Planck 2013 results. III. LFI systematic uncertainties
  We present the current estimate of instrumental and systematic effect
uncertainties for the Planck-Low Frequency Instrument relevant to the first
release of the Planck cosmological results. We give an overview of the main
effects and of the tools and methods applied to assess residuals in maps and
power spectra. We also present an overall budget of known systematic effect
uncertainties, which are dominated sidelobe straylight pick-up and imperfect
calibration. However, even these two effects are at least two orders of
magnitude weaker than the cosmic microwave background (CMB) fluctuations as
measured in terms of the angular temperature power spectrum. A residual signal
above the noise level is present in the multipole range $\ell<20$, most notably
at 30 GHz, and is likely caused by residual Galactic straylight contamination.
Current analysis aims to further reduce the level of spurious signals in the
data and to improve the systematic effects modelling, in particular with
respect to straylight and calibration uncertainties.
