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tokens: data. Therefore, as a first step, we have computed the autocorrelation function (Equation ( 9 )) on timescales of up to 100 years for different directions on the celestial sphere. These autocorrelation functions can be formally approximated by a superposition of several components decreasing exponentially with different characteristic times T i . In contrast to the standard deviation, the characteristic decay time of exponents depends not only on the direction on the celestial sphere but also on the lower limit of integration over coordinates ( R min ), as can be seen from Figure 4 . Note, that the behavior of autocorrelation functions for both galactic models (DB and BS) is similar; therefore, in the following analysis, we are restricted only by the DB model. With increasing of R min , i.e., with increasing the minimal impact parameter, the characteristic decay time of exponents becomes larger, which seems quite natural. At the same time, varying the parameters of calculation does not affect an exponential form of the normalized autocorrelation function. Figure 4. Normalized autocorrelation functions calculated for the DB model for two directions in the sky at different minimal impact parameters. For the autocorrelation function in a form of a decaying exponent , the PSD is simplywhere T is characteristic time of the exponent, and . From this formula and Figure 5, it is clearly seen that on the reasonable observing timescales (up to one hundred years) the PSD index equals −2 for any direction on the celestial sphere and does not depend on the characteristic time. Thus, if an analysis of the apparent celestial positions of extragalactic sources, including the reference sources of the ICRF, reveals a component with a slope of −2 in their power spectra, this component can be explained by the collective influence of stars and compact objects in the Galaxy. Figure 5. Spectral index of PSD calculated for two exponents with the characteristic time of 100 and 1000 days. For the analysis of astrometric observations of staggering precision, it is important to know an expected value of the jitter of reference sources, arising due to the local non-stationarity of the Galactic gravitational field, for a specific time interval. Intuitively, the larger the time interval between observations, the greater the relative offset of sources can be. If we know the behavior of the autocorrelation function of the studied process in time, we can predict the expected value of the jitter (dispersion) for any time interval. For illustration purposes, the dependence of the coefficient (Equation ( 3 )) on time is shown in Figure 6 . This coefficient has been calculated for the DB model for different directions in the sky, and for . To obtain maps of the conditional standard deviation (see Equation ( 3 )), the corresponding coefficients were determined for all sky directions for three time intervals (three months, one year, and ten years) and convolved with the