# Small Target Radiometric Performance of Drone-Based Hyperspectral Imaging Systems
## 3 Methodology  
### Experiment Overview  
In this section, we will estimate the spectral radiance of point targets (with a fixed mounted HSI system) so as to demonstrate the usability of convex mirrors to understand the energy conservation (or lack thereof) within orthorectified imagery.  
Our experiment was designed to use industry standard Lambertian targets and point targets to investigate the radiometric properties of orthorectified HSI imagery. The Lambertian targets include three field-deployable Permaflect panels, manufactured by Labsphere Inc., with average hemispherical reflectance factors of 7% (dark gray), 25% (gray), and 50% (white). The selection of Lambertian targets is based on previous studies indicating that the global VNIR reflectance levels of natural land surfaces are typically less than 60% [24]. The point targets were protective aluminum-coated convex mirrors with a 25 mm radius of curvature (\(-12.5\) mm focal length) and average specular reflectance (i.e., spectrally flat) greater than 85% from Edmund Optics. All properties related to the mirrors were identical (i.e., radius of curvature and reflectance) and independently measured by Labsphere Inc. The point targets were selected such that saturation did not occur for any pixel, which can be achieved by having the GSD and radius of curvature close in value.  
During collection, the drone made simple down and back overpasses with all targets deployed close to the center of the sensor's FOV. The drone flew at a nominal height of 32 m, resulting in a nominal GSD of 2.1 cm. The integration time was optimized (i.e., spectral signal peaks around 75% saturation) to 5.5 ms from a nadir observation of a secondary 50% Permaflect panel prior to take-off. The overpasses began at 11:15 a.m. local time with all 18 overpasses completed within 8 min. The solar zenith at the time of collection was 42 degrees. All images used were orthorectified using the same DEM, instrument calibration data, and processing software.  
Both the point target configuration and Lambertian target layout can be seen in Figure 3. The Lambertian panels were placed off to the side of the point targets. The larger dark gray and white panels were 1 m \(\times\) 1 m in size, whereas the gray panel was 0.5 m \(\times\) 1 m. With a GSD of 2.1 cm, there is a minimum of 23 full pixels on the panels. The point targets are identified by a mirror ID. For example, the label 12.5 MM-2 denotes mirror number 2 with a focal length of 12.5 mm. The point targets were deployed on a low reflectance (roughly 2%) background panel (0.9 m \(\times\) 1 m) in groupings of four with a minimum of 14 pixels between mirrors. The ASD field spectroradiometer, not shown in this image, was placed nearby in the middle of a cleared field away from the experiment area.  
Figure 3: The Lambertian targets are within the yellow box, and the point targets (i.e., convex mirrors) are within the orange box. The orange arrow in both images helps define mirror placement within the imagery. The image on the right depicts mirror placement and assigned mirror ID, which is linked to the descriptive mirror properties and subsequent results (e.g., 12.5 MM-2).