1. Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, 200030 Shanghai, China
2. University of Chinese Academy of Sciences, 19A Yuquanlu, Beijing 100049, China
3. Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, Nanjing 210008, China
4. Konkoly Observatory, MTA Research Centre for Astronomy and Earth Sciences, Konkoly Thege Miklós út 15-17, H-1121 Budapest, Hungary
5. Joint Institute for VLBI ERIC, Postbus 2, NL-7990 AA Dwingeloo, the Netherlands
6. Department of Astrodynamics and Space Missions, Delft University of Technology, Kluyverweg 1, NL-2629 HS Delft, the Netherlands
7. Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 305-348, Republic of Korea
8. National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan