# UVSQ-SAT, a Pathfinder CubeSat Mission for Observing Essential Climate Variables
## 2 Scientific Rationale of the UVSQ-SAT Mission
### Scientific Requirements  
As explained in detail in Section 2.1, measuring the absolute value of the Earth's energy imbalance and its variability over time appears to be a very difficult challenge. The relevant scientific goal is to be able to detect any long term trend with a target accuracy of 1/10 of the expected signal of 0.5-1.0 \(\mathrm{Wm}^{-2}\) in the global mean during a decade [10; 11; 12]. This issue could be solved through better absolute calibration of the sensors since several satellites will be needed to carry out these measurements with satellites' temporal overlap to realize inter-calibrations. Table 1 presents the scientific objectives to be achieved by future space based instrumentations onboard small satellites with onboard calibration systems for EEI observations. These EEI scientific objectives are extremely relevant and have not been achieved so far. At the present stage, the UVSQ-SAT CubeSat is a demonstrator, expecting future developments and improvement that would then really allow making use of CubeSat technology for these scientific purposes. EEI expected performances of the UVSQ-SAT CubeSat are given in Table 1.  
The absolute value of UV SSI and its variability during more than one decade are also challenging (Section 2.2). Accurate observations are fundamental to consolidate the reconstruction models of the solar spectral irradiance. Spectral And Total Irradiance REconstruction for the Satellite Era (SATIRE-S) [37] highlights a weak long term trend (Figure 1) of UV solar spectral irradiance over the past 40 years for solar minima (inter-cycles), which can be real or not. The relevant scientific goal is to be able to detect any long term trend with a target stability per decade of \(\pm 3.4\)\(10^{-5}\) Wm\({}^{-2}\)nm\({}^{-1}\) (Table 1) at 215 nm (\(\sim\)1/10 of the expected trend (Figure 1) of the inter-cycles during a decade). These solar observations with satellites temporal overlap to realize inter-calibrations are important since an analysis of radionuclides [23] concluded that the Sun will enter a state of significantly lower activity within the next 50 to 100 years. These accurate observations are also important for long term reconstructions over centuries where only proxies of solar activity are available as input for the reconstruction models. Indeed, the physical assumptions that go into the models lead to considerable discrepancies [38]. At the present stage, the UVSQ-SAT CubeSat is a demonstrator that must show that these accurate SSI continuous measurements are possible with small satellites using new compact and robust disruptive technologies. UV solar spectral irradiance in the Herzberg continuum expected performances of the UVSQ-SAT CubeSat are given in Table 1. \begin{table}
\begin{tabular}{l l l} \hline
**Requirements** & **Scientific Relevance** & \\ \hline Essential Climate Variable (ECV) & Absolute uncertainty & Stability per decade \\ \hline EEI measurements & \(\pm 1\) Wm\({}^{-2}\) at 1\(\sigma\) & \(\pm 0.1\) Wm\({}^{-2}\) at 1\(\sigma\) \\ SSI at 215 nm & \(\pm 1.7\)\(10^{-4}\) Wm\({}^{-2}\)nm\({}^{-1}\) & \(\pm 3.4\)\(10^{-5}\) Wm\({}^{-2}\)nm\({}^{-1}\) \\  & ( \(\pm 0.5\)\% at 1\(\sigma\)) & ( \(\pm 0.1\)\% at 1\(\sigma\)) \\ \hline
**Requirements** & **UVSQ-SAT Performances** & \\ \hline Essential Climate Variable (ECV) & Absolute uncertainty & Stability per year \\ \hline EEI measurements & \(\pm 15\) Wm\({}^{-2}\) at 1\(\sigma\) & \(\pm 5\) Wm\({}^{-2}\) at 1\(\sigma\) \\ SSI at 215 nm & \(\pm 8.5\)\(10^{-4}\) Wm\({}^{-2}\)nm\({}^{-1}\) & \(\pm 1.7\)\(10^{-4}\) Wm\({}^{-2}\)nm\({}^{-1}\) \\  & ( \(\pm 2.5\)\% at 1\(\sigma\)) & ( \(\pm 0.5\)\% at 1\(\sigma\)) \\ \hline \end{tabular}
\end{table} 
Table 1: Scientific requirements for Earth’s energy imbalance and solar spectral irradiance in the Herzberg continuum (high scientific relevance and UltraViolet and infrared Sensors at high Quantum efficiency onboard a small SATellite (UVSQ-SAT) expected performances). EEI, Earth’s Energy Imbalance; SSI, Solar Spectral Irradiance.