1. Unmanned System Research Institute, Northwestern Polytechnical University, Xi’an, China
2. School of Electronic, Electrical, and Communication Engineering, University of Chinese Academy of Sciences, Beijing, China
3. College of System Engineering, National University of Defense Technology, Changsha, China
4. Fraunhofer Institute for Computer Graphics Research (IGD), Darmstadt, Germany
5. Department of Automation, Tsinghua University, Beijing, China