## 2 Materials and Methods
### MISTIC Winds Observing Concept, Requirements, and MISTIC Instrument Concept
This range also includes channels strongly influenced by clouds and aerosols, and by the trace gases CO, and N2O in the free troposphere. In Figure 4, the R and P branches of the CO2 emission dominated region centered on 2385 cm\({}^{-1}\) that are used for retrieval of the temperature profile are indicated by A. A very high transmission \"super window\" channel, which has utility in observing aerosols is indicated by B. Relatively high brightness channels near C are dominated by surface and boundary-layer water vapor emission. Spectral channels, such as D, are more sensitive to lower troposphere water vapor emission. Lower brightness channels, such as those indicated by E and F, are more sensitive to mid-troposphere and upper troposphere water vapor emission respectively. G indicates the spectral coverage range for the airborne MISTiC instrument. H indicates the spectral coverage range for the planned space instrument (1750 cm\({}^{-1}\) to 2450 cm\({}^{-1}\)). As detailed in Appendix A, the airborne unit employed an off-the-shelf detector array with a nominal cutoff above 1850 cm\({}^{-1}\). However, employing large-area spatial averaging allowed a demonstration of the spectrometer down to approximately 1830 cm\({}^{-1}\) with the airborne unit. The adopted spectral range specifically excludes the very long-wave spectral channels also dominated by CO2 emission, for to include these would dramatically increase the required electrical power, instrument size, and exceed the accommodation capacity of an ESPA-class microsatellite. AIRS algorithms have advanced to the point where the temperature profile retrieval employs the shorter-wavelength CO2 emission channels to retrieve troposphere temperature, with the long-wave channels supporting cloud-clearing and the retrieval of stratospheric temperature [25]. By mission design, observation of stratospheric temperature is not essential for MISTiC, since these variables will be well-observed by the NASA/NOAA Joint Polar Satellite System (JPSS) and EUMETSAT IASI instruments for decades to come. Figure 4: Spectral coverage range and resolution for MISTiC was selected to provide IR sounding of the troposphere. Key spectral regions are shown on this example spectrum from IASI, and described in the text. This spectrum that has been filtered so that the spectral feature shapes will be similar to those expected the MISTiC space instrument. An important requirement consideration for MISTIC is the selection of the lower frequency limit (upper wavelength limit) of the spectral range. This requirement strongly influences the retrievable information content and data quality for the moisture profile in the upper troposphere, and also strongly influences the size, and power demands of the instrument, due to the exponential dependencies of detector characteristics on bandgap and temperature. Under the NASA Instrument Incubator Program (IIP), the impact on retrieved layer RMS error for both the temperature and water vapor concentration was evaluated for different values of the lower-frequency band limit, ranging from 1950 down to 1650 cm\({}^{-1}\). For this study, \"truth\" was selected to be a set of AIRS instrument retrieved temperature and moisture vertical profiles collected for cloud-free footprints over the Pacific Ocean, and retrieved using the AIRS Version 6 algorithm, which includes channels from the full spectral range of AIRS, including those in the very long wavelength infrared (VLWIR). A regression method was used by the NASA GSFC Sounder Research Team to perform this trade-study analysis. Spectral resolving power of 700:1 was used for the MISTIC spectral response function in this study. The key results of this trade study are shown in Figure 5. One conclusion is that reasonably accurate temperature vertical profiles can be provided up to a pressure-height of 200 hPa, using just the CO\({}_{2}\) emission band centered on 4.3 \(\upmu\)m (2383 cm\({}^{-1}\)).