## 2 Materials and Methods
### MISTIC Winds Observing Concept, Requirements, and MISTIC Instrument Concept  
One of the central ideas behind MISTIC Winds is the use of a constellation of micro-satellites, each hosting a miniature infrared spectral sounding instrument, to enable much more frequent observations of the three-dimensional atmosphere than a single instrument in LEO could provide. A constellation concept for these observation employs groups of three satellites in a (nearly) common orbital plane, with nodal crossing times separated by a short interval-nominally 10 min. One such wind-triplet group is shown, in blue, in Figure 1. Each group of three instruments observes a motion-vector wind triplet of hyperspectral images, analogous to three sequential images of a region by a GEO met-imager such as GOES, that are currently used to derive AMVs. In Figure 1, the light blue rectangles indicate the approximate ground-projection of the hypercube, which is acquired as the slit image of a spectrometer is scanned in a direction perpendicular to the polar orbital track (an alternative mission possibility, more frequent IR sounding, but without wind observations was also considered, and its orbits are shown in Figure 1 in goldenrod, but not discussed further). AMV winds, such as those shown in Figure 2, represent the current derived product, with horizontal wind speed and direction being determined from tracer observations in three radiance images. In the hyperspectral AMV case, the feature tracked would either be a cloud or a geometric  
Figure 1: Hyperspectral motion vector winds and infrared (IR) sounding constellations employing low earth orbit sun-synchronous orbit (LEO SSO) constellations. A winds constellation (blue) of three microsatellites in each of two orbital planes provides vertically resolved tropospheric winds and IR soundings globally, every six hours. feature of the retrieved water vapor field. Hyperspectral AMVs will build on methods developed for multispectral AMVs, but will feature more layers, with better vertical resolution than available from current multispectral imager-derived AMVs. A potentially more powerful, but more data-intensive wind retrieval method borrowed from the computer vision community, Optical Flow, is currently being developed by several groups [21] for use in wind field retrieval from meteorological imaging sensors. This paper will focus on the hyper-spectral extension of the current AMV method, but MISTiC observations are ideally suited for an Optical Flow-based retrieval method as well. Six MISTiC satellites in this constellation type would provide vertically-resolved wind velocity measurement, globally, with a refresh rate of six hours, together with high spatial resolution vertical temperature and moisture soundings of the troposphere at the same six-hour refresh rate as one example. Each MISTiC instrument would collect a series of hyper-cubes as it progresses along the orbital track. A MISTiC hypercube includes 480 along-track spatial samples, more than 1000 spatial samples cross-track, and 580 spectral samples. The vertically resolved tropospheric wind is obtained through higher-level data processing of the hyperspectral image data, using methods drawn from both IR sounding and feature-tracking AMV methods, or optical flow methods, using data from all three hyper-cubes provided by the satellite triplet. The MISTiC instrument is a dispersive imaging spectrometer, which combines features of BAE Systems' airborne and ground-based hyperspectral imaging systems developed for the Department of Defense, with spectral resolution, range, spectral calibration stability, and spectral knowledge requirements derived from the NASA AIRS instrument requirements and GOES-R Hyperspectral Environmental Suite (HES) infrared sounding requirements. A detailed physical model view of this instrument is shown in Figure 3. The instrument will be carried aboard an ESPA-class micro-satellite in polar sun-synchronous orbit.