  The Near-Infrared Spectrograph (NIRSpec) on board of the James Webb Space
Telescope will be the first multi-object spectrograph in space offering
~250,000 configurable micro-shutters, apart from being equipped with an
integral field unit and fixed slits. At its heart, the NIRSpec grating wheel
assembly is a cryogenic mechanism equipped with six dispersion gratings, a
prism, and a mirror. The finite angular positioning repeatability of the wheel
causes small but measurable displacements of the light beam on the focal plane,
precluding a static solution to predict the light-path. To address that, two
magneto-resistive position sensors are used to measure the tip and tilt
displacement of the selected GWA element each time the wheel is rotated. The
calibration of these sensors is a crucial component of the model-based approach
used for NIRSpec for calibration, spectral extraction, and target placement in
the micro-shutters. In this paper, we present the results of the evolution of
the GWA sensors performance and calibration from ground to space environments.
