Simulation bias. The mean of the residual maps smoothed to 1° is a measure of persistent error for this particular realization of systematics. The magnitude of the bias can be compared to the total error shown in Fig. I.1. The dipole residual in the 30 GHz polarization maps is consistent with a small relative calibration error between the radiometers, and reflects the degeneracy between calibration and the large scale polarization due to the Planck scan strategy. The error translates to 44 GHz by means of the polarization prior. HFI maps exhibit a ringing structure coming from the transfer function residuals and a faint ADCNL error above and to the right of the Galactic centre.