A Blackbody Design for SI-Traceable Radiometry for Earth Observation

Spaceborne measurements pinned to international standards are needed to monitor the earth’s climate,quantify human influence thereon, and test forecasts of future climate change. The International System ofUnits (SI, from the French for Système International d’Unités) provides ideal measurement standards forradiometry as they can be realized anywhere, at any time in the future. The challenge is to credibly proveon-orbit accuracy at a claimed level against these international standards. The most accurate measurementsof thermal infrared spectra are achieved with blackbody-based calibration. Thus, SI-traceability is obtainedthrough the kelvin scale, making thermometry the foundation for on-orbit SI-traceable spectral infraredmeasurements. Thermodynamic phase transitions are well established as reproducible temperature standards and form the basis of the international practical temperature scale (International Temperature Scaleof 1990, ITS-90). Appropriate phase transitions are known in the temperature range relevant to thermalinfrared earth observation (190–330 K) that can be packaged such that they are chemically stable over thelifetime of a space mission, providing robust and traceable temperature calibrations. A prototype blackbodyis presented that is compact, highly emissive, thermally stable and homogeneous, and incorporates a smallgallium melting point cell. Precision thermal control of the blackbody allows the phase transition to beidentified to within 5 mK. Based on these results, the viability of end-to-end thermometric calibration ofboth single-temperature and variable-temperature blackbodies on orbit by employing multiple-phasechange cells was demonstrated.