# Small Target Radiometric Performance of Drone-Based Hyperspectral Imaging Systems
## 5 Discussion
### Overall Point Target Performance  
Figure 11 illustrates the main findings when comparing point and Lambertian targets over multiple overpasses. Even though all the point targets are similar (i.e., mirror-to-mirror variation less than 4%), their standard deviations are significantly greater than the Lambertian targets that span the detector's dynamic range (i.e., the HSI system was optimized for a 50% reflector). The main limitation of the use of Lambertian targets for radiometric performance is their overall spatial uniformity when compared to point targets. Lambertian targets can isolate an HSI system's absolute radiometric performance, but the high spatial uniformity masks orthorectification artifacts. The use of point targets provides the best technique to examine the system-level radiometric performance of both the instrument and orthorectification. Figure 11a demonstrates that the radiometric performance over Lambertian targets is less affected by orthorectification errors due to their high spatial uniformity compared to point targets. Within Figure 11, the general spectral shapes require further explanation. Excluding values at both ends of the wavelength range (i.e., close to 400 nm and 1000 nm), we see a significant trend of worsening measurement dispersion at the bluer wavelengths for point targets. We speculate that the reasoning for this is the interplay between orthorectification and the wavelength-dependent SPSF sharpness (Figure 17) [23]. Depending on how the point target is imaged, the orthorectification struggles with sharper targets compared to targets having a larger spatial extent. This trend continues until the target becomes much larger than the SPSF or, in this comparison, a Lambertian target, where the image-to-image  
Figure 16: After orthorectification, the spectral radiance around 700 nm was significantly modified compared to the rest of the spectrum. The red boxes in the right image indicate single pixels that were interpolated during the orthorectification process. The dotted and solid curves correspond to the unorthorectified and orthorectified point targets, respectively. repeatability is small (see Figure 11b). Radiometric errors induced by orthorectification on small targets not only exist for the point targets but will manifest in any small target within the scene. Insight into these observations and the ability to extract radiometric inconsistencies of small targets is only possible with high-contrast point targets (i.e., mirrors). Further wavelength-dependent orthorectification errors are discussed in Section 5.3. A lesser known effect that may account for the unreal spectral features seen in the point target results in Figure 11a is the potential for polarization sensitivity. As mentioned in Section 2.3, the point targets are convex mirrors coated with protected aluminum. The aluminum layer is a metal thin film and has the potential to reflect polarized light up to the sensor at specific solar zenith angles. Since the HSI system has a reflective holographic grating, there is an inherent grating efficiency for the various polarization states. If the HSI system reimages the point target at various polarization states and orthorectified to a single target, the observed spectral features could potentially exist in the results shown in Figure 11a [15; 26]. Figure 12 demonstrates how fixed mounted HSI systems image the ground scene where platform motion significantly impacts the ground sampling. This observation is seen for all the point targets with the exception of a few outliers, to be discussed in Section 5.3, even though the spectral radiance is underrepresented in the unorthorectified image. The main contribution to this over-estimation is orthorectification errors induced by platform motion (e.g., re-imaging the same point target) and interpolation artifacts within the orthorectification process (e.g., pixel values manipulated).