The Near Infrared Spectrograph (NIRSpec) is one of four science instruments on board the James Webb Space Telescope (JWST). NIRSpec offers multi-object, fixed slit, and integral field spectroscopy. There are eight optical elements mounted on the grating wheel assembly (GWA), six gratings, a double-pass prism, and a mirror. The precise knowledge of the position and tilt of these elements is critical for target acquisition and an accurate extraction and calibration of science data. We present the concept of calibrating the position/tilt sensors during the NIRSpec flight model ground calibration campaign, the performance of the sensors and first results concerning the GWA repeatability.