Evolution in time of the zeroth-order coefficients of the large-scale component of the BP geometric calibration. The units on the ordinate axis are ms (the TDI period is 1 ms). The units on the abscissa axis are OBMT revolutions (one revolution corresponds to approximately 6 h). The OBMT range covers the entire science acquisition period for Gaia DR1, i.e. between 25 July 2014 and 16 September 2015. Different colours are used to indicate different CCD rows (CCD rows 1 to 7 from red to violet, lighter colours for the preceding field of view, darker ones for the following field of view). Each large-scale calibration covers a time range of about 20 revolutions (5 days).