Three-dimensional models of the CME-driven shock at four different times (11:05, 11:10, 11:15, and 11:20 UT) as viewed from the solar north pole. The blue line pointing away from the Sun is the centre line of the shock, which is used to estimate the radial speed of the shock. The blue line perpendicular to it shows an example of the distance of a point on the shock surface from the centre line. The shortest distance of each point in Fig. 6a is used to estimate the lateral expansion speed of the shock. Here, the distance between the location of the radio source in Box 1 at 11:20 UT and the centre line is shown.