Model for the astrometric precision of individual Gaia measurements (i.e., field-of-view crossings) as a function of apparent G-band magnitude. DR3 astrometric uncertainties from B. Holl et al. (2023a) are shown in black, along with the interpolation used by the Gaiamock code (K. El-Badry et al. 2024; see Section 3). Our simple piecewise model (Equation (9); red) was fitted to the B. Holl et al. (2023a) data. For bright stars (G < 14), the astrometric uncertainty is approximately constant, whereas for fainter stars (G > 14), the astrometric uncertainty rises with G in the manner expected due to photon-counting noise.