  To reliably detect the cosmic microwave background (CMB) anisotropy is of
great importance in understanding the birth and evolution of the Universe. One
of the difficulties in CMB experiments is the domination of measured CMB
anisotropy maps by the Doppler dipole moment from the motion of the antenna
relative to the CMB. For each measured temperature the expected dipole
component has to be calculated separately and then subtracted from the data. A
small error in dipole direction, antenna pointing direction, sidelobe pickup
contamination, and/or timing synchronism, can raise significant deviation in
the dipole cleaned CMB temperature. After a full-sky observational scan, the
accumulated deviations will be structured with a pattern closely correlated to
the observation pattern with artificial anisotropies on large scales, including
artificial quadrupole, octopole etc in the final CMB map. Such scan-induced
anisotropies on large scales can be predicted by the true dipole moment and
observational scan scheme. Indeed, the expected scan-induced quadrupole pattern
of the WMAP mission is perfectly in agreement with the published WMAP
quadrupole. With the scan strategy of the Planck mission, we predict that
scan-induced anisotropies will also produce an artificially aligned quadrupole.
The scan-induced anisotropy is a common problem for all sweep missions and,
like the foreground emissions, has to be removed from observed maps. Without
doing so, CMB maps from COBE, WMAP, and Planck as well, are not reliable for
studying the CMB anisotropy.
