  The Planck-ATCA Co-eval Observations (PACO) have provided flux density
measurements of well defined samples of AT20G radio sources at frequencies
below and overlapping with Planck frequency bands, almost simultaneously with
Planck observations. We have observed with the Australia Telescope Compact
Array (ATCA) a total of 482 sources in the frequency range between 4.5 and 40
GHz in the period between July 2009 and August 2010. Several sources were
observed more than once. In this paper we present the aims of the project, the
selection criteria, and the observation and data reduction procedures. We also
discuss the data in total intensity for a complete sample of 189 sources with
S(20 GHz)>500 mJy, Galactic latitude |b|>5deg, and declination <-30deg, and
some statistical analysis of the spectral behaviour and variability of this
sample, referred to as the "bright PACO sample". Finally we discuss how these
data could be used to transfer absolute calibrations to ground based telescopes
using the CMB dipole calibrated flux densities measured by the Planck
satellite, and we provide some test fluxes on bright calibrators.
