The compact spectral irradiance monitor flight demonstration mission

Accurate, long-term solar spectral irradiance (SSI) measurements are vital for interpreting how solar variability impacts Earth's climate and for validating climate model sensitivities to spectrally varying solar forcing. The Compact Spectral Irradiance Monitor (CSIM) 6U CubeSat successfully launched on Dec. 3rd, 2018 as part of the SpaceX SSO-A: SmallSat Express Mission ultimately achieving a sun-synchronous 575 km orbit. CSIM brings new, emerging technology advancements to maturation by demonstrating the unique capabilities of a complete SSI mission with inherent low mass and compact design. The instrument is a compact, two-channel prism spectral radiometer incorporating Si, InGaAs, and extended InGaAs focal plane photodiodes to record 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. Pre-launch component level performance characterizations and final instrument end-to-end absolute calibration achieved low combined standard uncertainty (u_c<0.5%) in irradiance. These calibrations were performed in the LASP Spectral Radiometer Facility (SRF), a comprehensive spectral irradiance calibration facility utilizing a tunable laser system tied to an SI-traceable cryogenic radiometer. On-orbit, optical degradation corrections to better than 0.05% / year uncertainty are achieved by comparing periodic, simultaneous solar measurements of the two CSIM channels operating with significantly different solar exposure duty cycles. Operational overlap of CSIM with existing SSI measurements validate concepts for maintaining critical long-term solar data records.