Context : CMB experiments aiming at a precise measurement of the CMB polarization , such as the Planck satellite , need a strong polarized absolute calibrator on the sky to accurately set the detectors polarization angle and the cross-polarization leakage .
As the most intense polarized source in the microwave sky at angular scales of few arcminutes , the Crab nebula will be used for this purpose .

Aims : Our goal was to measure the Crab nebula polarization characteristics at 90 GHz with unprecedented precision .

Methods : The observations were carried out with the IRAM 30m telescope employing the correlation polarimeter XPOL and using two orthogonally polarized receivers .

Results : We processed the Stokes I , Q , and U maps from our observations in order to compute the polarization angle and linear polarization fraction .
The first is almost constant in the region of maximum emission in polarization with a mean value of \alpha _ { Sky } = 152.1 \pm 0.3 ^ { \circ } in equatorial coordinates , and the second is found to reach a maximum of \Pi = 30 % for the most polarized pixels .
We find that a CMB experiment having a 5 arcmin circular beam will see a mean polarization angle of \alpha _ { Sky } = 149.9 \pm 0.2 ^ { \circ } and a mean polarization fraction of \Pi = 8.8 \pm 0.2 % .

Conclusions :