## 2 Materials and Methods
### MISTIC Winds Observing Concept, Requirements, and MISTIC Instrument Concept
Figure 2: Motion vector winds from MODIS, which observes portions of the polar region approximately every 100 min. Wind tracers are identified using automated algorithms that identify trackable features from a sequence of images. The positions and direction are indicated by wind-barbs, with flags encoded for speed. Color indicates approximate altitude of the AMV. Key design elements of the instrument are identified in Figure 3. An internal scan mirror scans the slit-image field of view across the surface of the earth from east to west as the satellite progresses through the orbit. Like AIRS, the instrument will periodically observe both deep space, and an on-board blackbody source to provide the data used to calibrate the raw radiances, correcting for detector non-uniformity effects. Sensitive spectrometers operating in this portion of the infrared band require cooling for both the spectrometer optics and the detector array. The MISTiC spectrometer is passively cooled by using radiator panels that reject heat to space. A sun-synchronous orbit (SSO) with appropriate nodal crossing times is used so that the spacecraft and instrument are always provided with a surface for this passive radiator that is shielded from solar illumination and earth emission, without requiring spacecraft attitude adjustments. The detector is actively cooled using a miniature pulse-tube cryo-cooler to ensure both sufficient on-orbit life and to minimize vibration-induced spectral observation errors. The overarching design goal adopted for MISTiC Winds is to provide high-quality observations of proven scientific and meteorological value, while making dramatic reductions in size, mass, power, and cost for the instruments. This goal is pursued in a comprehensive and balanced way that includes:  
Tailoring requirements for the 3D winds and temperature/humidity sounding to emphasize observations from the troposphere, by adapting infrared resolving power, infrared spectral range, while minimizing instrument size, mass, power, and overall space segment cost. Using innovative instrument opto-mechanical and thermal design methods and adopting AIRS-proven orbit and passive spectrometer cooling to minimize instrument electrical power. Leveraging advanced instrument technology and the rapidly developing and increasingly capable CubeSat and microsat-satellite technology base including miniature coolers. As mentioned above, relocating hyperspectral IR observations originally envisioned for GEO to a LEO constellation that can provide comparable refresh rates would provide a major reduction in required resources. A second critical requirements adjustment is to constrain spectral coverage range and resolution to those attributes needed to observe the troposphere. This choice emerges from a system-of-systems perspective, which recognizes that thermodynamic soundings of the stratosphere will be provided operationally by other sounders, allowing MISTiC Winds observations to focus on the troposphere. MISTiC spectral band selection leverage scientific and algorithmic advances of the AIRS Science Team, which uses portions of this band for vertical temperature sounding, and whose retrieval algorithms have mastered the challenges posed by non-local thermal equilibrium and surface reflectance in this band [25]. Figure 4 is an example spectrum from an IASI overpass that illustrates the planned spectral range for MISTiC observations, and key atmospheric features within this range. It includes spectral radiances  
Figure 3: MISTiC Winds IR instrument concept, shown with nadir up (**left**) and down (**right**). strongly influenced by CO2 emission in the troposphere (near 2385 cm\({}^{-1}\)) from which the vertical temperature profile is extracted, and radiances strongly influenced by atmospheric moisture fields (between 1750 and 2000 cm\({}^{-1}\)), that, together with the temperature profile, can be used to extract vertical moisture profile.