On-orbit Traceable Blackbody Emissivity Measurement using the Heated Halo Method

In order to establish a global benchmark climate record and to test interdecadal climate forecasts, satellite measurements of high spectral resolution infrared radiance must be made with high accuracy and with uncertainty estimates that can be proven beyond a doubt. Meeting this stringent requirement is the purview of the Climate Absolute Radiance and Refractivity Observatory (CLARREO), a high priority recommendation of the National Research Council’s Decadal Survey. Technology development for CLARREO is being undertaken at the Space Science and Engineering Center at the University of Wisconsin to demonstrate high accuracy measurements of spectral infrared radiance and on-orbit diagnostics necessary to prove uncertainty claims while deployed in space. Blackbody cavities impart the most accurate calibration for spaceborne infrared sensors, provided that their temperature and emissivity is traceably determined on-orbit. The Heated Halo provides a robust and compact method to measure the spectral emissivity of a blackbody in situ. A carefully baffled heated cylinder is placed in front of a blackbody in an infrared spectrometer system, and the combined radiance of the blackbody and Heated Halo reflection is observed. Knowledge of key temperatures and the viewing geometry allow the blackbody cavity spectral emissivity to be calculated. We present the results from this methodology implemented with an existing ground-based and airborne spectrometer system, and we compare our findings to Monte Carlo models and alternate experimental methods of emissivity determination.