Distribution of uncertainty on the weighted mean G value as a function of the same G magnitude. The orange line shows the mode of the distribution. This plot is restricted to all sources with between 90 and 110 G observations. The magenta line shows the equivalent results from Gaia DR1 as shown in Fig. 17 of Evans et al. (2017). The green line shows the expected uncertainties for sources with 100 G-band CCD transits and for a nominal mission with perfect calibrations. The red line shows the same uncertainty function, but with a calibration uncertainty of 2 mmag added in quadrature to the individual observations. The dashed black line has a slope of 0.4 and indicates that the faint end is sky dominated. The distribution has been normalized along the magnitude axis, i.e. scaled so that each magnitude bin has the same number of sources in order to show features along the whole magnitude range.