1. Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, P. R. China
2. Department of Astronomy, Nanjing University, Nanjing 210008, P. R. China
3. Max Planck Institute for Gravitational Physics, D-30167 Hannover, Germany
4. European Space Research and Technology Center, 2200 AG Noordwijk, The Netherlands
5. China Academy of Space Science, Beijing 100094, P. R. China