## 2 Data and Methods
### Study Methods
#### 2.2.4 Rail Road Valley Playa (RRVP) PICS--Surface Reflectance at a Desert Site  
RRVP is among the PICSs endorsed by the Committee on Earth Observing Satellites (CEOS) to serve as a standard reference for the post-launch calibration of space-based optical imaging sensors [40]. RRVP is located in a large, dry lakebed in central Nevada and has a dry climate, typical of the high desert of the western US [41]. The site is characterized by high reflectance, relatively high spatial and temporal uniformity, high midday sun elevation, and minimal cloud cover. The surface layers and composition are relatively smooth and spatially homogeneous, consisting of compacted clay-rich lacustrine deposits [42]. More description of the site and detailed spectral evaluations are available in [PERSON] et al. [41], [PERSON] et al. [42], and [PERSON] et al. [43; 44]. Because of its large size, RRVP is used for sensors with larger footprints (1-10 km) and is automated with instrumentation used extensively for the vicarious calibration of terrestrial imaging sensors covering the VNIR and SWIR wavelength ranges [43; 44].  
The Hyperion time series collection at RRVP was processed following the procedures outlined in [PERSON] et al. [45]. Hyperion TOA radiances were converted to surface reflectance using the Atmospheric CORrection Now (ACORN) software [46] and the module designed for pushbroom imaging spectrometers with cross-track spectral calibration variation. According to the date of acquisition, apparent surface reflectance was derived using either mid-latitude summer or winter atmospheric models. To preserve the original spectral properties and variability, the images were not geographically or geometrically rectified. This was valid as a prior study established that at RRVP, Moran I statistics vary between 0.81 and 0.95 across Hyperion's spectral range (1 = strong positive spatial autocorrelation, 0 = spatially uncorrelated data), which was attributed to variation in soil moisture affects and differences in the mineral composition of the surface [45]. Band subsetting was used to remove uncalibrated and overlapping bands and bands adjacent to water absorption features, resulting in subsets having 171 bands (Table 4).  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Array** & **Bands** & **Wavelengths (nm)** \\ \hline VNIR & 8–57 (49) & 426-926 \\ SWIR1 & 7–120 (41) & 993–1346 \\ SWIR2 & 129–165 (36) & 1518–1800 \\ SWIR3 & 179–224 (5) & 1942–2395 \\ Total & (171) & \\ \hline \hline \end{tabular}
\end{table}
Table 4: Subset of bands used in the Rail Road Valley Playa (RRVP) study.  
Time series of Hyperion data were used to determine spectral stability during the period previous to the precession change beginning in 2011 or during the changing satellite precession. Thirty-seven radiometrically corrected Level 1R images from 2001-2015 were converted to surface reflectance using the ACORN software. Mean reflectance and standard deviation (SD) were calculated for select wavelength bands (Figure 4). These wavelengths were chosen as representative of the spectral properties throughout the VNIR and SWIR wavelengths for RRVP. The mean was calculated with data from 2001-2008. Twenty-three pre-precessional and fourteen post-precessional images (see Figure 1 for dates) were used for the statistics of the change as delta reflectance (\(\Delta\)p).