1. Information Networks Division, Defence Science and Technology Organisation, Edinburgh SA 5111, Australia
2. Intelligence Surveillance Reconnaissance Division, Defence Science and Technology Organisation, Edinburgh SA 5111, Australia
3. Department of Mathematics, Southern Illinois University, Carbondale, IL 62901-4408, United States
4. Institut für Informatik und angewandte Mathematik, University of Bern, Bern CH-3012, Switzerland