CLARREO Pathfinder Mission to ISS: Demonstrating Greatly Increased Accuracy for Reflected Solar Space Based Observations: Calibration and Intercalibration

The CLARREO (Climate Absolute Radiance and Refractivity) Pathfinder mission is a new mission started by

NASA in 2016. CLARREO Pathfinder will fly a new generation of high accuracy reflected solar spectrometer in

orbit on the Inernational Space Station (ISS) to demonstrate the ability to increase accuracy of reflected solar

observations from space by a factor of 3 to 20. The spectrometer will use the sun and moon as calibration

sources with a baseline objective of 0.3% (1 sigma) reflectance calibration uncertainty for the contiguous

spectrum from 350nm to 2300nm, covering over 95% of the Earth's reflected solar spectrum. Spectral

sampling is 3nm with resolution of 6nm. The spectrometer is mounted on a 2-axis gimbal enabling a new ability

to use the same optical path to view the sun, moon, and Earth. Planned launch is 2020 with at least 1 year on

orbit to demonstrate the new capability. The mission will also demonstrate the ability to use the new

spectrometer as a reference transfer spectrometer in orbit to achieve intercalibration of reflected solar

instruments to within 0.3% (1 sigma) using space, time, spectral, and angle matched observations across the

full scan width of remote sensing instruments. Intercalibration to 0.3% will be demonstrated across the full scan

width of the NASA CERES broadband radiometer and the NOAA VIIRS imager reflected solar spectral

channels. This mission will demonstrate reflected solar intercalibration across the full swath width as opposed

to current nadir only intercalibration used by GSICS (Global Space Based InterCalibration

System). Intercalibration will include a new capability to determine scan angle dependence of polarization

sensitivity of instruments like VIIRS. The high accuracy goals of this mission are driven primarily by the

accuracy required to more rapidly and accurately observe climate change signals such as cloud feedback (see

Wielicki et al. 2013 Bulletin of the American Meteorological Society). The new high accuracy and intercalibration

capability will also be very useful for serving as a reference calibrator for constellations of operational

instruments in Geostationary or Low Earth Orbit (e.g. land resource imagers, ocean color, cloud imagers). The

higher accuracy will enable operational sensors to more effectively serve as climate change sensors.  Recent studies

of the economic value of a higher accuracy climate observing system (Cooke et al. 2014, 2016, Hope 2015) estimate

a value of ~ $10 Trillion U.S. dollars.  CLARREO will provide a critical anchor of such a system.