# UVSQ-SAT, a Pathfinder CubeSat Mission for Observing Essential Climate Variables
## 3 Materials and Methods
### The Space Segment: The UVSQ-SAT CubeSat Payload
#### 3.2.2 The DEVINS Sensors  
To measure with accuracy (see Table 1) the solar spectral irradiance in the Herzberg continuum (200 to 242 nm), we will use disruptive new UVC detectors in Ga\({}_{2}\)O\({}_{3}\). Photodetectors based on monoclinic (\(\beta\)) Ga\({}_{2}\)O\({}_{3}\) have been demonstrated [3]. With a bandgap of \(\sim\)4.9 eV \(\beta\)-Ga\({}_{2}\)O\({}_{3}\), the films were naturally solar blind without any need for alloying with a third element [41]. Further, it was shown that their spectral response peak could be tuned between about 230 and 255 nm [3] and with a bandpass of 40 nm or so at FWHM. These sensors were radiation hard, which avoids the need for cooling and the associated contaminant trapping/lifetime issues of incumbent devices. Moreover, the gain of these devices is already more than two orders of magnitude higher than that of commercial SiC based equivalents. This indicates a potential for operation at lower voltages/powers (5 V in the case of UVSQ-SAT). They also have a strong responsivity (\(\sim\) 4A/W) and excellent dynamics. These previous works formed a solid basis for DEVINS to implement sensors with adapted characteristics and DEVINS \(\beta\)-Ga\({}_{2}\)O\({}_{3}\) based prototypes with a 215-220 nm peak and 40 nm bandpass to cover the Herzberg continuum are currently being developed. These will be small photodiodes using the kind of standardized metal semiconductor package used for transistors and some integrated circuits (base diameter of 8.9 mm, cap diameter of 8.1 mm, cap height of 6.3 mm). They will not require a cooling system to be sensitive to the Herzberg continuum. This aspect should prevent them from degradation due to contamination [33]. They will be designed to measure the UV solar spectral irradiance from a 180\({}^{\circ}\) field of view angle with an active area of 1 mm \(\times\) 1 mm. Figure 4 shows the DEVINS photodiodes' manufacturing process. The DEVINS technology will be validated in the framework of the UVSQ-SAT mission (check in orbit that the detector degradation is weak despite the exposure to radiation (both ionization and displacement-damage effects)). UVSQ-SAT will use a total of four DEVINS photodiodes. \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameter** & **Requirements** \\ \hline ERS signal range & \(-500\) Wm\({}^{-2}\) to +1500 Wm\({}^{-2}\) \\  & (\(-100\) \(\mu\)V per Wm\({}^{-2}\) to +300 \(\mu\)V per Wm\({}^{-2}\)) \\ ERS resolution & 18 bits (\(\sim\)1.5 mV) \\ ERS noise detection & \(\pm 0.25\) Wm\({}^{-2}\) (\(\pm 50\) mV) \\ ERS time response & \(<\)50 ms \\ \hline Temperature range & \(-60\)\({}^{\circ}\)C to +90 \({}^{\circ}\)C \\ Resolution & 18 bits (5.7 10\({}^{-4}\)\({}^{\circ}\)C) \\ Temperature noise & \(<\pm 0.1\)\({}^{\circ}\)C \\ \hline Acquisition time & Better than 10 s \\ \hline \hline \end{tabular}
\end{table} 
Table 4: ERS technical requirements. For all sensors, it is necessary to monitor the Temperature (\(T\)) with a high accuracy to take into account the temperature radiation lost (Stefan–Boltzmann law). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Absolute Uncertainty** & **Stability per Year** \\ \hline OLR & \(\pm 10\) Wm\({}^{-2}\) at 1\(\sigma\) & \(\pm 1\) Wm\({}^{-2}\) at 1\(\sigma\) \\ OSR & \(\pm 10\) Wm\({}^{-2}\) at 1\(\sigma\) & \(\pm 5\) Wm\({}^{-2}\) at 1\(\sigma\) \\ TSI & \(\pm 0.5\) Wm\({}^{-2}\) at 1\(\sigma\) & \(<\pm 0.1\) Wm\({}^{-2}\) at 1\(\sigma\) \\ \hline EEI = TSI/4 \(-\) (OLR + OSR) & \(<\pm 15\) Wm\({}^{-2}\) at 1\(\sigma\) & \(\sim\pm 5\) Wm\({}^{-2}\) at 1\(\sigma\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: ERS scientific requirements. Total solar irradiance is considered as accurately known. For DEVINS data processing, we will use the six coarse platform photodiodes located on each solar panel for determining the Sun's direction.