1. Department of Space, Earth and Environment, Chalmers University of Technology , Onsala Space Observatory, SE-439 92 Onsala, Sweden
2. Shanghai Astronomical Observatory, Key Laboratory of Radio Astronomy, Chinese Academy of Sciences , 200030 Shanghai, China
3. National Radio Astronomy Observatory , P.O. Box O, Socorro, NM 87801, USA
4. Joint Institute for VLBI ERIC (JIVE) , Oude Hoogeveensedijk 4, NL-7991 PD Dwingeloo, the Netherlands
5. Faculty of Aerospace Engineering, Delft University of Technology , Kluyverweg 1, NL-2629 HS Delft, the Netherlands
6. Kavli Institute for Astronomy and Astrophysics, Peking University , 100871 Beijing, China
7. Department of Astronomy, School of Physics, Peking University , 100871 Beijing, China
8. U.S. Naval Research Laboratory , 4555 Overlook Ave SW, Washington, DC 20375, USA
9. CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China , 230026 Hefei, China
10. School of Astronomy and Space Sciences, University of Science and Technology of China , Anhui, 230026 Hefei, China