## 4 Discussion
### Discussion of MISTiC Winds Requirements and Their Relation to Other Instrument Requirements
Infrared spectral sounding vertical resolution in the troposphere is approximately 100 hPa. The AIRS moisture retrieval shown in Figure 16 provides a good illustration of this, in comparison with the moisture vertical profile obtained from the near-by radiosonde shown in Figure 15. The radiosonde shows a sharply defined higher-concentration water vapor layer (in dew-point temperature) over a range of approximately 80 hPa in pressure height, centered near 560 hPa. The AIRS retrieved water vapor profile is somewhat broadened, but it is still sufficiently sharp so that the position of the peak water vapor concentration can reasonably be located to a precision of 30 hPa. The pressure height of the peak in water vapor concentration obtained in the AIRS retrieval is in good agreement with the radiosonde. The vertical resolution of water vapor retrievals obtainable from MISTIC is expected to be similar to those obtained by AIRS, as shown in by trade-study results in Figure 5. Full-scale development of this observing system would include a deeper exploration of this capability, for a variety of thermodynamic and wind speed profiles so that these requirements and feasibility assessments can be refined. Although the above example discussed the feasibility of height assignment improvements for water vapor tracers, hyperspectral observation should improve cloud AMV height assignment also. It enables this improvement in two ways: By providing access to the observed local temperature vertical profile co-located with the AMV feature, and by providing access to highly transparent narrow spectral channels for the brightness temperature measurements that are more accurate than those using the broader multi-spectral imagers. To ensure that feature tracking error portion of the total wind-speed error remains small (within a 1 m/s allocation), the current levels of image-sequence quality provided by GOES 16 ABI and its sister instruments on Himawari 8 and 9 are also needed for MISTIC. In this feature-tracking application, image quality is driven most strongly by two attributes-ground sampling distance (GSD) and the spatial oversampling ratio. The GOES ABI instruments provide \(\sim\)2 km GSD at nadir for thermal IR bands, and approximately 3.5 km at edge of scan. MISTIC is designed to have a 1.4 km GSD cross-track at nadir, and also approximately 3.5 km at edge of its scan. GOES ABI requirements (as have those for all prior GOES instruments) also called for sufficient spatial over-sampling (i.e., the ratio of the spatial point spread function FWHM to the sampling distance). For feature-tracking through a sequence of images ( as is done for AMVs or for image loops from GOES), it is important that the tracked features do not change in apparent size, shape, or centroid, as the feature it moves from image to image, as such changes will cause a wind velocity observation errors. The MISTIC spatial over-sampling ratio is approximately 1.5:1 along-track-similar to GOES 16 ABI, and more than 2:1 in the along-scan direction. Feature position change-tracking to half the GSD, or finer, is enabled under such sampling conditions, and feature brightness fluctuations are minimized. In addition, MISTIC Winds observes feature position changes with two separate instruments on different platforms in order to measure feature velocity. However, the attitude knowledge provided by contemporary micro-satellite-compatible star trackers and ephemeris knowledge are sufficiently accurate, therefore, these potential error terms in AMV tracer velocity are quite small. These considerations, together with the radiometric sensitivity requirements identified in Table 1 and shown in Figure 8 should be sufficient to support the <1 m/s tracking error capability for both cloud field and water vapor tracer features, for the planned observation separation period of 10 min. Optical Flow methods are under consideration by several groups, as a tracking method potentially capable of improvements in the density and accuracy of passive wind observations.