Planck 2015 results. V. LFI calibration
  We present a description of the pipeline used to calibrate the Planck Low
Frequency Instrument (LFI) timelines into thermodynamic temperatures for the
Planck 2015 data release, covering four years of uninterrupted operations. As
in the 2013 data release, our calibrator is provided by the spin-synchronous
modulation of the cosmic microwave background dipole, but we now use the
orbital component, rather than adopting the Wilkinson Microwave Anisotropy
Probe (WMAP) solar dipole. This allows our 2015 LFI analysis to provide an
independent Solar dipole estimate, which is in excellent agreement with that of
HFI and within $1\sigma$ (0.3% in amplitude) of the WMAP value. This 0.3% shift
in the peak-to-peak dipole temperature from WMAP and a global overhaul of the
iterative calibration code increases the overall level of the LFI maps by 0.45%
(30 GHz), 0.64% (44 GHz), and 0.82% (70 GHz) in temperature with respect to the
2013 Planck data release, thus reducing the discrepancy with the power spectrum
measured by WMAP. We estimate that the LFI calibration uncertainty is now at
the level of 0.20% for the 70 GHz map, 0.26% for the 44 GHz map, and 0.35% for
the 30 GHz map. We provide a detailed description of the impact of all the
changes implemented in the calibration since the previous data release.
