IPP marks overlap with each other. It should be noted that the distance between the observing sites is around 48 km. It is the shortest distance between radio observatories from which data are used in this study. The calculated distance between the IPPs at the 250-km altitude is nearly 17 km during the event. It means that the focusing conditions are almost the same, resulted in almost identical SCs.

In Figure \[Figure5\](c) the crests C1, C2 and the valley are hardly recognized. We fix the coordinates of IPPs for the instant at 08:05 UT. Then, we put them on dTEC map obtained at 08:10 UT in panel (d). There the same crests C1, C2 and the valley can be discerned clearly. We can easily demonstrate that both crest-valley-crest structures in panels (c) and (d) are the same. If we assume that TID velocity is of 200 m/s, then in 5 mins the TID wavefront would propagate at a distance of about 60 km. This distance is within one pixel of the dTEC map. Thus, we relate the TEC perturbation to the SCs.

Uncertainties in IPP positions
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There are two obvious uncertainty sources in the determination of IPP coordinates on 