dTEC map. The first one is the solar position. To get the solar zenith and azimuth angles at each observing site for equation \[Eq1\], we used online routine (<https://midcdmz.nrel.gov/solpos/spa.html>) described in [@Reda2004]. It calculates solar coordinates in the period from the year -2000 to 6000. The declared uncertainties are of $\pm0.0003^\circ$. Therefore, the position error of an IPP arising from this routine is negligible.

The second source of uncertainties in an IPP location on dTEC map is related to the effect of ionospheric refraction on radio ray propagation. The expressions for latitude and longitude of an IPP in equation \[Eq1\] are frequency-independent. Indeed, they are valid in the GPS measurements where electromagnetic waves at gigahertz range are exploited. The ionosphere is almost transparent in this frequency range. In our study, radio observations below 300 MHz were analyzed and the ionosphere affects the wave propagation. We can estimate the angular deviation of a radio ray introduced by the ionosphere with TIDs. @Bougeret1981 collected the peak-to-peak angular positional fluctuations from observations of different sources: solar sources (type I bursts and quiet Sun), strong radio sources (Cygnus A, Cassiopea), and radio beacons of space satellites. The fluctuations were believed to be caused by 