Planck early results: the low frequency instrument data processing

We describe the data processing pipeline employed by the Low Frequency Instrument (LFI) Data Processing Centre (DPC) to create and characterize the frequency maps used by the ERCSC (Early Release Compact Source Catalogue) first product of Planck to become public. In particular, we discuss the various steps involved in reducing the data, starting from telemetry (TM) 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. The 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 where the sky signal is removed using a Generalized Least Square map-making algorithm. The measured 1/f noise knee-frequencies range from ~ 100 mHz at 30 GHz to a few tens of mHz at 70 GHz. 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 ? -10 dB level using Jupiter transits which are also used for the geometrical calibration of the focal plane.