## 4 Results
### Data Comparison with M3 TRIOS System--Matchup Analysis  
The 29 matchups between the PANTHYR and M3 TRIOS systems, interpolated to typical multispectral wavelengths, are shown via scatterplots of \(E_{d}(\lambda)\), \(L_{d}(\lambda)\), \(L_{u}(\lambda)\), and \(\rho_{w}(\lambda)\), and associated  
Figure 5: Sample photos taken from the camera aligned with the radiance sensor (for azimuth angles not used for measurements in this paper). (**a**) Bird blocking field of view; (**b**) more subtle measurement contamination; (**c**) floating vegetation and platform shadow on water; (**d**) unexpected water target contamination. linear regression statistics in Figure 7, including root-mean-square difference (RMSD) and mean average relative difference (MARD). The MARD of \(E_{d}(\lambda)\) calculated over all wavelengths was 3.1% and the scatterplot (Figure 7a) suggests some spectral variability of differences with lower PANTHYR \(E_{d}(\lambda)\) for 410 nm and 440 nm and higher PANTHYR \(E_{d}(\lambda)\) at 620 nm and 675 nm. A full uncertainty analysis is not yet available for these instruments/systems as deployed at AAOT, and no corrections were applied for instrument artefacts such as imperfect cosine response, straylight, thermal sensitivity, non-linearity or polarization effects, or deployment imperfections, e.g., for optical perturbations from higher mast structures. Operational deployment of the PANTHYR system for fiducial reference measurements will require such an uncertainty analysis but is beyond the scope of this technology-proving paper. However, the observed differences for \(E_{d}(\lambda)\) are not a cause for concern at this stage of testing. The reader is referred also to Reference [34] for a more detailed discussion of differences between measurements of \(E_{d}(\lambda)\) from different instruments in typical field conditions. The scatterplot (Figure 7b) of \(L_{d}(\lambda)\) sky radiance measurements shows strong correlation but for two distinct groups of data. Analysis of the metadata recorded for both systems revealed that these distinct groups of data correspond to different azimuth angles of the PANTHYR and M3 TRIOS systems; they are pointing at very different portions of the sky and, in some cases, the PANTHYR system is pointing at a much brighter sky with cirrus clouds. Since both of the obvious two groups of data corresponded to cases where PANTHYR and M3 TRIOS were measuring at different azimuth angles because the systems were deployed on different sides of the AAOT platform and, hence, pointed at the water in different azimuth angles, there is no cause for concern that neither group of points has a slope of one. There were only two matchups where PANTHYR and M3 TRIOS were measuring at the same azimuth angle, but this was considered insufficient for detailed analysis. Figure 6: Water reflectance spectra for each of the 30 stations (gray lines) from (left) the three-radiometer TRIOS/RAMSES system (M3 TRIOS) system and (right) the PANTHYR system. Mean spectra over all 30 stations are superimposed for the PANTHYR (orange dashed line) and the M3 TRIOS system (blue dashed lines) and for all 10 measurements from the AERONET-OC system (green dotted line joining dots where the multispectral data exist). The scatterplot (Figure 7c) of \(L_{u}(\lambda)\) water (plus reflected skyglint) radiance measurements shows strong correlation (\(r^{2}=0.992\)) with low systematic differences (slope = 0.97). The MARD of 6.5% was affected by the different azimuth angles of the two systems since this parameter includes reflected skyglint, typically 2.5-3.0% of \(L_{d}(\lambda)\). The scatterplot (Figure 7d) of \(\rho_{w}(\lambda)\) water reflectance (after skyglint correction) shows high correlation between the PANTHYR and M3 TRIOS systems (\(r^{2}=0.996\)) with low systematic differences (slope = 0.97).