The Allen Telescope Array Pi GHz Sky Survey - III: The ELAIS-N1, Coma,
  and Lockman Hole Fields
  We present results from a total of 459 repeated 3.1 GHz radio continuum
observations (of which 379 were used in a search for transient sources) of the
ELAIS-N1, Coma, Lockman Hole, and NOAO Deep Wide Field Survey fields as part of
the Pi GHz Sky Survey (PiGSS). The observations were taken approximately once
per day between 2009 May and 2011 April. Each image covers 11.8 square degrees
and has 100 arcsecond FWHM resolution. Deep images for each of the four fields
have rms noise between 180 and 310 uJy and the corresponding catalogs contain
~200 sources in each field. Typically 40 - 50 of these sources are detected in
each single-epoch image. This represents one of the shortest cadence, largest
area, multi-epoch surveys undertaken at these frequencies.
  We compare the catalogs generated from the combined images to those from
individual epochs, and from monthly averages, as well as to legacy surveys. We
undertake a search for transients, with particular emphasis on excluding false
positive sources. We find no confirmed transients, defined here as sources that
can be shown to have varied by at least a factor 10. However, we find one
source which brightened in a single-epoch image to at least six times the upper
limit from the corresponding deep image. We also find a source associated with
a z = 0.6 quasar which appears to have brightened by a factor of about three in
one of our deep images, when compared to catalogs from legacy surveys.
  We place new upper limits on the number of transients brighter than 10 mJy:
fewer than 0.08 transients / sq. deg. with characteristic timescales of months
to years; fewer than 0.02 / sq. deg. with timescales of months; and fewer than
0.009 / sq. deg with timescales of days. We also plot upper limits as a
function of flux density for transients on the same timescales.
