## 4 Discussion
### Discussion of MISTiC Winds Requirements and Their Relation to Other Instrument Requirements
However, large-displacement optical flow estimation algorithms assume a relatively high degree of grey value constancy, gradient constancy, and image smoothness in the image data from image to image. These image quality attributes need to be ensured for these wind retrieval methods as well, and in order to accomplish this, the spatial resolution and spatial over-sampling considerations described above for AMV tracking with MISTiC are essentially the same as those needed to support optical flow-based wind field retrieval methods. Spatial resolving power is directly related to the potential density of AMVs that can be observed and tracked. Useful water vapor tracers may be as small as the spatial response function of the instrument. For MISTiC, the SRF FWHM is between one and two GSDs, and the entry in Table 2 relates to the number of SRF-scale targets that could be tracked. Cloud AMVs require more spatial information to ensure that the same cloud is being tracked between images, and so the potential density of cloud AMVs is much smaller, as shown in Table 2. The actual density of AMVs depends on the specific atmospheric conditions. In the future, Optical Flow methods may allow a higher density of tracking points in both cloud and clear conditions. Validation of current AMV observation methods widely employs comparisons with radiosonde observations as \"truth\", and validation of the MISTiC Winds observation method would employ this approach as well. However, there are additional error terms that arise in this comparison, that are due to potential errors in the radiosonde data, and to the displacements in either space or time between radiosonde observations. One important difference will be the simultaneous observation of the vertical thermodynamic field profiles both by the MISTiC Winds system, and independently by the radiosondes, which should improve the ability to understand the causes of differences between radiosonde and satellite-derived wind observations. The GOES-ABI-like image sequence requirements adopted for MISTiC differ in important ways from the traditional way IR sounder spatial response attributes are specified. IR sounders are envisioned as providing spatially un-correlated, independent \"soundings\" of the atmosphere. Such sounding instruments are designed so that the individual ground footprints (and atmospheric columns) observed do not overlap. In some sounding instrument designs such as IASI, there is a substantial gap between spatial samples. In image-quality terms though, the images such instruments produce are significantly under-sampled spatially. While spatial under-sampling is fully acceptable for single-frame sounding observations, spatial under-sampling is not desirable at all for image-sequence tracking or optical flow estimation, due to the radiance and radiance gradient errors it introduces. The finer spatial resolution of MISTiC relative to current hyperspectral sounders, although selected in order to support AMV wind observations, will also provide benefits for observing the spatial structure of the atmospheric fields. This capability will enable a larger fraction of cloud-free observations than current operational sounders observe, along with an improved capability to observe multi-level moisture field gradients. The most significant spectral range difference between the requirements proposed for MISTiC and those of hyperspectral sounders currently in orbit is that MISTiC spectral range does not include the longwave infrared (LWIR) atmospheric window or very longwave (VLWIR) CO2 emission-dominated regions of the infrared spectrum. If it did, the size, mass, and power accommodation needs of the instrument would approximately triple, requiring a substantially larger spacecraft host. The MISTiC spectral range includes 4.3 \(\upmu\)m (2390 cm\({}^{-1}\)) region channels, which are in the range used by the AIRS science team algorithms for retrieving the temperature vertical profile.