2.1 ACS NIR spectrometer
TGO was launched in March 2016 and was inserted into Mars’ orbit in October 2016. Routine science began in April 2018 after 18 months of aerobraking that formed a circular 400-km orbit. ACS is a set of three high-resolution spectrometers, Near-InfraRed (NIR), Mid-InfraRed (MIR), Thermal InfraRed (TIRVIM), operating in the spectral range from 0.65 to 17 μm onboard the TGO mission.
The NIR spectrometer combines an acousto-optical tunable filter (AOTF), used as a monochromator that can be commanded to isolate a specific wavelength bandpass whose width corresponds to the free spectral range of the echelle grating that operates at high diffraction orders. It covers the 0.7–1.7 µm spectral range using diffraction orders 101 through 49 with high resolving power of about 28000 (Trokhimovskiy et al., 2015b; Korablev et al., 2018). During an occultation, ACS NIR measures ten pre-selected diffraction orders every 2 s. Extremely high spectral resolution allows ACS NIR to sound different atmospheric constituents as H2O, CO, CO2, and weak band of O2 (Korablev et al., 2018; Fedorova et al., 2020; 2021; 2022). The instantaneous FOV of NIR in the direction perpendicular to the limb is small (~0.02°) and limited by the slit width. The instantaneous FOV (IFOV) corresponds to a vertical resolution of 500–600 m at the tangent altitude of the line of sight. The time to measure one diffraction order is 0.2 s, yielding an effective vertical resolution better than 1 km for each diffraction order. The number of detector lines measured in a single occultation and its position on the detector depends on the pointing direction (NOMAD or ACS MIR-driven) and the available downlink volume. It varies from 6 to 30 lines; the solar image size on the detector limits the upper boundary. Averaging 30 detector lines does not worsen vertical resolution, since that dimension of FOV is oriented along the limb. The SNR for an individual pixel and pure solar signal in the AOTF maximum is ~600 for the MIR pointing and ~300 for the NOMAD pointing.
The calibration of the ACS NIR from laboratory measurements and first in-flight data are summarized in Trokhimovskiy et al., 2015a; (Korablev et al., 2018). It included the AOTF characterization dependence to temperature and spectral range, signal-to-noise ratio estimations, blaze function and instrumental line shape and resolving power variation with diffraction order and detector position. These calibrations were used in the retrievals of temperature and water vapor by Fedorova et al. (2020). The present study includes just one update, an improved characterization of the AOTF profile within the diffraction order 49 to better describe the order overlapping from far sidelobes. The AOTF profile at ~1.565–1.58 µm, close to the long-wavelength bound of the detector, has not been characterized in the laboratory accurately enough and was improved using in-flight solar spectra.
The data processing of ACS NIR in occultation mode was described in Fedorova et al. (2020). The transmittance was obtained as a ratio of spectra measured inside the atmosphere to reference solar spectrum that is averaged spectra outside the atmosphere (above 140-150 km) for each detector line separately. We averaged 6–25 lines inside the solar disc with the maximal signal, excluding the solar image edges, resulting in an SNR of 800–3000, depending on the occultation. Another major improvement compared to previous retrievals, the geometry of the observations was calculated with the latest version of the attitude kernels. The TGO onboard clock was recently found to have a gradually increasing shift with respect to the Universal Time Coordinated (UTC), up to several seconds at maximum, but used here geometry has correction of this anomaly.