# Compact Midwave Imaging System: Results from an Airborne Demonstration
## 3 Results
### Ground Point Validation
We see  
Figure 11: Statistical comparisons for (**a**) stereo-DEM height difference vs. XT pixel; (**b**) DEM height vs. uncorrected stereo height; (**c**) DEM height vs. corrected stereo height; (**d**) XT velocity vs. XT pixel; (**e**) uncorrected XT velocity vs. probability distribution function (PDF); (**f**) corrected XT velocity vs. PDF. Figure 10: Histogram of retrieved XT wind and height shows low clouds with small XT velocities and a larger population of clear-sky ground retrievals. in Figure 11b and e that the biases for the uncorrected CTH and XT winds are \(-11\) m and \(0.23\) m s\({}^{-1}\), respectively, which affects their overall accuracy. When the biases are corrected, the standard deviations reduce to \(20.0\) m and \(0.27\) m s\({}^{-1}\) (Figure 11c,f) for the CTH and XT winds, respectively. Thus, systematic error compensation is needed to achieve the best results for this case. The leading hypothesis for the origin of the systematic errors is imperfect optical calibration initially done in the laboratory and adjusted in flight using mapped features such as roads. Our analysis of differences between stereo and DEM heights for ground points demonstrates a methodology that can be used in future space flights for geometric calibration or validation. A scatterplot of XT wind vs. zero-wind height (Figure 12a) reveals an apparent correlation between the two fields. When the data are plotted along the transect shown in Figure 12c, an interesting pattern emerges. The XT winds in the cloudy regions (closed black diamonds) are well correlated (Figure 12b) with the zero-wind cloud-top height (CTH; red triangles). As discussed by [PERSON] et al. (2019), if a platform making stereo observations where the AT wind is not negligible, the errors of retrieved zero-wind stereo CTH and unaccounted AT wind are highly correlated. If the CMS flight maintains a fixed angle with respect to wind direction and the wind direction does not vary, the wind speed variation would result in a significant AT wind component that induces a stereo CTH error. Since the AT and XT winds are correlated under the fixed directional flight, the AT-CTH error correlation manifests itself as the XT-CTH correlation as seen in Figure 12a,b. The magnitude of AT winds can be assumed based on the value required to minimize the change in altitude of the cloud-top height. When that assumption was applied for this  
Figure 12: Analysis of Hold F2 for (**a**) scatter plot of XT wind vs. zero-wind height; (**b**) XT winds and heights as a function of row pixel #; and (**c**) wind-corrected height. The red line is a transect along which the data in (**b**) are plotted. The open and closed diamonds depict XT winds for surface pixels and cloudy pixels, respectively. For cases in which the pixels are cloudy, the red and blue triangles depict zero-wind heights and wind-corrected heights. When the AT winds are assumed to be zero (i.e., zero wind), an anomalous correlation between XT winds and zero-wind heights appears. When more realistic AT winds are assumed as described below, the correlation largely disappears. case (V\({}_{\text{at}}\) = 2.5 m/s), the XT-CTH correlation disappeared, as shown with the blue-triangle symbols for the wind-corrected heights and XT winds in Figure 12b. The geometry for this situation is illustrated in Figure 13.