First M87 Event Horizon Telescope Results. III. Data Processing and
  Calibration
  We present the calibration and reduction of Event Horizon Telescope (EHT)
1.3mm radio wavelength observations of the supermassive black hole candidate at
the center of the radio galaxy M87 and the quasar 3C 279, taken during the 2017
April 5-11 observing campaign. These global very long baseline interferometric
observations include for the first time the highly sensitive Atacama Large
Millimeter/submillimeter Array (ALMA); reaching an angular resolution of 25
micro-as, with characteristic sensitivity limits of ~1 mJy on baselines to ALMA
and ~10 mJy on other baselines. The observations present challenges for
existing data processing tools, arising from the rapid atmospheric phase
fluctuations, wide recording bandwidth, and highly heterogeneous array. In
response, we developed three independent pipelines for phase calibration and
fringe detection, each tailored to the specific needs of the EHT. The final
data products include calibrated total intensity amplitude and phase
information. They are validated through a series of quality assurance tests
that show consistency across pipelines and set limits on baseline systematic
errors of 2% in amplitude and 1 degree in phase. The M87 data reveal the
presence of two nulls in correlated flux density at ~3.4 and ~8.3 giga-lambda
and temporal evolution in closure quantities, indicating intrinsic variability
of compact structure on a timescale of days, or several light-crossing times
for a few billion solar-mass black hole. These measurements provide the first
opportunity to image horizon-scale structure in M87.
