1. Faculty of Computer Science, Electrical Engineering and Mathematics, University of Paderborn, D-33095 Paderborn, Germany
2. Control and Dynamical Systems, MC 107-81, California Institute of Technology, Pasadena, CA 91125, USA
3. Department of Mechanical and Aerospace Engineering, Princeton University Engineering Quad, Olden Street, Princeton, NJ 08544-5263, USA
4. Navigation and Mission Design, Jet Propulsion Laboratory, California Institute of Technology, M/S 301-140L, 4800 Oak Grove Drive, Pasadena, CA 91109, USA