  The Muon g-2 Experiment (E989) at Fermilab measures the muon magnetic
anomaly, aiming to resolve the greater than 3$\sigma$ discrepancy between the
previous measurement and the Standard Model calculation with an improved
precision of 140 part-per-billion (ppb). In E989, the muon beam is stored in a
ring magnet. The spin precession frequency $\omega_{a}$ is measured by counting
the decay positrons in 24 calorimeters, and the magnetic field is measured by
nuclear magnet resonance (NMR) probes. An in-vacuum field scanning system
consisting of NMR probes and read-out electronics has been implemented to
measure the magnetic field applied to the muon beam. An additional 378 NMR
probes, placed at fixed locations outside the vacuum chamber, monitor the field
drift in between field scans. A high-accuracy probe was designed for
calibrating the probes in the scanner. In this presentation, the magnetic field
measurement hardware system and analysis methods will be described in detail.
The progress of the Run-1 data analysis and improvements in Run-2 will be
presented as well.
