The 2007 and 2018 U.S. Academy of Sciences Decadal Surveys for Earth Science recommended a major improvement in calibration for both reflected solar and infrared space based instruments (NRC, 2007). The 2007 report recommended a Tier 1 mission called CLARREO (Climate Absolute Radiance and Refractivity Observatory) to serve as reference calibration spectrometers in orbit. These instruments would anchor activities such as the Global Space Based InterCalibration System (GSICS) which has requested CLARREO or equivalent reference spectrometers (Goldberg et al. 2011). The Global Climate Observing System (GCOS, 2017) has made similar recommendations. The recent 2018 Decadal Survey listed similar reference radiance intercalibration as one of its Most Important Targeted Observables. The spectrometers are designed to achieve SI standard traceable accuracy in orbit of 0.3% at 95% confidence for the reflected solar spectrum, and 0.07K at 95% confidence for the infrared spectrum (NRC, 2007; Wielicki et al. 2013). While the full CLARREO mission has been delayed due to limits in NASA funding, instrument and science development has continued.
The presentation will summarize progress and current status of a CLARREO reference calibration mission. Ground demonstration instruments have been built for both the infrared and reflected solar spectrometers, with both instruments having successfully reached TRL-6. The reflected solar instrument has demonstrated greatly improved accuracy in orbit using 30km high altitude balloon flights (Kopp et al. 2017). The infrared instrument has been demonstrated in two laboratory test implementations. Methodologies for high accuracy inter-calibration have also been demonstrated and published, considering requirements for instrument offset, gain, non-linearity, and polarization dependence.
A CLARREO Pathfinder mission for the reflected solar spectrometer entered Phase A development in January 2016. The low cost Pathfinder mission (< $100 million U.S. dollars) is designed to demonstrate the new higher accuracy SI traceability on orbit as well as the intercalibration of multiple sensors. The spectrometer is based on the University of Colorado LASP HySICS balloon demonstration instrument, and is planned for launch as an attached payload on the International Space Station (ISS) in early 2022. Accuracy goal for the Pathfinder is 0.3% (k=1) or a factor of 3 to 10 better than current reflected solar instruments. The Pathfinder instrument on the ISS would be capable of obtaining time/space/angle/spectral matched data across the full scan swath of low earth orbit and geostationary orbit reflected solar instruments such as CERES, MODIS, VIIRS, Landsat, Sentinel 2a/b, GERB, SEVERI, ABI, as well as land imager constellations and ocean color sensors. The spectrometer has contiguous spectral response with 3 nm sampling from 350 nm to 2300 nm. Instantaneous field of view is 500 m, with a swath width of 70 km. A 2-axis gimbal allows matched angle of view for the full swath of scanning instruments.
While the world still lacks an observing system specifically designed for climate observations, the CLARREO and CLARREO Pathfinder missions represent a foundation for the much higher accuracy observations critical to narrowing uncertainty in future climate change.
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