1. National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181–8588, Japan
2. National Time Service Center, Key Laboratory of Precise Positioning and Timing Technology, Chinese Academy of Sciences, Xi'an 710600, China
3. INAF – Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5, I-50125 Firenze, Italy
4. Max-Planck-Institut für Radioastronomie, Auf demHügel 69, D-53121 Bonn, Germany
5. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
6. Shanghai Astronomical Observatory, 80 Nandan Road, Shanghai 20030, China
7. Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Am Mühlenberg 1, D-14476 Potsdam-Golm, Germany
8. School of Astronomy and Space Sciences, Nanjing University, Nanjing 210093, China