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.