# Compact Midwave Imaging System: Results from an Airborne Demonstration
## 2 Materials and Methods  
### Instrument  
CMIS is a pushframe imager with a single lens assembly that relies on the forward motion of the satellite (or aircraft) to create a two-dimensional (2D) image swath (Figure 1a). The optical design of pushframe imagers is based on a set of passband filters inserted into the optical path directly over a focal plane array (FPA) such that each filter stripe maps into a unique row (line), or a continuum of rows, on the array. As shown in Figure 1b, the CMS filter design includes three stripes at 3.75 \(\upmu\)m to provide fore (+20\({}^{\circ}\)), nadir (0\({}^{\circ}\)), and aft (-20\({}^{\circ}\)) views, plus two stripes at 2.25 \(\upmu\)m and 4.05 \(\upmu\)m in between. A given wavelength stripe on the filter covers 32 rows of pixels on the FPA at the same wavelength in order to allow for time-delay integration (TDI) to build up signal if needed to image dim targets. As the satellite moves along its orbital track, the imager takes samples of pixels in a cross-track (XT) swath and maps each pixel from each stripe to a unique spot on the ground. This is shown in the CMIS'snapshot' in Figure 2a. As the satellite (or aircraft) moves forward, the instrument builds up a continuous ground swath for each stripe, with different relative coverage times by each stripe for a given swath. At typical orbital speeds ~7.7 km s\({}^{-1}\) in low-earth orbit [6], the nadir and aft 3.75-\(\upmu\)m stripes image a given ground swath at 20 s and 40 s, respectively, after the time that imagery for the same area was collected by the 3.75-\(\upmu\)m fore stripe. This design allows us to measure the apparent change in position of a cloud between the fore and nadir views and again between the nadir and aft views, as shown in Figure 2b. These apparent changes in position, referred to as disparities, are used to derive atmospheric motion vectors (AMVs) and cloud-top heights (CTHs). The first step in the process is to perform geolocation and apply a calibration to derive radiances for each stripe. Radiances are then used to perform pattern matching for clouds and terrain between the fore and nadir, and the nadir and aft images. Texture, shape, and size of the individual cloud elements contribute to the correlation between patterns in different images used to identify a positive match. After cloud elements are matched between multiple images, disparities can be derived. The component of the disparity in the XT direction relates directly to XT wind speed, although CTH is needed to correctly calculate the velocity. Disparities in the along-track (AT) direction contain information about both the CTH and AT wind, which are difficult to separate. An independent view from another platform is needed to resolve this ambiguity. Nadir views in two platforms in a leader-follower formation together with times of observation for both platforms can be used to determine the wind velocity in the AT direction. This constrains the fraction of AT disparity due to AT winds, thereby allowing retrieval of CTH. Given CTH, XT winds can be retrieved for the given XT disparities. This method is very simple conceptually, but numerical methods are required to perform optimal fits over large scenes. These methods are described in [PERSON] et al. [4] and adapted for an aircraft as described below. Figure 3 presents the three spectral bands chosen for the CMIS instrument, superimposed on a MODTRAN atmospheric transmission spectrum. The bands are located in spectral windows where the atmosphere has high transmission. The band at 3.75 \(\upmu\)m provides day and night (24/7) images for stereo sensing. The band at 4.05 \(\upmu\)m can be used to derive temperatures, including those for land and sea surfaces, volcanic plumes, and wildfires.