1. Graduate School of Information Science and Electrical Engineering, Kyushu University, Fukuoka, Japan
2. Research & Development Group, Hitachi, Ltd., Ibaraki, Japan
3. Centre for Applied Autonomous Sensor Systems, Örebro University, Örebro, Sweden
4. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
5. Faculty of Information Science and Electrical Engineering, Kyushu University, Fukuoka, Japan