Statistics of the errors of the astrometric source parameters versus the frequency of the injected GW. Left: standard deviations of the errors. Right: averages of the errors. Top to bottom: errors in position (α and δ), parallax (ϖ), and proper motion (µ_α* and µ_δ). All statistics are normalised to the maximum astrometric amplitude of the GW effect, ∆_max . The coloured bars show the range of statistics obtained in the five simulations for each frequency (see Sect. 4); for improved visibility the coloured lines connect the average statistics in the corresponding bars. The red tick marks on both lower and upper horizontal axes of the left pictures show the positions of the GW frequencies and periods shown in Table 2.