Redundant-Baseline Calibration of the Hydrogen Epoch of Reionization
  Array
  In 21 cm cosmology, precision calibration is key to the separation of the
neutral hydrogen signal from very bright but spectrally-smooth astrophysical
foregrounds. The Hydrogen Epoch of Reionization Array (HERA), an interferometer
specialized for 21 cm cosmology and now under construction in South Africa, was
designed to be largely calibrated using the self-consistency of repeated
measurements of the same interferometric modes. This technique, known as
"redundant-baseline calibration" resolves most of the internal degrees of
freedom in the calibration problem. It assumes, however, on antenna elements
with identical primary beams placed precisely on a redundant grid. In this
work, we review the detailed implementation of the algorithms enabling
redundant-baseline calibration and report results with HERA data. We quantify
the effects of real-world non-redundancy and how they compare to the idealized
scenario in which redundant measurements differ only in their noise
realizations. Finally, we study how non-redundancy can produce spurious
temporal structure in our calibration solutions--both in data and in
simulations--and present strategies for mitigating that structure.
