# Compact Midwave Imaging System: Results from an Airborne Demonstration
## 3 Results
### Ground Point Validation  
The Hold F flight contains mostly clear-sky land pixels over farmland in the vicinity of Durham, NC during the day. This case offers the opportunity to validate against static targets (i.e., terrain) with known altitudes. The aircraft made two passes over the collection area, but a readout error affected imagery from the first pass (Hold F1), rendering only the second (F2) useable. The ground sampling for this case is 12.2 m (AT) \(\times\) 22.8 m (XT) per pixel and the time difference between forward-nadir and aft-nadir looks is a nearly constant \(\pm\)19 s. Feature templates of 16 \(\times\) 16 pixels were used to obtain disparity matches at 20373 sites. The  
Figure 8: Calibrated radiance for Case N from the St. Lucie Nuclear Power Plant (27.35\({}^{\circ}\)N, 80.24\({}^{\circ}\)W) on the east coast of Florida during Science Flight #2 on 19 Jan 2021. Radiance from (**a**) 4.05 \(\upmu\)m, (**b**) 3.75 \(\upmu\)m (nadir) are presented. Notice the warm-water outlet and cold-water inlet flowing from and to the nuclear facility, respectively. retrieval model solved for height and velocity with prior AT velocity set at zero, as it should be for the ground. Figure 9a features radiance measurements from the 3.75-\(\upmu\)m nadir stripe where the aircraft route of flight was from left to right. The radiance across the left two-thirds of the image is fairly uniform due to flat terrain, as indicated by the zero-wind heights shown in Figure 9b. The radiances in the right 1/3 of the image are higher due to the high reflectivity of clouds in the MWIR. Under the assumption of zero AT winds, the XT winds (Figure 9c) were retrieved with values for this case ranging between \(-\)3.0 and 2.4 m/s. The bias-corrected XT winds are very close to zero in the no-cloud region (i.e., the terrain is not moving) as they should be. We also note that patterns of what appears to be mass divergence with positive XT winds adjacent to negative XT winds in the cloudy region (Figure 9c). Figure 9d presents the zero-wind heights only for heights below 400 m in order to facilitate comparison with digital elevation map (DEM) heights (Figure 9e). The white area in Figure 9d (i.e., no data) is the region obscured by clouds. Comparison between Figure 9d,e shows that the CMS stereo retrieval replicates the small 50- to 100-m variations in terrain height well. Figure 10 shows a histogram of all retrievals passing the quality criteria. The clouds clearly separate from the clear-sky ground-points, which can be classified by their retrieved height (<500 m) and speed (<5 m/s). The ground-point class consists of 16,518 sites also  
Figure 9: Analysis of hold F2 on 27 Jan 2021 for (**a**) CMS calibrated radiance; (**b**) CMS zero-wind stereo height; (**c**) CMS XT winds; (**d**) zero-wind heights with color contouring to highlight surface elevation differences; and (**e**) surface height from a digital elevation model (DEM). The direction of flight is from left to right, which is toward the northwest near Durham, NC, USA. meeting the retrieval quality criteria. The retrieved heights of ground points are then compared to the USGS one-arc-second digital elevation model (DEM) and the retrieved XT velocity is compared to zero. The next step is to evaluate the statistical performance of CMIS retrievals for Hold F. Figure 11a-c show a point-by-point comparison between the initially retrieved stereo heights and the DEM from Figure 9e. Since the DEM represents height above the geoid, we subtracted the geoid height above the WGS-84 ellipsoid, which is equal to \(-32\) m at the center of the collection, to represent the terrain height above the ellipsoid. Finally, the DEM is averaged to a scale, which is comparable to that of the templates. Figure 11a,d show the XT residuals for the initial stereo heights and XT winds, respectively. These are a systematic bias that can be determined empirically as a function of the XT pixel number and removed from the initial stereo height and XT wind retrievals.