A Compact SSI Instrument (CSIM)

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. Consistent with the emphasis on rapid technology advances and the need for smaller, lower risk Earth science instrumentation, we are developing a compact SSI instrument (CSIM) that will offer significant implementation flexibility for future alternative spaceflight opportunities, including CubeSat missions. This instrument, utilizing new manufacturing and material advances, will be a cost-effective and lower risk alternative to obtain high priority Earth science SSI measurements and will provide an SI-traceable tie to existing and future satellite records. The CSIM utilizes a straightforward optical design in a compact, folded geometry that overcomes costly, high tolerance fabrication requirements while reducing the overall calibration risks associated with previous designs. It utilizes a compact two-channel design incorporating Si and InGaAs photodiodes and independent carbon-nanotube (CnT) based electrical substitution radiometers as absolute detectors for full on-orbit calibration maintenance capability. The final CubeSat ready instrument will be fully characterized and calibrated to achieve unprecedented levels of absolute accuracy (uc<0.25% combined standard uncertainty) and high spectral stability (0.01-0.05% per year relative uncertainty) across a continuous wavelength region spanning 200 – 2400 nm (96% of the total solar irradiance). System level performance characterizations and final end-to-end absolute calibration will be accomplished with the LASP Spectral Radiometer Facility (SRF), a comprehensive LASP-NIST jointly developed spectral irradiance calibration facility utilizing the SIRCUS tunable laser system tied to an SI-traceable cryogenic radiometer.