Implementation of CubeSat Solar Irradiance Measurements: CSIM Solar Spectral Irradiance Continuity and On-Orbit Validation

Solar irradiance is one of the longest and most fundamental of all climate data records derived from space-based observations. The continuous 39-year total solar irradiance (TSI) observational data record is the result of several overlapping instruments flown on many different missions where observational overlap is central to establishing a reliable composite solar radiation data record. The long-term, continuous measurements of the nearly full spectrum solar spectral irradiance (SSI) began with the Spectral Irradiance Monitor (SIM) on the SORCE mission launched in 2003 providing continuous SSI observations for over 14 years – critical measurements dependent on a single instrument operating at more than twice its design lifetime. After more than a decade of delays and programmatic changes, the first implementation of NASA’s Total and Spectral Solar Irradiance Sensor (TSIS-1) is scheduled for launch to the International Space Station in November 2017. The TSIS-1 mission provides continuation of both the TSI and SSI observations with improved versions of the LASP Total Irradiance Monitor (TIM) and SIM, respectively, and will establish the observational link to the SORCE end-of-mission solar data record. In addition to the TSIS-1 SIM, the Compact Spectral Irradiance Monitor (CSIM) is scheduled for flight in mid-2018 on a 6U CubeSat bus and will provide additional SSI observational overlap and risk reduction. The CSIM flight opportunity will bring new, emerging technology advancements to maturation by demonstrating the unique capabilities of a complete SSI mission with inherent low mass and compact design. Furthermore, both the TSIS-1 SIM and CSIM have their absolute calibrations tied to a common NIST-traceable spectral irradiance calibration scale (LASP Spectral Radiometer Facility) and therefore establish end-to-end verification of the absolute SSI uncertainties. The simultaneous, independent operations of three unique SSI instruments will allow for unprecedented spectral stability validations and provide insight into SORCE SIM spectral trend uncertainties that have yet to be validated by any direct measurement comparisons. Finally, we discuss an observational strategy for data overlap (both TSI and SSI) with a focus on future mission designs implementing redundant and overlapping small satellite concepts involving low-risk, cost efficient approaches to maintain critical long-term solar data records.