  The Planck Low Frequency Instrument will recover polarization by differencing
the outputs from radiometers sensitive to orthogonal polarizations. We contrast
the systematic errors that afflict such a system with those that affect
correlation polarimeters; the Planck design has some important advantages when
measuring very weak signals such as the CMB. We also review systematic effects
arising from the choice of scan strategy for all-sky mapping missions like
Planck.[For the Planck-LFI consortium].
