1. School of Computer Science, Xi’an Polytechnic University, Xi’an, China
2. Istituto di Scienza e Tecnologie dell‘Informazione, ‘‘A. Faedo’’, CNR, Pisa, Italy
3. School of Computer Science and Technology, Xidian University, Xi’an, China
4. Dipartimento di Ingegneria dell’Informazione, University of Pisa, Pisa, Italy
5. Institute of navigation and intra-satellite link technology, China Academy of Space Technology, Beijing, China