## 1 Introduction  
Global Navigation Satellite Systems (GNSS) form part of a technological basis for different applications [1]. Their data is widely used for fundamental research tasks in different fields, for example in geodynamics [2], radio propagation environment including the GNSS remote sensing (GNSS-RO) [3] and GNSS Reflectometry of Earth surface (GNSS-R) [4]. In particular, one of the important geophysical studies that was carried out on the basis of GNSS signal processing was the study of the Earth's ionosphere and upper atmosphere [5, 6, 7] and their impact on different levels of applications [8, 9, 10, 11, 12].  
In many cases the main measured parameter of satellite vehicle (SV) signals is the carrier phase that is characterized by the lowest multipath noise level and the highest measurement accuracy. Various stochastic techniques report normally distributed carrier phase noise of 2 mm and code pseudo range noise of 0.5-0.8 m [13]. Such precise measurements allow us to detect the effects of rather weak geophysical events and eventually reconstruct the structure of the ionosphere.  
Determination of optimum sensitivity of the carrier phase lock loop (PLL) is an important task for the remote sensing of the ionosphere. The PLL sensitivity depends on both the internal noises and the sampling rate of carrier phase measurements [14]. The PLLsensitivity is considered as an optimal if the probability of detection of a weak event at the background of the uninformative noise within the phase variation spectrum is the highest at a given noise level and a given sampling rate.  
On the other hand, the higher boundary in the spectrum of the phase variations (at which weak disturbances can still be detected) may be considered as the boundary of the regular ionosphere. Studies that are focused on estimation of this boundary and its dependence on observation conditions are of particular importance for geophysical research advances.  
[PERSON] and [PERSON] (2015) suggested the \"deviation frequency\" term to denote the boundary between the variable part of the amplitude and phase scintillation spectrum and the part of uninformative noise in the spectrum. Developing this idea, we suggest introducing the concept of the \"characteristic deviation frequency\" during the observation period. The characteristic deviation frequency is defined as the most probable value of the deviation frequency (fd) under current local conditions. We assume that the characteristic deviation frequency is an estimate of the optimal sensitivity of the PLL under the current conditions.  
It is worth pointing out that the spectral slope of the phase variations is mostly defined with large-scale ionospheric structures such as manifestations of the acoustic-gravity waves in the form of large-scale travelling ionospheric disturbances. Such disturbances can be caused by auroral oval border pulsations (during geomagnetic storms and substorms), earthquakes of high magnitude, tsunamis, cyclones and other power processes in the lower atmosphere. Thus, the deviation frequency as a break point between the phase variations spectra (with the spectral slope \(<0\)) and the noise part of the spectra (with the spectral slope \(-0\)) varies depending on presence or absence of the large-scale ionospheric disturbances. This allows us to assume that the deviation frequency can be considered as a promising index to estimate the boundary of non-variability of the ionosphere.  
The aim of this work is to estimate the characteristic deviation frequency under particular observation conditions. The research tasks are to reveal the features of variations of the characteristic deviation frequency for GPS, GLONASS, Galileo and SBAS satellites, types of satellite signals, time of day and the level of geomagnetic activity.