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We investigate the influence of random variations of the Galacticgravitational field on the apparent celestial positions of extragalacticsources. The basic statistical characteristics of a stochastic process(first-order moments, an autocorrelation function and a power spectral density)are used to describe a light ray deflection in a gravitational field ofrandomly moving point masses as a function of the source coordinates. We map a2D distribution of the standard deviation of the angular shifts in positions ofdistant sources (including reference sources of the International CelestialReference Frame) with respect to their true positions. For different Galacticmatter distributions the standard deviation of the offset angle can reachseveral tens of $\mu as$ (microarcsecond) toward the Galactic center,decreasing down to 4-6 $\mu as$ at high galactic latitudes. The conditionalstandard deviation (`jitter') of 2.5 $\mu as$ is reached within 10 years athigh galactic latitudes and within a few months toward the inner part of theGalaxy. The photometric microlensing events are not expected to be disturbed byastrometric random variations anywhere except the inner part of the Galaxy asthe Einstein--Chvolson times are typically much shorter than the jitteringtimescale. While a jitter of a single reference source can be up to dozens of$\mu as$ over some reasonable observational time, using a sample of referencesources would reduce the error in relative astrometry. The obtained results canbe used for estimating the physical upper limits on the time-dependent accuracyof astrometric measurements.

Influence of the Galactic gravitational field on the positional accuracy of extragalactic sources