CLARREO (Climate Absolute Radiance and Refractivity Observatory) is a high-priority NASA decadal survey mission, originally selected as such by the National Research Council in 2007. The CLARREO mission is intended to provide a metrology laboratory in orbit to accurately quantify and attribute Earth's climate change (see List of climate research satellites). The mission is also designed to transfer its high accuracy to other spaceborne sensors. It would serve as a reference calibration standard in orbit, making climate trends apparent in their data sets by 2055, within a 30-year time frame after its planned launch in the 2020s. These measurements may go on to enable testing, validation, and improvement of climate model prediction.
Due to funding cuts in announced for the 2012 budget, the CLARREO mission was significantly scaled back, while remaining spaceborne projects were eyed to fill the gap. In the President's financial year 2016 budget request, a smaller CLARREO Pathfinder (CPF) mission was provided $76.9M to demonstrate essential measurement technologies of the CLARREO Tier 1 Decadal Survey mission. That funding will potentially support the flight of the Reflected Solar (RS) spectrometer, which is one piece of the full Decadal Survey-recommended mission, hosted on the International Space Station (ISS) from December 2023. The Trump administration unsuccessfully tried several times to defund the mission.
Mission concept
Below is the mission concept presented at the Mission Concept Review in November 2010. CLARREO was then envisioned to consist of four observatories on two dual-manifested launches on Minotaur IV+ vehicles. Three Instruments (two of each) Infrared (IR) Spectrometer Reflected Solar (RS) Spectrometer Global Navigation Satellite System-Radio Occultation (GNSS-RO) Four Observatories, two dual-manifested launches on Minotaur IV+ vehicles July 2018: Two infrared observatories, each with GNSS-RO May 2020: Two reflected solar observatories 609 km polar orbits (90 degree inclination)
Alternative mission concepts were developed to accommodate reduced available funding. A CLARREO mission on the International Space Station, to include one each of the RS and IR spectrometers, was found to provide the best science value for the lowest feasible cost. Due to the ISS orbital inclination of 51.65 degrees, CLARREO on ISS measurements would not include the polar regions, resulting in the mission being unable to track global spectral benchmarks compared to the version of the mission presented at the Mission Concept Review.
Pathfinder mission concept
In 2016, a Pathfinder mission to the full CLARREO mission received funding. "The allocated funds support the flight of a Reflected Solar (RS) spectrometer, hosted on the International Space Station (ISS) in the 2020 time frame. ... The CPF is a Class D mission with 1 year of operations on orbit and 1 year for analysis of acquired data." The mission is designed to demonstrate essential measurement technologies in orbit that can be used to reduce the risk of a full CLARREO mission. CLARREO Pathfinder has two primary objectives: to demonstrate high accuracy on orbit traceable to International Systems of Units (SI) and to transfer that accuracy to other spaceborne sensors. Pathfinders threshold objective compared to the full baseline CLARREO mission is relaxed by a factor of two from 0.15% to 0.3% (k=1).
Reference intercalibration
Current satellite-based sensors are not designed to meet the accuracy requirements needed for climate change detection. Many sensors used for climate measurements were designed to meet operational weather needs and are not optimized for climate sampling. These sensors, along with older instruments designed for climate, lack the on-board ability to test for systematic errors on orbit. The CLARREO mission will meet these goals through careful consideration of the instrument design, calibration traceability at all stages of development and operation, with spectral, spatial and temporal sampling focused specifically on the creation of climate records. Then after development of new cross-calibration methodologies far more accurate than those achieved today, CLARREO may serve as an in-orbit standard to provide reference intercalibration for missions like the broadband Clouds and the Earth's Radiant Energy System (CERES), operational sounders including the Cross-track Infrared Sounder (CrIS) and Infrared Atmospheric Sounding Interferometer (IASI), and imagers such as the Visible Infrared Imaging Radiometer Suite (VIIRS) and Advanced Very-High-Resolution Radiometer (AVHRR).