## 3 Instrument Description, Calibration, and Relevance to ERB Measurement Challenges
### Times Series Used for the Flux Maps Reconstruction  
Time-series data from Uvsq-Sat and Inspire-Sat provide observations of the OSR and OLR at the TOA. The overlapping operational periods of these two CubeSats missions provide an opportunity to observe Earth's radiative fluxes at different solar local times and to analyze their diurnal and seasonal variability. Figure 2 shows the temporal evolution of radiative fluxes from Uvsq-Sat used in this study, including the TSI from the SATIRE-S reconstruction model. The Naval Research Laboratory Total Solar Irradiance (NRLTSI2) model [41; 42] is not shown here, but it represents an alternative source, providing results similar to those of the SATIRE-S model. Both models can be used interchangeably.  
Figure 2 (Top) illustrates the upward trend in TSI flux, reflecting the increasing solar activity of Solar Cycle 25, which initiated in December 2019. The OSR flux (Figure 2, Middle) shows the day-night alternation, along with variability in the mean flux that highlights seasonal effects. In contrast, the OLR flux (Figure 2, Bottom) exhibits a less pronounced day-night cycle, while the seasonal variability is more evident. To reconstruct Earth's radiation flux maps, data were selected over specific time periods and under defined conditions (e.g., daytime, nighttime, both day and night, all-sky). Figure 3 presents an example of the spatial distribution of Uvsq-Sat observations in a global map, based on a selection covering the period from 1 March 2021 to 1 October 2024. A noticeably lower density of data points appears over France and Western Europe. This reduction in data availability is caused by radio frequency interference during satellite data downlinks to the LATMOS ground station located in Guyancourt (Yvelines, France), which affects the quality of measurements and results in fewer valid observations in this region. Additionally, no data samples are present over the poles due to the characteristics of the SSO Orbit. The orbital path does not pass directly over the extreme polar regions, and the satellite's viewing geometry further limits observations near the poles, leading to gaps in coverage in these areas.  
Figure 2: **(Top)** Daily TSI from SATIRE-S and its monthly mean. **(Middle)** Uvsq-Sat OSR at TOA, shown with 30-second resolution data and monthly mean. The OSR exhibits a semi-annual modulation, reflecting the solar declination cycle and hemispheric alternation in insolation and albedo. **(Bottom)** Uvsq-Sat OLR at TOA, also shown with 30-second resolution data and monthly mean. The OLR shows a dominant annual cycle linked to the seasonal variation of global surface temperature.  
It is important to note that the duration of the observation period significantly impacts data quality. The more data points are accumulated, the higher the accuracy of radiative flux estimates at each latitude and longitude. Generating global coverage maps from time series of OSR and OLR typically requires aggregating observations over a period of about 30 days. Due to their orbital characteristics and narrow swath widths, satellites like Uvsq-Sat and Inspire-Sat have limited instantaneous spatial coverage. By accumulating data over a month, it becomes possible to sample a wide range of latitudes and longitudes, effectively filling spatial gaps and enabling comprehensive global map. Using a satellite constellation provides a major advantage for maximizing observational coverage. Unlike a single satellite, a constellation comprises multiple satellites operating simultaneously in coordinated orbits, which increases both the frequency and spatial density of observations. This approach enables more complete and timely data collection across different latitudes and longitudes, thereby reducing spatial and temporal gaps in the datasets.