# Small Target Radiometric Performance of Drone-Based Hyperspectral Imaging Systems
## 5 Discussion
### Spectral Inconsistencies  
For most HSI applications, the overall signal accuracy has less of an impact on the algorithm performance compared to the spectral shape. For sub-pixel target detection or spectral unmixing algorithms, the reliance on the repeatability of the spectral component is critical to identify spatially unresolved objects. Significant issues can arise within algorithms if the spectral component of small targets is not conserved over all images. Figure 15 and Figure 16 are examples where the primary contribution to spectral distortions related to the point target signal is the orthorectification process and the interpolation scheme. During the orthorectification process, nearest neighbor or linear interpolation is implemented to construct the scene, but both of these schemes have their issues when encountering a point target. Nearest neighbor interpolation has the issue of replicating or replacing pixels within a scene. For point targets, this can drastically modify the radiometric and spectral integrity due to the inherent SPSF sharpness. Linear interpolation has a lesser effect on the signal level because it has an averaging effect but has significant issues by creating \"unphysical\" spectra [11]. In addition, both interpolation schemes will have differing performance when HS instrument defects are present (i.e., smile and keystone). HSkeystone will produce the worst effects for small spectral targets because pixels of varying spectral quantities will be replicated or replaced and will produce a modified spectrum. To further complicate this problem, the polarization sensitivity of HS instruments will add to spectral inconsistencies. During imaging, the ground was not sampled adequately and resulted in missing energy from the point target. However, during orthorectification, the linear interpolation derived more energy and spatially modified the point target. The orange curves within Figure 15 exhibit a more complicated orthorectification process that spectrally modified the mirror signal. Again, during imaging, the mirror's signal was not fully captured by improper sampling of the ground. But instead of adding energy to the mirror's signal, this is one of the few cases where the orthorectification caused a massive under-prediction of the mirror's spectral radiance. The main cause is the complete elimination of the pixel highlighted with the red \"X\" as seen in the left side of the figure. Because of the wavelength-dependent SPSF sharpness, the amount of energy missing after orthorectification affects the shorter wavelengths more. Furthermore, the red arrow (bottom right of figure) indicates a row of pixels that were linearly interpolated but does not fully account for all the missing energy in the blue end of the spectrum. When entire rows are interpolated (Figure 15), the instrument defects (i.e., keystone) can have negligible effects on the point target's overall signal because all keystone artifacts are replicated in the interpolation. This is not true when single pixels are interpolated as seen in Figure 16. More importantly, the signal reconstruction will be further impacted if the observed spectral radiance experiences a polarization dependence. The red boxes (right side of figure) highlight pixels that never existed in the unorthorectified image. Since these pixels are a weighted linear combination of their surroundings, the added energy does not combine to realistically replicate the predicted energy defined by the black spectral curve. Since the mirror's energy is a summed quantity, any instrument defects should not impact the overall spectral signature, even though single pixels may have a spectrum that has been modified by keystone effects. This is true as long as the ensquared energy for all wavelengths is captured in the bounding box. The important conclusion from this analysis is that single-pixel modification drastically affects the spectral component of small targets compared to entire rows being added.