PSS (2021) https://doi.org/10.1016/j.pss.2021.105410

Ian R. Thomas and Shohei Aoki and Loic Trompet and Séverine Robert and Cédric Depiesse and Yannick Willame and Guillaume Cruz-Mermy and Frédéric Schmidt and Justin T. Erwin and Ann Carine Vandaele and Frank Daerden and Arnaud Mahieux and Eddy Neefs and Bojan Ristic and Laszlo Hetey and Sophie Berkenbosch and Roland Clairquin and Bram Beeckman and Manish R. Patel and Jose Juan Lopez-Moreno and Giancarlo Bellucci.

 

The Nadir and Occultation for MArs Discovery (NOMAD) instrument is a 3-channel spectrometer suite on the ESA ExoMars Trace Gas Orbiter. Since April 2018, when the nominal science mission began, it has been measuring the constituents of the Martian atmosphere. NOMAD contains three separate spectrometers, two of which operate in the infrared: the Solar Occultation (SO) channel makes only solar occultation observations, and therefore has the best resolving power (∼20,000) and a wider spectral region covering 2.2–4.3 μm. The Limb, Nadir and Occultation (LNO) channel covers the 2.2–3.8 μm spectral region and can operate in limb, nadir or solar occultation pointing modes. The Ultraviolet–VISible (UVIS) channel operates in the UV–visible region, from 200 to 650 nm, and can measure in limb, nadir or solar occultation modes like LNO.

The LNO channel has a lower resolving power (∼10,000) than the SO channel, but is still typically an order of magnitude better than previous instruments orbiting Mars. The channel primarily operates in nadir-viewing mode, pointing directly down to the surface to measure the narrow atmospheric molecular absorption lines, clouds and surface features in the reflected sunlight. From the depth and position of the observed atmospheric absorption lines, the constituents of the Martian atmosphere and their column densities can be derived, leading to new insights into the processes that govern their distribution and transport. Surface properties can also be derived from nadir observations by observing the shape of the spectral continuum.

Many calibration measurements were made prior to launch, on the voyage to Mars, and continue to be made in-flight during the science phase of the mission. This work, part 2, addresses the aspects of the LNO channel calibration that are not covered elsewhere, namely: the LNO ground calibration setup, the LNO occultation and nadir boresight pointing vectors, LNO detector characterisation and nadir/limb illumination pattern, instrument temperature effects, and finally the radiometric calibration of the LNO channel. An accompanying paper, part 1 (Thomas et al., 2021, this issue), addresses similar aspects for SO, the other infrared channel in NOMAD. A further accompanying paper (Cruz-Mermy et al., 2021, this issue) investigated the LNO radiometric calibration in more detail, approaching the work from a theoretical perspective. The two calibrations agree with each other to within 3%, validating each calibration method.

 

 lno cal21

LNO detector signal on four detector bins as the FOV is slewed across the sunlit limb of Mars. A + B: The scan direction is parallel to the long edge of the FOV (i.e. perpendicular to the Mars limb); B + D: The scan direction is perpendicular to the long edge of the FOV (i.e. parallel to the Mars limb). The observation sequence is as follows: (1) the signal on each bin is zero when viewing dark space; (2) the signal peaks as the sunlit limb enters the FOV of the bin; (3) the slew continues to the non-illuminated region of the planet and then reverses direction; (4) the signal peaks again as the FOV views the sunlit limb; (5) zero single when viewing dark space; (6) the signal peaks as the sunlit limb enters the FOV. When slewing parallel to the long edge of the FOV, the bins hit the limb at different times (B); when slewing perpendicular, the bins hit the limb at the same time (D). The sunlit limb-crossing points, shown as vertical dashed lines, are analysed to determine the pointing vector of each bin and thus the entire FOV.