Evolution in time of the first- (distributed between 4.5 and 6 ms) and second-order coefficients (distributed between −0.5 and 0.5 ms) of the large-scale component of the BP geometric calibration. The units on the ordinate axis are ms.The units on the abscissa axis are OBMT revolutions (one revolution corresponds to about 6 h). The OBMT range covers the entire science acquisition period for Gaia DR1. Colour-coding is as in Fig. 1.