rms of the positional residuals Δ between DR1 and FOV and between DR1 and RF for chip1 (circles) and chip2 (triangles) in every 1 mas bin of σ_G. Three-sigma error bars are drawn under the assumption of a normal error distribution. These data are compared with the model uncertainty σ_Δ computed with ν = 0 in Eq. (2) (dashed lines) and with a value of ν that fits the rms at σ_G < 0.5 mas (dashed-dotted lines). Solid lines show the fit function Eq. (3).