5. Simulation of NOMAD-SO

5.1. Simulation dataset
5.1. Simulation dataset This second dataset has been generated with the most precise direct model, taking into account the full non-linear radiative transfer and instrumental effects to produce synthetic transmittance, highly comparable with actual observations. Synthetic transmittances were made for real NOMAD-SO observation files using the relevant geometry and instrument parameters to attempt to include the variability inherent in the true measurements. Model atmospheres for each occultation were developed from the GEM-Mars general circulation model (Neary and Daerden, 2018; Daerden et al., 2019). The output of the model were provided for 1 Martian day every 10 solar longitude, and 48 timesteps per Martian day. Atmospheric profiles were developed for each occultation by interpolating the model temperature and pressure to the solar longitude, local solar time, latitude, longitude, and tangent altitude relative to the areoid. To construct the simulated transmittance spectra, the high resolution irradiances were computed for each occultation assuming a spherically symmetry and the tangent atmosphere developed from GEM-Mars for several different abundance of methane and water, which were simulated as constant volume mixing ratios. The spectroscopic data for methane and water were taken from HITRAN 2016 using CO2 broadening (Gordon et al., 2017; Gamache et al., 2016; Fissiaux et al., 2014). The instrument forward model was then applied to each simulation by considering the AOTF bandpass, instrument Instrument Line Shape (ILS), blaze function, spectel to wavenumber calibration, and the contribution of light coming from the main order and nearby orders. The final synthetic transmittance spectra is the ratio of this low-resolution irradiance to the top-of-atmosphere low resolution irradiance.
This second dataset has been generated with the most precise direct model,
taking into account the full non-linear radiative transfer and instrumental effects
to produce synthetic transmittance, highly comparable with actual observations.
Synthetic transmittances were made for real NOMAD-SO observation files using
the relevant geometry and instrument parameters to attempt to include the
variability inherent in the true measurements.
Model atmospheres for each occultation were developed from the GEM-Mars
general circulation model (Neary and Daerden, 2018; Daerden et al., 2019). The
output of the model were provided for 1 Martian day every 10 solar longitude,
and 48 timesteps per Martian day. Atmospheric profiles were developed for each
occultation by interpolating the model temperature and pressure to the solar
longitude, local solar time, latitude, longitude, and tangent altitude relative to
the areoid.
To construct the simulated transmittance spectra, the high resolution irra-
diances were computed for each occultation assuming a spherically symmetry
and the tangent atmosphere developed from GEM-Mars for several different
abundance of methane and water, which were simulated as constant volume
mixing ratios. The spectroscopic data for methane and water were taking from
HITRAN 2016 using CO2 broadening (Gordon et al., 2017; Gamache et al.,
2016; Fissiaux et al., 2014). The instrument forward model was then applied
to each simulation by considering the AOTF bandpass, instrument Instrument
Line Shape (ILS), blaze function, spectel to wavenumber calibration, and the
contribution of light coming from the main order and nearby orders. The final
synthetic transmittance spectra is the ratio of this low-resolution irradiance to
the top-of-atmosphere low resolution irradiance.

|  | CH [ppt] 4 | H O [ppm] 2 | fraction of CH [%] 4 | noise level |
| --- | --- | --- | --- | --- |
| Value | 0; 100; 500; 1000 | 0; 10; 100 | 1; 5; 10; 50; 100 | 0.001; 0.0001 |

[Table: Table 2: Simulation parameters. Fraction of CH4 is fraction of spectra containing methane hidden in the simulation dataset.]