## 4 Conclusions  
We suggested the concept of the characteristic deviation frequency to determine the optimal sampling rate of the GNSS carrier phase data for the ionosphere studies. Current research considers that the characteristic deviation frequency is optimal if its further increase does not provide more information on the small-scale structure of the ionosphere.  
Our case study showed that the characteristic deviation frequency tends to be stable during the short observation intervals (1 h). However, its value can vary significantly during 24 h (within 15-21 Hz). This agrees with the current knowledge on the physical ionospheric structure [24; 25; 26; 27; 28].  
The maximal probability of the characteristic deviation frequency of 20 Hz at some moments did not exceed 0.8 under geomagnetically disturbed conditions and reached 0.9-1.0 under quiet conditions. The increase in this probability can indicate the increase in the spatial-temporal stability of the ionosphere under quiet conditions. Correspondingly, its decrease implies the presence of the disturbance effects that change the phase spectrum slope. This allows us to assume that the deviation frequency can be considered as a  
Figure 7: The deviation frequency variations (**upper** panels) and variations of the characteristic deviation frequency 20 Hz (**lower** panels) on 13 April (**left**) and 16 April (**right**).  
promising index to estimate the boundary of non-variability of the ionosphere at the presence or absence of large-scale ionospheric disturbances.  
The probability distribution of the characteristic deviation frequency obtained from SBAS data differs from the probability distribution of the other GNSS signals significantly. We showed that the carrier phase noise for SBAS signals is notably higher than for the signals of other GNSS. This is in accordance with the results of [23]. It is probable that the plasmasphere could impact the characteristic deviation frequency for SBAS signals. This hypothesis should be tested with further experiments and analysis of data. Moreover, the signals of geostationary SBAS satellites passed from the same narrow angular sector. This means that spatial variability of the ionosphere due to the line-of-site movement through different ionospheric/atmospheric regions did not impact on the deviation frequency variations significantly. This circumstance is absent for the radio propagation of signals from medium-orbit (and lower orbital) GNSS constellations. The data of geostationary BeiDou satellites which is characterized by the same phase noise level as GPS/GLONASS may be used for checking this hypothesis further.  
[PERSON] developed the conceptualization of this work; [PERSON] and [PERSON] designed the experiments; [PERSON] and [PERSON] provided the data processing, and performed the simulations and the experiments; all the authors participated in the analysis of the results; [PERSON] and [PERSON] prepared the manuscript with contributions from all the authors. All authors have read and agreed to the published version of the manuscript.  
This work was performed under the Russian Science Foundation Grant No. 17-77-20005.  
Not applicable.