True astrometric errors, their GBOT estimates, and the corresponding CRLB values for different values of the source-drifting parameter between zero and tentimesthe FWHM (along the U-axis in the top panel, and along the V-axis in the bottom panel). Each filled circle is the standard deviation of the “true astrometric errors” of one simulated image. Each open square and related error bar are the mean value and standard deviation of the GBOT estimates of the astrometric uncertainties. The dashed curves are the corresponding value of the CRLB computed with Eqs. (30) and (36) for the top and bottom panels, respectively. The lower curves are for bright sources of set S₁, the medium curves for sources of set S₂, and the upper curves for faint sources of set S₃ (see the details concerning the characteristics of each set in the text).