One Year of Operational Overlap of the Compact Spectral Irradiance Monitor (CSIM) with the Total and Spectral Solar Irradiance Sensor (TSIS-1) Spectral Irradiance Monitor (SIM)

Accurate, long-term solar spectral irradiance (SSI) measurements are vital for interpreting how solar variability impacts Earth’s climate. The Compact Spectral Irradiance Monitor Flight Demonstration (CSIM-FD) mission, a 6U CubeSat, successfully launched on Dec. 3^rd, 2018 (SpaceX SSO-A) achieving a sun-synchronous 575 km orbit. CSIM-FD advances new technology and demonstrates the unique capabilities of a complete SSI mission with inherent low mass and compact design. CSIM-FD records the solar spectrum daily across a continuous wavelength region spanning 200 – 2800 nm (>97% of the total solar irradiance).A new, novel electrical substitution radiometer (ESR) using vertically aligned carbon-nanotube (VACNT) bolometers serves as an absolute detector for periodic on-orbit spectral calibration corrections.The CSIM-FD mission has been highly successful in demonstrating the new technology for improving SSI measurement performance and providing new, highly accurate SSI observations and has produced several valuable outcomes. First, it demonstrates a viable 6U CubeSat SSI instrument that can potentially maintain the SSI data record, particularly as a gap mitigation strategy to provide measurement continuity. Second, it raises the TRL of several new technologies that are applicable design approaches to implement in future instruments. Finally, the timing of this flight overlaps with the TSIS-1 Spectral Irradiance Monitor (SIM) instrument. Operational overlap between these instruments is unique because both TSIS-1 SIM and CSIM have their calibrations and end-to-end validations tied to the same spectral irradiance traceability scale providing validation of the absolute SSI uncertainties and forwards future small satellite concepts for maintaining critical long-term solar data records.