[Ian: This paper concerns VEx/SOIR, but is included here on the NOMAD website because the same algorithm is used to calibrate SO and UVIS occultations]
Applied Optics (2016) Vol. 55, Issue 32, pp. 9275 - 9281 https://doi.org/10.1364/AO.55.009275; http://pdfs.semanticscholar.org/240b/016c68c9d1f4a7b323a253fd4cbc77a18102.pdf
Loic Trompet, Arnaud Mahieux, Bojan Ristic, Séverine Robert, Valérie Wilquet, Ian R. Thomas, Ann Carine Vandaele, and Jean-Loup Bertaux
The Solar Occultation in the InfraRed (SOIR) instrument onboard the ESA Venus Express spacecraft, an infrared spectrometer sensitive from 2.2 to 4.3 μm, probed the atmosphere of Venus from June 2006 until December 2014. During this time, it performed more than 750 solar occultations of the Venus mesosphere and lower thermosphere. A new procedure has been developed for the estimation of the transmittance in order to decrease the number of rejected spectra, to check that the treated spectra are well calibrated, and to improve the quality of the calibrated spectra by reducing the noise and accurately normalizing it to the solar spectrum.
Robert, S.; Vandaele, A.C.; Thomas, I.; Willame, Y.; Daerden, F.; Delanoye, S.; Depiesse, C.; Drummond, R.; Neefs, E.; Neary, L.; Ristic, B.; Mason, J.; Lopez-Moreno, J.-J.; Rodriguez-Gomez, J.; Patel, M.R.; Bellucci, G.; the NOMAD Team
NOMAD (Nadir and Occultation for MArs Discovery) is one of the four instruments on board the ExoMars Trace Gas Orbiter, scheduled for launch in March 2016. It consists of a suite of three high-resolution spectrometers – SO (Solar Occultation), LNO (Limb, Nadir and Occultation) and UVIS (Ultraviolet and Visible Spectrometer). Based upon the characteristics of the channels and the values of Signal-to-Noise Ratio obtained from radiometric models discussed in (Vandaele et al., 2015a and Vandaele et al., 2015b; Thomas et al., 2016), the expected performances of the instrument in terms of sensitivity to detection have been investigated. The analysis led to the determination of detection limits for 18 molecules, namely CO, H2O, HDO, C2H2, C2H4, C2H6, H2CO, CH4, SO2, H2S, HCl, HCN, HO2, NH3, N2O, NO2, OCS, O3. NOMAD should have the ability to measure methane concentrations <25 parts per trillion (ppt) in solar occultation mode, and 11 parts per billion in nadir mode. Occultation detections as low as 10 ppt could be made if spectra are averaged (Drummond et al., 2011). Results have been obtained for all three channels in nadir and in solar occultation.
Vandaele, A.C.; Willame, Y.; Depiesse, C.; Thomas, I.R.; Robert, S.; Bolsee, D.; Patel, M.R.; Mason, J.P.; Leese, M.; Lesschaeve, S.; Antoine, P.; Daerden, F.; Delanoye, S.; Drummond, R.; Neefs, E.; Ristic, B.; Lopez-Moreno, J.J.; Bellucci, G.; the NOMAD Team
The NOMAD instrument has been designed to best fulfil the science objectives of the ExoMars Trace Gas Orbiter mission that will be launched in 2016. The instrument is a combination of three channels that cover the UV, visible and IR spectral ranges and can perform solar occultation, nadir and limb observations. In this series of two papers, we present the optical models representing the three channels of the instrument and use them to determine signal to noise levels for different observation modes and Martian conditions. In this first part, we focus on the UVIS channel, which will sound the Martian atmosphere using nadir and solar occultation viewing modes, covering the 200-650nm spectral range. High SNR levels (>1000) can easily be reached for wavelengths higher than 300nm both in solar occultation and nadir modes when considering binning. Below 300nm SNR are lower primarily because of the lower signal and the impact of atmospheric absorption.
Thomas, I.R.; Vandaele, A.-C.; Robert, S.; Neefs, E.; Drummond, R.; Daerden, F.; Delanoye, S.; Ristic, B.; Berkenbosch, S.; Clairquin, R.; Maes, J.; Bonnewijn, S.; Depiesse, C.; Mahieux, A.; Trompet, L.; Neary, L.; Willame, Y.; Wilquet, V.; Nevejans, D.; Aballea, L.; Moelans, W.; De Vos, L.; Lesschaeve, S.; Van Vooren, N.; Lopez-Moreno, J.J.; Patel, M.R.; Bellucci, G.; the NOMAD Team
NOMAD is a suite of three spectrometers that will be launched in 2016 as part of the joint ESA-Roscosmos ExoMars Trace Gas Orbiter mission. The instrument contains three channels that cover the IR and UV spectral ranges and can perform solar occultation, nadir and limb observations, to detect and map a wide variety of Martian atmospheric gases and trace species. Part I of this work described the models of the UVIS channel; in this second part, we present the optical models representing the two IR channels, SO (Solar Occultation) and LNO (Limb, Nadir and Occultation), and use them to determine signal to noise ratios (SNRs) for many expected observational cases. In solar occultation mode, both the SO and LNO channel exhibit very high SNRs >5000. SNRs of around 100 were found for the LNO channel in nadir mode, depending on the atmospheric conditions, Martian surface properties, and observation geometry.
Vandaele A. C.; Neefs E.; Drummond R.; Thomas I. R.; Daerden F.; Lopez-Moreno J.-J.; Rodriguez J.; Patel M. R.; Bellucci G.; Allen M.; Altieri F.; Bolsée D.; Clancy T.; Delanoye S.; Depiesse C.; Cloutis E.; Fedorova A.; Formisano V.; Funke B.; Fussen D.; Geminale A.; Gérard J.-C.; Giuranna M.; Ignatiev N.; Kaminski J.; Karatekin O.; Lefèvre F.; López-Puertas M.; López-Valverde M.; Mahieux A.; McConnell J.; Mumma M.; Neary L.; Renotte E.; Ristic B.; Robert S.; Smith M.; Trokhimovsky S.; Vander Auwera J.; Villanueva G.; Whiteway J.; Wilquet V.; Wolff M.
The NOMAD spectrometer suite on the ExoMars Trace Gas Orbiter will map the composition and distribution of Mars׳ atmospheric trace species in unprecedented detail, fulfilling many of the scientific objectives of the joint ESA-Roscosmos ExoMars Trace Gas Orbiter mission. The instrument is a combination of three channels, covering a spectral range from the UV to the IR, and can perform solar occultation, nadir and limb observations. In this paper, we present the science objectives of the instrument and how these objectives have influenced the design of the channels. We also discuss the expected performance of the instrument in terms of coverage and detection sensitivity.
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