A FUNDAMENTAL LIMIT TO THE ACCURACY OF ASTROMETRIC MEASUREMENTS

The gravitational field of the Galaxy, while stationary on the average , becomes nonstationary due to the motion of its constituent bodies. T herefore, a light ray passing through the Galaxy undergoes random grav itational ''refraction.'' Estimates show that the magnitude of this re fraction may reach several tens of mu as. This effect may become impor tant in the construction of fundamental astrometric catalogs of the ne xt generation using Hipparcos-type specialized astrometric satellites, thus determining the highest achievable accuracy of astrometric measu rements. Since a similar accuracy has been achieved in present-day VLB I observations, this effect must be taken into account when performing such observations.