## 3 Prototype Model of LIBRA  
### Specification of LIBRA  
The LIBRA mission consists of one satellite carrying four payloads: an Infrared Spectrometer (IRS) with hyperspectral resolution, an Earth-Moon Imaging Spectrometer (EMIS) measuring in reflected solar radiation, a Total Solar Irradiance (TSI) instrument, and a Solar spectral Irradiance monitoring instrument Traceable to Quantum benchmark (SITQ). A GPS/Beidou Globle Navigation Satellite System Radio Occultation (GPS/BD GNSS RO) will be an optional instrument depending on the spacecraft bus capability. The detailed payload specifications of the LIBRA prototype model and the key technologies used are listed in Table 1. The IRS will be positioned on the spacecraft bus deck for nominal nadir views. Using an internal mirror, the IRS will enable nadir, off-nadir, internal calibration, and deep-space (zenith) observations. The EMIS and TSI will be co-boresighted and mounted to a two-axis gimbal to enable nadir (nominal operations) and off-nadir lunar and solar observations. The spacecraft bus should provide an independent means for acquiring necessary position and attitude information.  
The LIBRA experimental observatory has been designed for an operational lifetime of five years with consumables for eight years. The follow-up operational satellites will be designed for a long-term mission (20 years or more), with satellites in orbit overlapping in time. The spacecraft operations system will be designed to minimize the cost and risk of operations, including minimizing the number  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Instrument Name** & **Payload Requirements** & **Key Technology** \\ \hline \multirow{4}{*}{IRS} & Spectral range: 600–2700 cm\({}^{-1}\) & \\  & Spectral resolution: 0.5 cm\({}^{-1}\) & \\  & IFOV: 24 mrad & Miniature fixed-temperature \\  & Sensitivity: 0.1 K@270 K & phase-change cells \\  & Emissivity of BB: \(\geq\)0.999 & \\  & Measurement uncertainty: 0.15 K (k = 2) & \\ \hline \multirow{4}{*}{EMIS} & Spectral range: 380–2350 nm, & \\  & Spectral resolution: 10 nm, & \\  & Spectral precision: 0.5 nm, & \\  & Spatial resolution: 100 m, & Radiometer \\  & Coverage: 50 km, & \\  & Measurement uncertainty: 1\% (k = 2) & \\ \hline \multirow{4}{*}{TSI} & Spectral range: 0.2–35 μm, & \\  & Measurement uncertainty: 0.05\% (k = 2) & \\  & Long-term stability:0.005\% & \\ \hline \multirow{4}{*}{SITQ} & Spectral range: 380–2500 nm, & \\  & Spectral resolution: 3 nm (380–1000 nm), & \\  & 8 nm (1000–2500 nm) & Spontaneous Parametric \\ \cline{1-1}  & Spectral precision: 0.1–0.3 nm, & Down-Conversion \\ \cline{1-1}  & Self-calibration uncertainty: 0.2\%, & \\ \cline{1-1}  & Measurement uncertainty: 0.35\% (k = 2) & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Detailed payloads specifications of the LIBRA prototype model.  
of operators required to safely command and control the spacecraft, and maximizing use of spacecraft and ground system fault detection, reporting and protection tools.  
Other potential platforms considered for the LIBRA mission include the Chinese Space Station (CSS) as an instrument platform and independent small satellite missions with only one SI instrument each. Lagrangian orbit locations are also being investigated for complementary observations.