# Small Target Radiometric Performance of Drone-Based Hyperspectral Imaging Systems
## 4 Results
Figure 13 takes a closer look at two instances of the same point target (mirror ID: 12.5 MM-4) imaged within 2 min or three overpasses apart. The solid blue curve represents the point target imaged on the second overpass of the experiment and provides a mirror signal estimate within the uncertainty. However, not even 2 min later, the same mirror (now in solid orange curve) is estimated to be 25% brighter. Figure 12: From unorthorectified (**top**) to orthorectified (**bottom**) imagery, the overall predicted radiance is immense. The ensquared energy captured in the unorthorectified imagery varies significantly above and below the predicted entrance aperture-reaching spectral radiance, whereas in the orthorectified imagery, most of the curves overestimate the ensquared energy (i.e., nearest neighbor interpolation scheme). The orange box within the picture highlights the point target that was analyzed over the 18 overpasses. Figure 13: The same point target (12.5 MM-4) imaged at overpass #2 and overpass #5 signifies the radiometric inconsistency when observing a point target in quick succession. The blue curve is measured within the uncertainty, whereas the orange curve is over-predicted. Platform motion and orthorectification contribute to these inconsistencies. The dotted and solid curves correspond to the unorthorectified and orthorectified point targets, respectively. If we focus our attention to a mirror pair as represented in Figure 10b, an examination of oversampling the observed scene can be discussed further. Figure 14 demonstrates a major issue: imaging point targets with a fixed mounted pushbroom HSI system encountering significant platform motion. When point targets are re-imaged over multiple frames due to platform motion, the measured radiometric response will always be overestimated. The comparison between mirror pairs shows that orthorectification can be different over a small region on the ground. The previous examples demonstrated the inconsistent radiometric performance of orthorectification on the overall signal level. In the following examples, spectral distortions from the orthorectification process will be examined. Figure 15 illustrates a mirror pair after orthorectification, where we can see that the observed spectral radiance has changed. When a scene is not imaged properly (i.e., along-track is spatially undersampled), orthorectification can accurately represent the radiometric signal from a point target or inadvertently eliminate energy. Within Figure 15, the pixel highlighted with the red \"X\" is completely eliminated after orthorectification. The red arrows indicate full row interpolation after the imagery is orthorectified. Mirror 1 (blue curves) is a perfect example of linear interpolation positively affecting the mirror's spectral radiance. Whereas, mirror 2 (orange curves) demonstrates a major issue with the orthorectification as energy was eliminated. Figure 14: Mirror pairs (12.5 MM-7 and 12.5 MM-8) that are imaged simultaneously show similar behavior (signal and spatial smear) in the unorthorectified image. Whereas the orthorectified product adds uniqueness to their signal and spatial behavior. The dotted and solid curves correspond to the unorthorectified and orthorectified point targets, respectively. Figure 15: For mirror pairs, 12.5 MM-1 and 12.5 MM-2, significant orthorectification errors exist and act conversely on the spectral radiance. Interpolation contributes heavily to ensquared energy being added to the point target that was never captured. The dotted and solid curves correspond to the unorthorectified and orthorectified point targets, respectively. Figure 16 highlights issues when only single pixels are linearly interpolated and contributes to spectral inconsistencies in the spectral radiance of the orthorectified imagery. Single-pixel interpolation can increase the ensquared energy by forming linear combinations of surrounding pixels that do not spectrally match the target.