# Small Target Radiometric Performance of Drone-Based Hyperspectral Imaging Systems
## 6 Conclusions  
In this paper, the radiometric performance assessment on small targets was assessed for a fixed mounted drone-based hyperspectral imaging system. Convex mirrors were used to create point targets that were radiometrically connected to the solar spectrum. Point targets provided the ultimate radiometric test in reconstructing an accurate radiometric signal from orthorectified HS imagery. Results demonstrated the lack of physical realism in the orthorectification process when analyzing point targets. In retrospect, Lambertian targets showed excellent repeatability and self-consistency over multiple overpasses compared to point targets. There was an 18.1% chance that a point target was accurately predicted using radiometric equations defining the spectral radiance of a point target. Major issues originated from both the mounting of HSI systems and the orthorectification process itself. A radiometrically accurate HSI system tends to overestimate the true spectral radiance of small targets based on the nearest-neighbor interpolation scheme used when orthorectifying imagery. Due to the sub-pixel response of the imaging system to a point target, the orthorectification and interpolation can be assessed for deriving radiometrically accurate HS images for the most difficult object, a small target.  
A major challenge for drone-based HSI systems is stable along-track sampling (i.e., sampling in the direction of motion). Thus, future work should examine the relationship between orthorectification and along-track sampling over point targets from a linear translation stage. The primary focus of this experiment would be to recreate field conditions similar to drone-based campaigns but restricting the platform motion to the along-track direction. Limiting the platform motion in this direction would provide a technique for extracting the true correlation between orthorectification and small target radiometric performance at different along-track sampling rates. More importantly, different interpolation schemes could be examined to determine the radiometric and spectral integrity for small targets.  
Conceptualization, D.N.C. and E.J.I.; Data curation, D.N.C., T.D.B. and N.G.R.; Formal analysis, D.N.C.; Investigation, D.N.C.; Methodology, D.N.C., T.D.B. and N.G.R.; Resources, D.N.C., T.D.B. and N.G.R.; Software, D.N.C. and T.D.B.; Supervision, E.J.I.; Validation, D.N.C. and E.J.I.; Visualization, D.N.C.; Writing--original draft, D.N.C.; Writing--review and editing, D.N.C. and E.J.I. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
All relevant data can be requested by contacting corresponding authors due to privacy.  
The authors declare no conflicts of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} DIRS & Digital Imagery and Remote Sensing \\ VNIR & Visible-Near Infrared \\ LWIR & Long-Wave Infrared \\ LIDAR & Light Detection and Ranging \\ HSI & Hyperspectral Imaging \\ GSD & Ground Sampling Distance \\ QTH & Quartz-Tungsten Halogen \\ SPSF & Sampled Point Spread Function \\ NEI & Noise-Equivalent Irradiance \\ FOV & Field-Of-View \\ m & meters \\ cm & centimeter \\ NIR & Near Infrared \\ HS & Hyperspectral \\ SPARC & SPecular Array for Radiometric Calibration \\ GPS & Global Positioning System \\ IMU & Inertial Measurement Unit \\ \end{tabular}