  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%
