## 3 Instrument Description, Calibration, and Relevance to ERB Measurement Challenges
### Uvsq-Sat Map Reconstruction Examples  
Figures 4 and 5 show maps derived from data collected by a single Uvsq-Sat satellite operating in an SSO orbit. This satellite observed each location at a fixed local solar time, so the data represent cumulative measurements taken consistently at the same solar observation hour rather than instantaneous snapshots or full diurnal averages. Figure 4 presents the TOA all-sky SW flux (reflected solar radiation), and Figure 5 displays the TOA all-sky LW flux (thermal emission). No diurnal modeling or combination of data from multiple satellites was applied, allowing for consistent comparison of spatial and temporal variability at the satellite overpass times. Observations from Uvsq-Sat reveal that the OSR (Figure 4) displays marked spatial variability, largely influenced by surface albedo, cloud cover, atmospheric aerosol concentrations, and variations in the solar zenith angle. High-albedo surfaces--such as snow, ice, deserts, and bright clouds--reflect a significant portion of incoming solar radiation, contributing to elevated OSR values. This effect is typically observed in polar regions, where persistent snow and ice cover--especially during local summers--enhance reflectivity. However, accurately observing these regions by satellite is challenging, as low solar elevation angles and highly oblique viewing geometries reduce measurement precision--or even render observations impossible in areas with limited or no satellite coverage. In other regions, large desert areas like the Sahara and Arabian Peninsula maintain high OSR throughout the year due to their consistently bright, dry surfaces. In contrast, darker landscapes such as vegetated regions, urban areas, and soils with low albedo absorb more  
Figure 4: TOA all-sky shortwave (SW) flux from Uvsq-Sat: March 2021–October 2024. Figure 5: TOA all-sky longwave (LW) flux from Uvsq-Sat: March 2021–October 2024.  
sunlight, resulting in lower OSR values. Over the oceans, the OSR is relatively low under clear skies, as the dark water surface reflects only about 5-10% of incoming radiation. However, cloud cover, as a dominant modulator of OSR, can significantly enhance OSR in these regions. Thick and widespread cloud systems--particularly those associated with tropical convection, the InterTropical Convergence Zone (ITCZ), monsoonal flows, and midlatitude storms--reflect substantial amounts of solar radiation back to space. In many cases, their impact on the OSR exceeds that of surface albedo, especially over low-reflectivity regions such as oceans. Marine stratocumulus clouds in subtropical high-pressure zones, such as the eastern Pacific and Atlantic, also act as effective reflectors, producing OSR values comparable to bright continental surfaces. In contrast, clearer subtropical gyres or regions affected by ENSO-related subsidence display lower OSRs, reflecting the ocean's inherently absorptive nature. Additionally, atmospheric aerosols--including dust, smoke, and industrial pollutants--modulate the OSR by scattering sunlight. Moreover, the solar zenith angle, which varies with latitude, time of day, and season, influences both the intensity of incoming solar radiation and surface reflectivity. These combined factors contribute to the spatial and temporal variability of OSRs, underscoring the importance of satellite observations at different solar local times. As shown in Figure 5, the OLR displays notable spatial variability worldwide, largely governed by surface temperature, the composition of the atmosphere, and cloud dynamics. Since thermal radiation increases with temperature, warmer regions emit more longwave radiation. Consequently, the highest OLR values are typically found in hot, arid areas such as the Sahara Desert, the Arabian Peninsula, and parts of southern California, where clear skies and elevated surface temperatures facilitate efficient infrared emission to space. In contrast, polar regions exhibit much lower OLRs due to consistently cold surface temperatures and reduced thermal emissions.