The accuracy of the NIRSpec grating wheel position sensors

We present a detailed analysis of measurements collected during the first ground-based cryogenic calibration campaign of NIRSpec, the Near-Infrared Spectrograph for the James Webb Space Telescope (JWST). In this paper we concentrate on the performances of the NIRSpec grating wheel, showing that the magneto-resistive position sensors installed on the wheel provide very accurate information on the position of the wheel itself, thereby enabling an efficient acquisition of the science targets and a very accurate extraction and calibration of their spectra.