1. Department of Electronic Engineering, Tsinghua University, Beijing 100084, China
2. Department of Information Engineering, Niigata University, Niigata 950-2181, Japan
3. Computational Physics Inc., Springfield, VA 22151-2110, USA
4. Canada Centre for Remote Sensing, Natural Resources Canada, 588 Booth Street, Ottawa, ON, Canada K1A OY7
5. UIC-ECE Communications, Sensing & Navigation Laboratory, University of Illinois at Chicago, 900 W. Taylor Street, SEL (607) W-4210, M/C 154, Chicago, IL 60607, USA