Radial velocity at t = 20.5 h after the CME onset for the 2006 December 13 event. Five observers (dots) with their corresponding magnetic field lines connecting with the shock front are displayed in grey. The observer, corresponding to the near-Earth spacecraft, is the one in the middle. Their corresponding cobpoints are the circles on the shock front. Black lines across them indicate the shock normal whereas magenta lines show the radial direction.