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