of the linear structure of the system dynamics in the transformed variables. This tracking law is shown to achieve bounded tracking of t he output variables. Simulation results indicate the effectiveness of the tracking law in compensating for initial offsets from a reference trajectory.