## 2 Materials and Methods
### MISTIC Winds Observing Concept, Requirements, and MISTIC Instrument Concept
A second key result is that a lower limit of 1750 cm\({}^{-1}\) is sufficient to provide a water vapor concentration retrieval of sufficient accuracy in the troposphere. While a lower-limit of 1650 cm\({}^{-1}\) would provide slightly better water vapor retrievals, it would also drive out the lower-frequency detector spectral cutoff wavelength, and drive up the required instrument cooling power. A higher spectral cutoff such as 1950 cm\({}^{-1}\) would degrade water vapor concentration measurement accuracy in the mid-troposphere. Overall, the major benefit of this mission-specific tailoring is a reduction in spectral channel count by a factor of 3\(\times\) to 4\(\times\) for MISTIC relative to current hyperspectral sounders, and the exclusion of the most resource-intensive channels from the instrument. It has also since been recognized that the  
Figure 5: A spectral range trade study using atmospheric infrared sounder (AIRS) Version 6 retrievals for a diverse set of cloud-clear footprints over the Pacific showed that a lower spectral range cutoff of 1750 cm\({}^{-1}\) provided a good vertical coverage with needed accuracy for tropospheric temperature and water vapor. temperature vertical profile can be accurately retrieved with slightly coarser spectral resolution than used for AIRS, (adopting the effective spectral resolution of CrIS), and this fact has been taken into account in the selection of MISTiC Winds spectral resolution and sampling requirements. The combination of requirements choices, design, and the use of contemporary technology in MISTiC Winds allow tremendous reductions in instrument size. Figure 6 shows the NASA AIRS instrument, together with the MISTiC instrument concept at the same scale. The GOES-R HES Formulation Phase studies yielded GEO sounder instrument designs from three industry teams, each with size much larger than AIRS, and very much larger than the set of three MISTiC instruments needed to perform a (triplet) hyperspectral AMV observation. Key MISTiC instrument requirements developed under the IIP are shown in Table 1. These are followed by the key MISTiC Winds Level 2 data product requirements for vertically resolved wind, temperature, and water vapor observation capability in Table 2, also developed under the IIP. \begin{table}
\begin{tabular}{c c c} \hline \hline
**MISTiC Key Instrument Performance Requirements** & \\ \hline
**Instrument Characteristic** & **Value** & **Comments** \\ \hline Minimum Spectral Frequency & 1750 cm\({}^{-1}\) & 5.72 \(\upmu\)m \\ \hline Maximum Spectral Frequency & 2450 cm\({}^{-1}\) & 4.082 \(\upmu\)m \\ \hline Spectral Sampling & \(\sim\)2:1 & \(\sim\)580 spectral samples \\ \hline Spectral Resolving Power & \(>\) 700:1 & v/\(\delta\)v (similar to CrIS-Apodized) \\ \hline Spectral Calibration Knowledge & 1/100,000 & \(\delta\)\(\uplambda\)/\(\uplambda\) \\ \hline Angular Sampling (Spatial Sampling) & 0.0016 radians x 0.0016 radians & 1.38 km (@ Nadir) \\ \hline Orbital Altitude and Orbit Type & 705.3 km & Polar/Sun-Synchronous \\ \hline Angular Range (cross-track) & 1.570 radians & 90 Degrees—Same as AIRS \\ \hline Spatial Resolution & \(<\)3.0 km (geometric mean) & @ Nadir \\ \hline Radiometric Sensitivity & \(<\)200 mK (Ref 250 K scene) & (\(<\)150 mK @ 2380 cm\({}^{-1}\)) \\ \hline Radiometric Accuracy & \(<\)1\% & @ 300 K Scene Background \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Key MISTiC instrument requirements. Figure 6: A combination of key requirements changes (focusing on tropospheric sounding), instrument design innovation and key technology advances enable a dramatic reduction in instrument size between AIRS instrument (**left**) and MISTiC, shown as an artist’s concept (**right**). The AIRS and MISTiC requirements for temperature and water vapor sounding, while similar, are not identical. The difference in vertical coverage was noted above. A requirement area where MISTiC provides considerably higher performance than current IR sounders is in the area of spatial resolution and sampling density.