# UVsq-Sat NG, a New CubeSat Pathfinder for Monitoring Earth Outgoing Energy and Greenhouse Gases
## 3 Description of the Uvsq-Sat NG Space-Based Mission
### Calibration and Validation--Cal/Val
#### 3.2.2 Uvsq-Sat NG NIR Spectrometer for Monitoring GHG Concentrations in the Atmosphere  
Satellite observations provide the concentration of a respective GHG from the TOA down to the surface, referred to as the column average. On the other hand, ground-based in situ stations measure concentrations close to the surface. Both observations are valuable; however, they cannot be directly compared or used for cross-Cal/Val purposes. Satellites conduct accurate observations while being at altitudes of at least 500 km. When observing GHGs far from their sources and sinks, they must be capable of detecting very small variations in GHG concentrations, often lower than 0.5%. To measure such faint signals accurately, meticulous calibration and validation are crucial. This highlights the importance of implementing precise Pre-Launch Calibrations for satellite observations. The following description outlines the Pre-Launch Calibrations intended for Uvsq-Sat NG and its spectrometer. * The characterization of the spectrometer's pixels to accurately deduce the incident irradiance based on the digital signal output from specific pixels. This is to be performed because each pixel of the InGaAs linear image sensor of the Uvsq-Sat NG Spectrometer has a specific dark current, dark offset, and gain and might respond non-linearly to change in irradiance. * Photosensitivity and Quantum Efficiency (QE) as a function of wavelength of each pixel to be able to convert digital signal from the InGaAs linear image sensor of the Uvsq-Sat NG Spectrometer into input irradiance at a given wavelength. * The Measurement of Spectral Response Function (SRF) of the Uvsq-Sat NG Spectrometer to assess the weighting of a continuous light source via sensor response. This test will be performed using the LATMOS/OVSQ iHR320 monochromator (Horiba, Palaiseau, France), equipped with a holographic grating in the spectral range of 400-1300 nm (1200 grove/mm, blaze wavelength of 630 nm) and a ruled grating in the spectral range of 1000-2500 nm (600 g/mm, blaze wavelength of 1500 nm). The iHR320 monochromator will be positioned before an NIR lamp that requires cooling. At the exit of the iHR320 monochromator, a beamsplitter will divide the light, directing it towards both the Uvsq-Sat NG Spectrometer and a sensor reference. * The absolute calibration of the Uvsq-Sat NG Spectrometer using secondary standards for spectral irradiance. For this measurement, an absolute reference lamp will be placed directly in front of the Uvsq-Sat NG Spectrometer. To eliminate stray light, a mask will be used between the spectrometer and the lamp. A test will also be conducted at PTB to validate the absolute measurements. * The verification of the inverse square law for confirming the principle that the intensity of light from a source decreases as the square of the distance from the source increases. * The Bidirectional Reflectance Distribution Function (BRDF) and angular responses that help us to predict how light will be reflected or transmitted at varying angles. * The characterization of the stray light, which is essential for maintaining the sensitivity, dynamic range, and spectral resolution of the Uvsq-Sat NG Spectrometer. * Thermal characterization efforts will be included to examine how the Uvsq-Sat NG NIR Spectrometer's response varies with temperature. The significance of these calibrations lies in obtaining an insight into the precision required for the upcoming GHG measurements, while also instilling confidence in the prospective data observations from space. As an example, GOSAT observations entail systematic uncertainties of 4 ppm for XCO\({}_{2}\) (column average) and 34 ppb for XCH\({}_{4}\), at a spatial resolution of 1000 km over land. GOSAT-2 has improved the precision of measurements with systematic uncertainties of 0.5 ppm for XCO\({}_{2}\) and 5 ppb for XCH\({}_{4}\), achieving resolutions of 500 km over land and 2000 km over the ocean [27]. For OCO-2/3, the resolution is around 0.3% (1 ppm) for XCO\({}_{2}\).