## 3 Instrument Description, Calibration, and Relevance to ERB Measurement Challenges  
Uvsq-Sat [10] is a CubeSat developed by LATMOS and launched in January 2021 into an SSO orbit at approximately 540 km altitude. Its primary objective was to demonstrate the feasibility of using miniaturized instrumentation to monitor ERB, focusing on OSR and OLR. The satellite carries ERS sensors, thermopile-based detectors designed for broadband radiative flux measurements. Each of the six satellite faces is equipped with two ERS units--one coated with bonded Vantablack S-IR carbon nanotubes (Surrey NanoSystems Ltd., UK) for near-unity absorptivity across the UV-to-far-infrared range, and one with an optical solar reflector (Excelitas Technologies (as Qioptiq), UK) for spectral selectivity. These sensors capture radiative flux via thermoelectric conversion, without the need for scanning mechanisms, and offer a WFOV for instantaneous flux detection. A second CubeSat, Inspire-Sat [11], was launched in April 2023 into a similar SSO orbit at 508 km. Also developed by LATMOS, it carries the same ERS instruments. The mission goal is to extend ERB monitoring by providing temporal coverage at a different local solar time (10:30 a.m. for Inspire-Sat vs. 9:30 a.m. for Uvsq-Sat), thus improving sampling of the diurnal cycle in TOA fluxes when used in constellation. The ERS sensors underwent extensive pre-launch calibration. For characterizing the shortwave response (OSR), a radiometric calibration setup comprising a Xenon lamp coupled with a LATMOS Newtonian telescope was used to simulate solar illumination (30 Wm\({}^{-2}\)). For the longwave response (OLR), the LATMOS BX-500 blackbody--operated at controlled temperature set points between 323 K and 343 K--was used to test the instrument's ability to measure thermal radiative fluxes. Direct solar calibration was performed by integrating the CubeSats into the LATMOS/OVSQ INTRA Solar Tracker system, allowing exposure to solar irradiance (1000 Wm\({}^{-2}\)). This calibration was independently validated using ground-based pyranometers--specifically the Kimo SAM30 and Kipp & Zonen SMP6-V--to ensure consistency of the measurements. Additionally, the absolute calibration of the ERS was conducted on a dedicated test bench using a National Institute of Standards and Technology (NIST)-traceable 1000 W quartz halogen lamp (F-546) precisely aligned along the optical axis of Uvsq-Sat or Inspire-Sat using laser guidance. This lamp, calibrated by NIST and traceable to the International Temperature Scale of 1990 (ITS-90), provided a well-characterized spectral irradiance in the solar bands (approximately 1362 Wm\({}^{-2}\)). During calibration, the satellite instrument was positioned 50 cm from the lamp, and measurements were corrected for dark current. This procedure enabled the determination of the ERS's absolute response and transfer function, ensuring that the measurements were accurate. Finally, thermal vacuum chamber tests (LATMOS/OVSQ) were performed to evaluate sensor performance under space-like environmental conditions, with the chamber walls simulating the cold background of space and infrared radiation fields used to reproduce the thermal loads experienced in orbit. Together, these calibration and testing efforts yielded a pre-flight absolute uncertainty of approximately \(\pm 5\) Wm\({}^{-2}\) (1\(\sigma\)) for outgoing shortwave and longwave radiation measurements, supporting the reliability of ERS data throughout the mission [39]. In orbit, Uvsq-Sat and Inspire-Sat lack active onboard calibration systems (e.g., calibration lamps, reference gallium source [40], etc. ), limiting direct tracking of instrument degradation. However, the choice of materials--bonded Vantablack S-IR and optical solar reflector--was made to mitigate this, as these coatings have demonstrated extremely low degradation in low Earth orbit environments. [PERSON] et al. (2018) [8] provide a comprehensive review of the limitations inherent to non-scanner, WFOV instruments, such as those used on Uvsq-Sat and Inspire-Sat.