  A tremendous international effort is currently dedicated to observing the
so-called primordial B modes of the Cosmic Microwave Background (CMB)
polarisation. If measured, this faint signal imprinted by the primordial
gravitational wave background, would be an evidence of the inflation epoch and
quantify its energy scale, providing a rigorous test of fundamental physics far
beyond the reach of accelerators. At the unprecedented sensitivity level that
the new generation of CMB experiments aims to reach, every uncontrolled
instrumental systematic effect will potentially result in an analysis bias that
is larger than the much sought-after CMB B-mode signal. The absolute
calibration of the polarisation angle is particularly important in this sense,
as any associated error will end up in a leakage from the much larger E modes
into B modes. The Crab nebula (Tau A), with its bright microwave synchrotron
emission, is one of the few objects in the sky that can be used as absolute
polarisation calibrators. In this communication, we review the best current
constraints on its polarisation angle from 23 to 353 GHz, at typical angular
scales for CMB observations, from WMAP, IRAM XPOL, Planck and NIKA data. We
will show that these polarisation angle measurements are compatible with a
constant angle and we will present a study of the uncertainty on this mean
angle, making different considerations on how to combine the individual
measurement errors. For each of the cases, the potential impact on the CMB
B-mode spectrum will be explored.
