Remote Sensing (2023) https://www.mdpi.com/2602016

Fabrizio Oliva, Emiliano D’Aversa, Giancarlo Bellucci, Filippo Giacomo Carrozzo, Luca Ruiz Lozano, Özgür Karatekin, Frank Daerden, Ian R. Thomas, Bojan Ristic, Manish R. Patel, José Juan Lopez-Moreno, Ann Carine Vandaele, Giuseppe Sindoni

NOMAD is a suite of spectrometers on the board of the ESA-Roscosmos Trace Gas Orbiter (TGO) spacecraft and is capable of investigating the Martian environment at very high spectral resolution in the ultraviolet–visible and infrared spectral ranges by means of three separate channels: UVIS (0.2–0.65 μm), LNO (2.2–3.8 μm), and SO (2.3–4.3 μm). Among all channels, LNO is the only one operating at infrared wavelengths in nadir-viewing geometry, providing information on the whole atmospheric column and on the surface. Unfortunately, the LNO data are characterized by an overall low level of signal-to-noise ratio (SNR), limiting their contribution to the scientific objectives of the TGO mission. In this study, we assess the possibility of enhancing LNO nadir data SNR by applying the Minimum Noise Fraction (MNF), a well-known algorithm based on the Principal Components technique that has the advantage of providing transform eigenvalues ordered with increasing noise. We set up a benchmark process on an ensemble of synthetic spectra in order to optimize the algorithm specifically for LNO datasets. We verify that this optimization is limited by the presence of spectral artifacts introduced by the MNF itself, and the maximum achievable SNR is dependent on the scientific purpose of the analysis. MNF application study cases are provided to LNO data subsets in the ranges 2.627–2.648 μm and 2.335–2.353 μm (spectral orders 168 and 189, respectively) covering absorption features of gaseous H₂O and CO and CO₂ ice, achieving a substantial enhancement in the quality of the observations, whose SNR increases up to a factor of 10. While such an enhancement is still not enough to enable the investigation of spectral features of faint trace gases (in any case featured in orders whose spectral calibration is not fully reliable, hence preventing the application of the MNF), interesting perspectives for improving retrieval of both atmospheric and surface features from LNO nadir data are implied.

(A) MCD simulation of the CO-integrated column in kg/m². (B) LNO order 189 data averaged surface altimetry in km (from MOLA data, [37]). (C) Equivalent width of the CO 4267 cm⁻¹ absorption line computed on LNO order 189 original data. (D) Equivalent width of the CO 4267 cm⁻¹ absorption line computed on the MNF-processed data (N_e = 7). (E) Density scatter plot of the quantities in panels (A,C). (F) As in panel (E), but here the quantities in panels (A,D) are being compared.