# Small Target Radiometric Performance of Drone-Based Hyperspectral Imaging Systems
## 5 Discussion
### Radiometric Accuracy Concerns  
Figure 13 examines two overpasses of the same point target (mirror ID: 12.5 MM-4) imaged within 2 min. It has already been demonstrated that the atmosphere and HSI instrument stability cannot explain this drastic increase in perceived energy from the point  
Figure 17: The orthorectified point response at (**a**) 443 nm and (**b**) 844 nm illustrates the wavelength-dependent SPSF size for convex mirror, 12.5 MM-2. The sharpness of the SPSF impacts the radiometric performance after orthorectification. The colors of each pixel define the intensity values. Low and high values are dark blue and yellow, respectively.  
target. The only difference between the imaged point targets is the platform motion (fixed mounted HSI) and resulting orthorectification.  
If the unorthorectified curves (and point target imagery) are examined more closely, we can begin to form definitive conclusions on the reasoning for such drastic difference in the perceived spectral radiance within Figure 13. When the mirror was imaged on the second overpass, the perceived energy was lower than expected, even though the orthorectified signal was accurate according to the prediction. In this instance, the orthorectification properly interpolated the scene to add an adequate amount of energy back that was never physically observed. This is one of the largest contributors to the large measurement dispersion seen in the point targets. There is nothing within the post-processing algorithm that provides enough information to accurately inject the correct amount of energy that was never observed due to inconsistent sampling of the ground during imaging. For the orange dashed curve, the HSI system actually imaged the target or ground location too many times, and the orthorectification cannot realistically dispose of the excessive amount of energy from the point target. This is also observed in Figure 14 as well. This observation provides a missing link within orthorectification algorithms that can be easily seen when imaging high-contrast point targets (i.e., convex mirrors) of a known radiometric signal. If information is not observed or is in excess, orthorectification algorithms can only use IMU/GPS and interpolation schemes to correct the scene.  
As mentioned before, orthorectification does not have the correct information to correctly dispose of excess collected energy when a point target is re-imaged. As Figure 14 shows, the point target pairs are similarly affected in the unorthorectified imagery where both record roughly twice the expected mirror signal. However, a difference between the point targets can be seen after orthorectification, where the point targets no longer overlap. Even though the point targets were aligned in the across-track direction, the orthorectification affected the spectral radiance curves differently, further highlighting the volatility in localized regions.  
These results demonstrate a clear lack of realism in orthorectifying algorithms for pixel-level details for fixed mounted HSI systems. Multi-modal imaging capabilities and constrained budgets are situations where the radiometric accuracy of the HSI system is sacrificed for payload configuration. At a minimum, convex mirrors provide a low-cost solution for assessing the radiometric performance for any payload configuration. The most optimal mounting solution for HSI systems is a gimbal mount. The reduction in the HS instruments relative motion to the drone will reduce the motion-induced radiometric errors as seen in the results. A gimbal mounting solution will not fully remove all orthorectification errors. However, using the technique illustrated in this paper, the radiometric performance for small targets can be estimated.