The NASA climate science mission Climate Absolute Radiance and Refractivity Observatory (CLARREO), which is to measure Earth's emitted spectral radiance from orbit for 5 years, has an absolute accuracy requirement of 0.1 K (3σ) at 220 K over most of the thermal infrared. To meet this requirement, CLARREO needs highly accurate on-board blackbodies which remain accurate over the life of the mission. Space Dynamics Laboratory is developing a prototype blackbody that demonstrates the ability to meet the needs of CLARREO. This prototype is based on a blackbody design currently in use, which is relatively simple to build, was developed for use on the ground or on-orbit, and is readily scalable for aperture size and required performance. We expect the CLARREO prototype to have emissivity of ~0.9999 from 1.5 to 50 μm, temperature uncertainties of ~25 mK (3σ), and radiance uncertainties of ~10 mK due to temperature gradients. The high emissivity and low thermal gradient uncertainties are achieved through cavity design, while the SItraceable temperature uncertainty is attained through the use of phase change materials (mercury, gallium, and water) in the blackbody. Blackbody temperature sensor calibration is maintained over time by comparing sensor readings to the known melt temperatures of these materials, which are observed by heating through their melt points. Since blackbody emissivity can potentially change over time due to changes in surface emissivity (especially for an on-orbit blackbody) an on-board means of detecting emissivity change is desired. The prototype blackbody will include an emissivity monitor based on a quantum cascade laser to demonstrate the concept.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.