## 4 Instrument Design and Key Technologies  
### EMIS and TSI Make Use of the Space Cryogenic Absolute Radiometer (SCAR)  
In order to improve the measurement accuracy and long-term stability of the reflective solar band and the total solar irradiance observations, it is important to develop a new onboard calibration system with high accuracy. The LIBRA will provide a calibrated reflected solar spectrum and total solar irradiance based on the Space Cryogenic Absolute Radiometer (SCAR) [22], instead of a solar diffuser, standard lamps, vicarious calibration methods, and ground-based calibration techniques. The prototype payload under development since 2018 is illustrated in Figure 2. Essentially, the SCAR is an electrical substitution radiometer operating at 20 K [23]. The detector of SCAR is a blackbody cavity with super high absorptance. Incident light induces a temperature increase in the blackbody cavity by absorption. The power of the incident light can be obtained by precisely measuring the electrical power applied in order to maintain constant temperature, as the incident light is duty cycled and its power is replaced by the electrical power. The heating effects of the incident light and electrical heater are nearly equivalent at 20 K, reducing non-equivalence corrections.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Satellite** & \multicolumn{4}{c}{**Libra**} & \multicolumn{4}{c}{**Clarreo**} & \multicolumn{2}{c}{**Truths**} \\ \hline
**Instrument** & \multirow{2}{*}{**IR**} & \multirow{2}{*}{**RS**} & \multirow{2}{*}{**TS**} & \multirow{2}{*}{**SS**} & \multirow{2}{*}{**IR**} & \multirow{2}{*}{**RS**} & \multirow{2}{*}{**RS**} & \multirow{2}{*}{**TS**} & \multirow{2}{*}{**SS**} \\
**Type \({}^{*}\)** & & & & & & & & & & \\ \hline Spectral & 60–2700 & 380–2350 & 0.2–35 & 380–2500 & 200–2000 & 320–2300 & 380–2300 & 0.2–35 & 320–2450 \\ Coverage & cm\({}^{-1}\) & nm & \(\upmu\)m & nm & cm\({}^{-1}\) & nm & nm & \(\upmu\)m & nm \\ \hline Spectral & \multirow{2}{*}{0.5 cm\({}^{-1}\)} & \multirow{2}{*}{10 nm} & \multirow{2}{*}{–} & \multirow{2}{*}{3–8 nm} & \multirow{2}{*}{0.5 cm\({}^{-1}\)} & \multirow{2}{*}{8 nm} & \multirow{2}{*}{5–10 nm} & \multirow{2}{*}{–} & \multirow{2}{*}{1–10 nm} \\ Resolution & & & & & & & & & \\ \hline Measurement & 0.15 K & 1\% & 0.05\% & 0.35\% & 0.065 K & 0.3\% & 0.1\% & 0.02\% & 0.2\% \\ Uncertainty & (k = 2) & (k = 2) & (k = 2) & (k = 2) & (k = 2) & (k = 2) & (k = 2) & (k = 2) \\ \hline SI-traceability & Yes & Yes & Yes & Yes & Yes & Yes & Yes & Yes \\ \hline \multicolumn{10}{l}{\({}^{*}\) In the line ‘Instrument type\({}^{\prime}\), IR represents the instrument to measure the spectrally resolved infrared radiance, RS represents the instrument to measure the spectrally resolved reflectance of solar radiation, TS represents the instrument to measure the total solar irradiance, and SS represents the instrument to measure the spectrally resolved solar irradiance.} \\ \end{tabular}
\end{table}
Table 2: Comparison among radiometric benchmark satellite.  
Through the onboard calibration system, the radiometric scale of the reflected solar band and total solar irradiance can be traced to SCAR. The accuracy of the space cryogenic radiometric measurement, the in-orbit self-calibration, and the high spectral resolution radiance observation are the critical parts to the final performance of SCAR. The experimental prototype of SCAR has been in development since 2015. The Stryn-type Pulse Tube Cryocooler (SPTC) is used to obtain the 20 K working temperature. The SPTC is optimized to provide the refrigerating capacity of 350 mW at 20 K for space applications. The measurement uncertainty of the SCAR prototype is 0.029%, according to uncertainty analyses. The measurement uncertainty was verified by an indirect comparison with the ground-based cryogenic radiometer of the National Institute of Metrology (NIM) [24; 25].