Optimized in-flight absolute calibration for extended CMB surveys

Accurate measurements of the Cosmic Microwave Background (CMB) anisotropy call for high precision and reliability of the in-flight calibration. For extended surveys the CMB dipole provides an excellent calibration source at frequencies lower than ~ 200 GHz; however poorly known foreground emissions, such as diffuse galactic components, complicate the signal and introduce a systematic error in the calibration. We show that introducing a weight function that takes into account the uncertainty in the a priori knowledge of the sky, allows us to substantially improve the calibration accuracy with respect to methods involving galactic latitude cuts. This new method is tested for PLANCK-LFI radiometers at 30 and 100 GHz. On short time scales (less than 1 day) the absolute calibration of each channel can be recovered with an overall 1-2% accuracy. We also consider the effect of CMB anisotropy itself on the calibration, and find that knowledge of the CMB pattern on large scales is needed to keep the short-time scale calibration accuracy within 1%.