# UVsq-Sat NG, a New CubeSat Pathfinder for Monitoring Earth Outgoing Energy and Greenhouse Gases
## 4 Methodology for Determining ERB and GHGs with Uvsq-Sat NG  
The methods used for determining the ERB components from ERSs were explained in [8; 9; 10]. This section will therefore focus on the methodology used for determining GHGs' mixing ratios with Uvsq-Sat NG. Greenhouse gases (CO\({}_{2}\), CH\({}_{4}\)) as well as H\({}_{2}\)O present absorption lines [29] in the spectral range measured via the Uvsq-Sat NG Spectrometer. This is also the case for O\({}_{2}\) around 1270 nm that is used as a reference due to its 21% (0.21) constant mixing ratio in the dry atmosphere. The total column of each of the four gases can be determined by fitting the observed spectra with simulated spectra using the Levenberg-Marquardt (L-M) algorithm and the Monte Carlo method. Here is a description of the method employed to determine the atmospheric gas concentrations that the Uvsq-Sat NG mission is set to observe. * Recovering the atmospheric transmittance functions data files (CO\({}_{2}\), CH\({}_{4}\), O\({}_{2}\), H\({}_{2}\)O at different concentrations). NIR transmission spectra are simulated using the High-Resolution Transmission Molecular Absorption (HITRAN) Database [31]. Spectra are computed at very high resolution (\(\Delta\
u\) = 0.005 cm\({}^{-1}\)) in the wavenumber (\(\
u\)) interval 4500 to 10,000 cm\({}^{-1}\). For CO\({}_{2}\), CH\({}_{4}\), and O\({}_{2}\) that are well mixed in the atmosphere, the pressure (\(P\)) is set to 500 hPa, which represents an average value between the bottom and the top of the atmosphere. For H\({}_{2}\)O, which is more abundant in the lower atmospheric layers, the pressure is set to 800 hPa. The temperature (\(T\)) is 250 K and a Voigt profile is considered for the HITRAN simulation. The optical path of length (\(L\)) of the simulation is different for each gas (for instance, under nominal mixing conditions, the values considered are 6.228 m for CO\({}_{2}\), 0.028 m for CH\({}_{4}\), 3113.840 m for O\({}_{2}\), and 16.161 m for H\({}_{2}\)O). The absorption coefficient (\(KN_{a}\)) is used for each gas. It represents a normalization of the absorption coefficient \(Ka\) by the number N (cm\({}^{-3}\)) of absorbing molecules per unit gas volume. The dimensionless transmittance function (TR) for each gas is given by Equation (1). \[\mathrm{TR}(\
u,T,P,L)=\exp(-KN_{a}(\
u,T,P)\cdot L) \tag{1}\]  
Figure 4 (top panel) depicts the NIR dimensionless transmittance functions of various gases (TR(\(\
u,T,P,L\))) for a nominal mixing ratio (420 ppm for CO\({}_{2}\), 1.9 ppm for CH\({}_{4}\), 0.21 for O\({}_{2}\), and 1.0 precipitable cm for H\({}_{2}\)O) as a function of wavelength (\(\lambda\)). Other dimensionless  
transmittance functions have been computed using different mixing ratios (for instance, 430 ppm for CO\({}_{2}\), 2.0 ppm for CH\({}_{4}\), 0.22 for O\({}_{2}\), and 1.1 precipitable cm for H\({}_{2}\)O). Figure 4 (bottom panel) displays dimensionless transmittance functions' relative variations from a'maximum' mixing ratio to a nominal mixing ratio for each gas (CO\({}_{2}\), CH\({}_{4}\), O\({}_{2}\), and H\({}_{2}\)O as O\({}_{2}\), and H\({}_{2}\)O). To determine the sensitivity of the dimensionless transmittance functions to a change in the mixing ratio of a given gas, we compute the absorption spectrum for a nominal mixing ratio and for a mixing ratio with an increment added to this value (CO\({}_{2}\) (420 to 430 ppm), CH\({}_{4}\) (1.9 to 2.0 ppm), O\({}_{2}\) (0.21 to 0.22), and H\({}_{2}\)O (1.0 to 1.1 precipitable cm)). * **Step 2**: Normalization and convolution of atmospheric data to the Uvsq-Sat NG Spectrometer's resolution.