## 2 Materials and Methods
### MISTiC Instrument Design and Sensitivity Performance  
The core of the instrument is a field-imaging dispersive infrared spectrometer, shown in cut-away in Figure 7. A small scan mirror directs scene radiation into a small imaging optical assembly that focuses the light on the entrance slit of the spectrometer. The spectrometer optics disperse the light, pass it through optical filters, and present it to a 640\(\times\)480 format infrared focal plane array. This array is cooled by an active cooler to reduce noise and detector dark currents. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{2}{c}{**MISTiC Winds Key Observation Requirements**} & \multicolumn{1}{c}{**Value**} \\ \hline \multirow{6}{*}{**Vertically Resolved Motion**} & \multirow{2}{*}{Layer Wind Speed Uncertainty} & \(<\)2 m/s rms (Lower Troposphere) \\  & & \(<\)3 m/s (High Troposphere) \\ \cline{2-3}  & Layer Wind Direction Uncertainty (above & \\  & 10 m/s) & \(<\)10 degrees rms \\ \cline{2-3}  & Layer Height Pressure Height & \(<\)30 hPa rms (assuming 850–200 hPa \\  & Assignment Accuracy & wind shear \(<\)20 m/s) \\ \cline{2-3}  & Layer Effective Vertical Thickness & \(<\)100 hPa (FWHM) \\ \cline{2-3}  & Minimum Pressure Height of Highest & 350 hPa (WV)/500 hPa (C) \\  & Level & \\ \cline{2-3}  & Tracer Potential Density (Cloud-Free & \(>\)1 per 6 km2 per vertical layer (Water \\  & Conditions for Water Vapor Motion Vector, & \(>\)1 per 150 km2 per layer) (Cloud \\  & Cloud Contrast for Cloud Motion Vector) & AMVs @ nadir) \\ \hline \multirow{3}{*}{Temperature Vertical Profile} & \multirow{2}{*}{Layer Effective Vertical Thickness} & \(>\)100 hPa (–1 km) \\ \cline{2-3}  & Layer Temperature Accuracy & \(<\)1.25 K (Lower Troposphere) \\ \cline{2-3}  & Layer Water Vapor Concentration & \(<\)15\% (Lower Troposphere) \\ \cline{2-3}  & Accuracy & \(<\)15\% (Lower Troposphere) \\ \cline{2-3}  & Sounding Measurement Potential Density & \(>\)1 per 6 km\({}^{2}\) \\ \hline \multirow{2}{*}{Observation Frequency} & \multirow{2}{*}{Observation Refresh Period} & \(<\)6 h (two planes, each with three instruments) \\ \cline{2-3}  & & \\ \end{tabular}
\end{table} 
Table 2: Key vertically resolved wind and thermodynamic profile observation requirements. The spectrometer optical assembly's largest dimension is approximately 20 cm. The overall instrument assembly has a volume of approximately 12 U (1 U = 10 \(\times\) 10 \(\times\) 10 cm) and is compatible with hosting by an ESPA-class micro-satellite spacecraft with a combined (payload + spacecraft) size of approximately one cubic foot, or about 33 \(\times\) 33 \(\times\) 33 cm (stowed for launch). Total instrument power requirements are approximately 50 W (continuously), and total instrument payload mass is less than 15 kg, consistent with a total spacecraft mass of less than 50 kg. BAE's detailed radiometric performance model, adapted from models proven on AIRS and updated for GOES-R HES, has been used to estimate noise-equivalent spectral radiance (NESR) performance of the MISTiC instrument, and compare it to requirements originally developed by NASA and NOAA for GOES-R HES. MISTiC sensitivity estimates show solid performance margin against these NESR requirements. These estimates are shown for each spectral sub-band in Figure 8.  
In addition to radiometric sensitivity, a key requirement for an IR sounding instrument is the stability and knowledge of its spectral calibration. AIRS has provided outstanding spectral calibration stability, a primary goal for its thermal design. Microsatellites cannot supply large amounts of electrical power. In order to minimize on-orbit power demand by the MISTiC instrument, passive radiative  
Figure 8: MISTiC Winds instrument radiometric sensitivity performance estimates show solid margin against the adopted requirements in Table 1. MISTiC radiometric sensitivity requirements are comparable to those of AIRS, which required NEAT < 200 mK, referenced to 250 K scene temperature. Figure 7: MISTiC IR spectrometer opto-mechanical concept shown in cut-away.