Calibration of NOMAD on ExoMars Trace Gas Orbiter: Part 3 - LNO validation and instrument stability
Introduction
The ExoMars program consists of two missions designed to study the trace gases of the martian atmosphere but also to acquire information on potential ongoing geological and biological processes on the surface of Mars (Vago et al., 2015). Since April 2018, the four instruments aboard the ESA/Roscosmos ExoMars Trace Gas Orbiter mission has acquired observations of both the atmosphere and surface of Mars. Among them the NOMAD instrument (Nadir and Occultation for MArs Discover), led by the Belgian Institute for Space Aeronomy (BIRA-IASB), is a suite of three spectrometers spanning the UV and IR spectral range: SO (Solar occultation), LNO (limb, nadir, and occultation) and UVIS (ultraviolet–visible). The three channels work separately but are all controlled via a single main electronic interface (Neefs et al., 2015). The two first channels are infrared spectrometers based upon the SOIR (Solar Occultation in the InfraRed) instrument aboard the Venus Express mission (Nevejans et al., 2006).
The LNO channel is a compact high-resolution echelle grating spectrometer with an acousto-optic tunable filter (AOTF) working in the infrared domain from 2.3 μm to 3.8 μm (4250-2630 cm−1) with a resolving power (λ/Δλ) of around 10 000, specially designed for nadir observation. With such high resolving power combined with the near-circular orbit of TGO permitting 12 orbits in one sol, promoting a global coverage of the planet, the NOMAD-LNO instrument is perfectly suited to study the martian surface and atmosphere.
The main objective of this article is to propose an original calibration procedure, adaptable for the full dataset of NOMAD-LNO. This calibration is complementary to the one proposed by Thomas et al. (2021) who developed a fully empricial method using in-flight data. In their paper the LNO ground calibration, occultation and nadir boresight pointing vectors, detector characterisation and illumination pattern are covered. A combination of several observation of the sun is used to derive instrument temperature effects such as the shape and intensity of a LNO spectrum. The radiometric calibration is done by assuming temporal stability of the instrument and directly using solar observation to calibrate nadir observation.
In this paper we will not assume temporal stability of the instrument. Our approach is thus able to investigate the temporal evolution of the instrumental sensitivity, which is expected to vary due to degradation by energetic particles. This approach will be based on an empirical continuum removal to take into account the departure between actual blaze function and its theoretical form. By construction, our approach is thus more robust but may fail to model some instrumental effect such as the temperature dependence of the blaze and AOTF transfer function on the raw continuum of an LNO spectrum. The main calibration of NOMAD-LNO is well described in Thomas et al. (2021). This complementary work aims to validate the calibration of LNO but also to give additional information about instrumental transfer function and instrumental line shape.
Section snippets
NOMAD LNO instrument
The optical design of the LNO spectrometer is identical to that of SO and therefore very similar to SOIR (Nevejans et al., 2006; Vandaele et al., 2013), it is a combination of a high-dispersion echelle grating along with an AOTF and a cooled detector. The main advantage of using an echelle grating is that the full height of the detector can be used to register spectral lines (Neefs et al., 2015), which greatly improves the SNR after column binning. In the spatial direction, the detector
Solar fullscan
The NOMAD-LNO fullscans are solar observation made for calibration purposes. The instrument, normally in nadir position, is pointing toward the sun. The choice of using solar fullscans was made for two reasons. First, there are not enough miniscans to cover all diffraction orders with a significant amount of data while fullscans always cover the whole spectral range which allows testing the time dependence of the calibration. Second, it is important to estimate the instrumental sensitivity over
Method
The calibration aim was to build a model to estimate the spectral conversion (wavenumbers in cm−1 for each spectels of the detector) and the photometric sensitivity (conversion factor from ADU to spectral radiance). The model must be versatile enough to face the uncertainties of some instrumental functions, such as the AOTF transfer function and the grating blaze function.
The method used here is based on the usual comparison between a real solar observation and a simulated solar spectrum. The
Justification of the continuum removal
All modeled effects are not able to perfectly fit both the overall shape of the spectra and the absorption lines of the observation as shown in Fig. 5. In this figure, we applied the calibration procedure with/without the continuum removal (Fig. 5 A and B). One can see that without the continuum removal, the fitting procedure is dominated by the large-scale feature and unfortunately not coherent with detailed solar line shape. Such results imply that in the present state of instrument
Results
Fit of order 189. An interesting way to check the validity of the model is to compare observations and simulations at the spectrum scale. Fig. 6 shows such comparison for order 189 on the whole of a fullscan (here 14/03/2 019), as an example. Here the spectra are flat due to the continuum removal step which allows apprehending only the level of the spectrum and the position of the bands. The 13 observation sequences are shown in a shade of gray while the optimized simulation after the inversion
Calibration pipeline
With such an approach any nadir observation can be calibrated to spectral radiance, the pipeline to calibrate a raw nadir spectrum is as follows: first, the raw spectrum is normalized following equation (1) with the spectral resolution (eq. (15)). Second, we remove the continuum using eq. (18) to get a flat spectrum. Then, knowing the temperature at the time of the measurement from housekeeping and using coefficients a and b from eq. (24), we apply the sensitivity factor to convert the
Conclusion
We propose an alternative calibration method for the LNO data using reference solar spectra with the advantage of being able to investigate the correlations between the instrumental sensitivity and the temperature of the instrument. By having done this, we can understand the potential temporal variations of the instrument due to its aging. The method is based on the adjustment of a synthetic spectrum to the solar data acquired with NOMAD-LNO fullscan operation mode, which allows a calibration
Author statement
Guillaume. Cruz Mermy: Methodology, Investigation, Software, Validation, Writing. Frédéric Schmidt: Methodology, Investigation, Software, Validation, Supervision. Ian R. Thomas: Investigation, Software, Validation, Supervision. Frank Daerden: Validation, Supervision. Bojan Ristic: Validation, Supervision. Manish R. Patel: Validation, Supervision. Jose Juan Lopez-Moreno: Validation, Supervision. Giancarlo Belluci: Validation, Supervision. Ann Carine Vandaele: Validation, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). We would like to thank everyone involved in the ExoMars project. Funding: This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000 103 401, 4000 121 493), by Spanish Ministry of Science and
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