  We describe the data processing pipeline of the Planck Low Frequency
Instrument (LFI) data processing centre (DPC) to create and characterize
full-sky maps based on the first 15.5 months of operations at 30, 44 and 70
GHz. In particular, we discuss the various steps involved in reducing the data,
starting from telemetry packets through to the production of cleaned,
calibrated timelines and calibrated frequency maps. Data are continuously
calibrated using the modulation induced on the mean temperature of the cosmic
microwave background radiation by the proper motion of the spacecraft. Sky
signals other than the dipole are removed by an iterative procedure based on
simultaneous fitting of calibration parameters and sky maps. Noise properties
are estimated from time-ordered data after the sky signal has been removed,
using a generalized least square map-making algorithm. A destriping code
(Madam) is employed to combine radiometric data and pointing information into
sky maps, minimizing the variance of correlated noise. Noise covariance
matrices, required to compute statistical uncertainties on LFI and Planck
products, are also produced. Main beams are estimated down to the -20 dB level
using Jupiter transits, which are also used for the geometrical calibration of
the focal plane.
