  On April 1st, 2019, the Advanced Laser Interferometer Gravitational-Wave
Observatory (aLIGO), joined by the Advanced Virgo detector, began the third
observing run, a year-long dedicated search for gravitational radiation. The
LIGO detectors have achieved a higher duty cycle and greater sensitivity to
gravitational waves than ever before, with LIGO Hanford achieving
angle-averaged sensitivity to binary neutron star coalescences to a distance of
111 Mpc, and LIGO Livingston to 134 Mpc with duty factors of 74.6% and 77.0%
respectively. The improvement in sensitivity and stability is a result of
several upgrades to the detectors, including doubled intracavity power, the
addition of an in-vacuum optical parametric oscillator for squeezed-light
injection, replacement of core optics and end reaction masses, and installation
of acoustic mode dampers. This paper explores the purposes behind these
upgrades, and explains to the best of our knowledge the noise currently
limiting the sensitivity of each detector.
