# Landsat Surface Product Validation Instrumentation:
## 5 Mirror Empirical Line Method (MELM)
### Sentinel-2A Experiment  
In addition to the primary BigMAC validation activities using the small point source and Lambertian reference panels with the UAS-based imaging systems, SPARC was used to target Sentinel-2A MSI 10 m bands during a coincident overpass (30 August 2021, 12:19 Local Time). The automated FLARE system (Figure 18A) and manual SPARC mirrors (Figure 18B) were used to generate high and low LER targets, respectively. Level 1C ToA reflectance imagery for the targets was downloaded (Copernicus Open Access Hub) and the mirror signals extracted using a proprietary software (Labsphere, Inc., North Sutton, NH, USA). These signals were then used to perform a MELM regression against the mirrors' known LER (\(\lambda\)), producing the gain and offset factors used to convert the ToA imagery to a surface reflectance product (Figure 19) in the same manner as described for the UAS spectral data but convolved over the spectral response functions of the Sentinel-2A MSI bands (Figure 18C). Pixels from a region of interest corresponding to the BigMAC natural target area (Figure 18B, green square) were extracted, averaged, and compared to the ASD surface reflectance data measured during the experiment. When convolved against the Sentinel-2A MSI spectral response functions (Figure 18C), the ASD and Sentinel-2A surface reflectance values showed reasonable agreement over the VNIR range (Table 12). The agreement is within 6% except for band 08. The robust performance of the SPARC/FLARE system is illustrated by the fact that the two targets were separated over 30 km and it was able to extract the surface reflectance of the alfalfa field within expected uncertainties compared to ground truth measurements. This supports the capability for using the MELM method for producing reliable large area surface reflectance products.  
\begin{table}
\begin{tabular}{l l l l} \hline \hline  & \multicolumn{3}{c}{**Surface Reflectance**} \\ \hline Sentinel-2A MSI Band & CW (nm) & FLARE Sentinel-2A Image & EROS Ground Truth \\ \hline
02 & 493 & 1.4\% & 3.3\% \\ \hline
03 & 560 & 5.7\% & 5.5\% \\ \hline
04 & 665 & 1.6\% & 4.5\% \\ \hline
08 & 831 & 28.4\% & 26.0\% \\ \hline \hline \end{tabular}
\end{table}
Table 12: Alfalfa surface reflectance derived from the SPARC calibrated Sentinel-2A surface reflectance product and in situ measurements resolved to Sentinel-2A bands.  
Figure 17: (**Left**) Validation of mirror reflectance with the colors corresponding to the mirrors in Figure 16. (**Right**) Two-point MELM applied to an in-scene red felt panel with ground truth data from a field spectrometer.  
Figure 19: Example MELM surface reflectance regressions for Sentinel-2A MSI band 3 for a plot of at-sensor relative image radiance (x -axis) against calculated MELM target LER factor (y-axis). The SPARC target LER (blue dots) produce for each band a gain and offset that were used to derive the band-resolved spectral reflectance of the alfalfa test target pixels in Figure 18.  
Figure 18: Automated FLARE station (**A**) and manual SPARC mirrors (**B**), indicated by the red circles, were utilized to perform a surface-reflectance MELM regression against satellite assets, particularly Sentinel-2A MSI, level 1C imagery. The derived gain coefficients were then applied to estimate the surface reflectance of the alfalfa field target at the BigMAC field site (green Region of Interest). The SPARC calibrated retrievals show reasonable agreement with in situ surface reflectance as measured by EROS (**C**). The blue, green, red, and brown curves represent the relative spectral responses of the Sentinel-2A MSI blue, green, red, and NIR bands, respectively.