  The straylight contamination due to the Galactic emission (GSC, Galaxy
Straylight Contamination) entering at large angles from the antenna centre
direction may be one of the most critical sources of systematic effects in
observations of the cosmic microwave background (CMB) anisotropies by future
satellite missions like {\sc Planck} and MAP. We describe here different
methods to evaluate the impact of this effect and compare it with other
systematics of instrumental and astrophysical origin. The contributions within
few degrees from the beam centre dominate the GSC near the Galaxy plane. The
antenna sidelobes at intermediate and large angles from the beam centre
dominate the GSC at medium and high Galactic latitudes. We find a GSC peak at
$\sim 15\mu$K and a GSC angular power spectrum above that of the white noise
for multipoles $\ell \lsim 10$ albeit smaller than that of CMB anisotropies by
a factor larger than $\sim 10$. Although the GSC is largest at low Galactic
latitudes, the contamination produced by far pattern features at medium and
high Galactic latitudes, peaking at $\sim 4\mu$K, has to be carefully
investigated, because the combination of low amplitude of Galaxy emission in
those regions with the extremely good nominal {\sc Planck} sensitivity
naturally makes high Galactic latitude areas the targets for unprecedentedly
precise estimation of cosmological CMB anisotropy.
