## 3 Discussion of Results
### Deviation Frequency Estimates for Different GNSS Signal Components  
Figure 3 shows the histograms of distribution of the deviation frequency (fd) for signal components L1X and L5X of GALILEO satellites during 24 h. The deviation frequency value varied within 15-22 Hz during both days. The most probable deviation frequency in either day of two and for all signal components was 20 Hz. The character of distribution shown for L1X and L5X components under the same conditions has no fundamental difference. At the same time, the carrier phase noise for the L5X component is significantly lower than for the L1X component (Table 2). Moreover, the histograms constructed for the control day of 13 April and the geomagnetically disturbed day of 16 April differ essentially. Histograms for the quiet day are characterized by more gradual rise at their left part. In contrast, the most probable deviation frequency is pronounced more clearly at the histograms for the disturbed day. Figures 4 and 5 present the similar histograms for the signal components of GLONASS satellites and GPS satellites, respectively. It is worth noting that regardless of the significant difference in the carrier phase noise (see Table 2) the results for GPS, GLONASS and GALILEO signals are quite similar. The difference in the phase noise did not affect the characteristic deviation frequency. Figure 6 shows the distribution of deviation frequency of carrier phase for L1X and L5I components of SBAS satellite signals during 24 h. The measurements were obtained for three SBAS satellites whose angular characteristics are given in Table 3.  
According to Table 3, all observed SBAS satellites were concentrated in one narrow southwest sector and at low elevation angles. Consequently, spatial variability of the ionosphere due to the line-of-sight movement through different ionospheric/atmospheric regions may not be taken into account. The histogram form in Figure 6 differs significantly from the histograms shown in Figures 3-5. The SBAS deviation frequency varied within 13-20 Hz, which is smaller than one of GPS, GLONASS or Galileo. The characteristic deviation frequency can be determined with confidence only in one case of four in Figure 6 (in panel b). For example, the characteristic deviation frequency in Figure 6a is 18 Hz with probability of 0.25, and it is 15 or 16 Hz with the probability of 0.22. In contrast, in Figure 6b the characteristic deviation frequency is pronounced much clearer at 18 Hz. Other two histograms in Figure 6 show no clear maximum. On the quiet day of 13 April we recorded smaller deviation frequencies as compared to 16 April. Under disturbed conditions the maximum is more pronounced and varied within 18-19 Hz (Figure 6b,d). Figure 4: The same as in Figure 3 but for the GLONASS signal components (Table 1). Deviation-frequency distribution on 13 April 2021 (**a**,**c**,**e**,**g**) and 16 April 2021 (**b**,**d**,**f**,**h**) for GLONASS signals. Figure 5: The same as in Figure 3 but for the signal components of GPS signals (Table 1). Deviation frequency distribution on 13 April 2021 (**a**,**c**,**e**,**g**,**i**) and 16 April 2021 (**b**,**d**,**f**,**h**,**j**) for GPS signals. Figure 7 (upper panels) shows diurnal variations in the most probable deviation frequency for all signal components and for all navigation systems (excluding SBAS). We compute one fd value per one hour. The local noon and the night period indicated by the red arrow and blue rectangle, respectively. The left panels show the results for the quiet day, and the right panels show those for the disturbed day. The lower panels of Figure 7 present the diurnal variations of the relative number of cases when _fd_ = 20 Hz, calculated hourly as follows  
\[N_{CFd}=\frac{N_{20}}{N_{TOT}} \tag{5}\]  
where \(N_{20}\) is the number of cases when _fd_ = 20 Hz during one hour; \(N_{TOT}\) is the total number of all fd values derived from all phase variations spectra for all satellites in view for all signal components and for all navigation systems (excluding SBAS) during one hour.