# 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
The accuracies achieved with these satellites are excellent. They underline the importance of having stable instruments and implementing excellent corrections (due to systematic and random biases). Indeed, satellite retrievals may encounter spatial bias, which could result from environmental and/or instrumental factors, as well as temporal drift in the satellite sensor. Addressing both spatial bias and temporal drift is essential. Moreover, passive methods face a significant challenge as undetected aerosol layers or thin ice clouds can lead to systematic measurement errors of uncertain magnitude. These errors arise due to the intricacy of retrieval algorithms and limited access to independent measurements for validation. This observation approach also highlights the necessity of having known solar spectra with good accuracy [28; 29; 30]. Significant angles for the calibration and correction of atmospheric effects in remote sensing include solar azimuth and viewing azimuth. It is important to address and correct various biases resulting from surface albedo, aerosols, clouds, interfering gases, airglow, the CO\({}_{2}\) 'Ring effect', viewing geometry, sensor drift, and stray light. In the context of the Uvsq-sat NG mission, certain specific biases could be evaluated through specific in-flight calibrations (rapid rotation of the satellite to capture the quasi-same scene from multiple angles) associated with the satellite's attitude due to its ADCS. Others can be estimated and corrected. Another issue is related to the aging of instruments in orbit, which represents a significant problem. To mitigate such effects, the Uvsq-Sat NG NIR Spectrometer will have an in-flight calibration system that can illuminate the spectrometer linear image sensor with known irradiance to assess instrument aging and correct the data. Several laser sources will enable the execution of this task. The goal of Uvsq-Sat NG is not to surpass the accuracy of OCO-2 but rather to achieve comparable accuracy using a significantly more cost-effective SmallSat platform. This approach opens the possibility of establishing a constellation with measurements taken at different local times. The initial SmallSat serves as a demonstrator, proving the feasibility of precise CO2 measurements from a SmallSat. The objective with the constellation is to compile a regional-scale climatology of CO2 concentrations to identify sources and sinks of CO2 from both natural and anthropogenic origins. One of the most challenging experimental issues is to demonstrate carbon sequestration on a given ecosystem. The validation process will involve a comparison of the satellite data with ground-based CO2 measurements obtained from networks such as the Total Carbon Column Observing Network (TCCON). This validation will help us to identify any potential regional biases and to enhance the retrieval algorithm. The expected performance within the scope of the Uvsq-Sat NG mission as well as that of the future constellation are detailed in Table 1. Uvsq-Sat NG is expected to achieve accurate and stable GHG measurements over time (absolute measurements of \(\pm\)4.0 ppm with an annual stability of \(\pm\)1.0 ppm for CO2, absolute measurements of \(\pm\)25.0 ppb with an annual stability of \(\pm\)10.0 ppb for CH4). Uvsq-Sat NG will be able to rely on TCCON, which is the current reference network for the satellite Calibration and Validation of GHG observations. With a narrow field of view, the Uvsq-Sat NG NIR Spectrometer will enable the observation of ground scenes with a ground footprint diameter of approximately 5 km.