# Small Target Radiometric Performance of Drone-Based Hyperspectral Imaging Systems
## 4 Results  
This section presents the main findings and results of the experiment. In our analysis below, standard orthorectification processes are assumed. However, the exact process and interpolation schemes are irrelevant to fully understand and quantify the results because point targets provide for a \"black box\" approach. In fact, instrument and post-processing anonymity further demonstrates the usefulness of the SPARC technique. Nothing more than the ground sampling distance (GSD) is required to characterize small target radiometric performance. Analyzing the unorthorectified imagery provides a baseline to determine how much energy was originally collected for comparison with the orthorectified results. More importantly, the observed spatial pattern of point targets helps to aid in the discussion of the observed results. The experimental design is key for extracting two types of results discussed in this section, highlighted in Figure 10. When the small point target is imaged over multiple frames (e.g., over 18 overpasses), the radiometric repeatability of small targets can be examined, as seen in Figure 10a, whereas when two similar point targets are imaged together,  
Figure 8: The spectral radiance of the mirror backgrounds (**right**) is extracted and plotted (**left**). Even though there is little variation between the background panels, the background subtraction is kept separate. The saw-tooth structure within the spectral radiance is an artifact of the HS sensor and is only observed at low radiance signals where read noise dominates. Figure 9: The summation of a point target over the SPSF image (**left**) defines the ensquared energy and can be connected to the predicted entrance aperture-reaching spectral radiance. The spectral radiance for the observed ensquared energy on the (**left**) is plotted on the (**right**). The color of each pixel within the SPSF image define the intensity values. Low and high values are dark blue and yellow, respectively. as seen in Figure 9(b) over a small region, the radiometric stability from orthorectification can be assessed. Both assessment strategies will allow for the investigation of radiometric errors induced on point targets by platform motion and orthorectification. Over the entire experiment, eight point targets were imaged during 18 overpasses, accounting for 144 point target observations in total. Of the 144 observations, only 26 point targets (or 18.1%) were within the uncertainty of the predicted entrance aperture-reaching spectral radiance, at a 95% confidence level. When assessing the use of a new technology (i.e., point targets) for investigating the radiometric accuracy of drone-based HSI systems, it is critical to compare results to standardized methods (i.e., Lambertian targets). In our experimental plan, Lambertian panels were deployed that spanned roughly 2% to 50% reflectance to compare image-to-image radiometric stability. Figure 10(a) illustrates the spectral radiance standard deviation of each mirror given 18 observations (i.e., overpasses). Figure 10(b) illustrates the spectral radiance standard deviation of a single pixel, for each of the four Lambertian panels, given 18 observations. Figure 12 is an observation of a single mirror's spectral radiance across the 18 overpasses. A general trend exists when observing the difference between the unorthorectified (top) and orthorectified (bottom) imagery: the absolute radiometric signal of small targets is generally overestimated for fixed mounted HSI systems. Figure 11: Assessing the measurement dispersion within the orthorectified imagery of each (**a**) point target and (**b**) the Lambertian targets across the 18 overpasses. The Lambertian targets show little variation compared to the point targets. Figure 10: Point targets were examined from two perspectives highlighted in the orange boxes: (**a**) tracking a single mirror over the 18 overpasses and (**b**) comparing mirror pairs from a single image.