1. Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, SE-439 92 Onsala, Sweden
2. Joint Institute for VLBI ERIC (JIVE), Oude Hoogeveensedijk 4, NL-7991 PD Dwingeloo, the Netherlands
3. Shanghai Astronomical Observatory, Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 200030 Shanghai, China
4. Department of Astrodynamics and Space Missions, Delft University of Technology, Kluyverweg 1, NL-2629 HS Delft, the Netherlands
5. Max-Planck-Insitut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany