Calibration of NOMAD on ESA's ExoMars Trace Gas Orbiter: Part 1 – The Solar Occultation channel

Nadir and Occultation for MArs Discovery (NOMAD) is a 3-channel spectrometer suite that is currently orbiting Mars onboard ESA's ExoMars Trace Gas Orbiter, measuring the composition of the Martian atmosphere in unprecedented detail. Of the three channels, two operate in the infrared: the Solar Occultation (SO) channel observes gas species in the 2.2–4.3 ​μm spectral region in solar occultation mode, while the Limb, Nadir and Occultation (LNO) channel observes in the 2.2–3.8 ​μm spectral region and can operate in limb-, nadir- and solar occultation-pointing modes. The Ultraviolet–VISible (UVIS) channel operates in the UV–visible region, from 200 to 650 ​nm. Both infrared channels have a spectral resolution typically an order of magnitude better than previous instruments orbiting Mars, to measure molecular absorption lines and therefore determine the abundances of constituents of the Martian atmosphere and the processes that govern their distribution and transport. To maximise the full potential of the instrument, a wide range of calibration measurements were made prior to launch and continue to be made in-flight. This work, part 1, addresses the aspects of the SO channel calibration that are not covered elsewhere, namely: the SO channel ground calibration setup, boresight pointing vector determination, detector characterisation, detector illumination pattern and saturation levels, and an investigation of the instrument line shape. An accompanying paper, part 2, addresses similar aspects for LNO, the other infrared channel in NOMAD (Thomas et al., 2021, this issue).