On-orbit absolute calibration of temperature with application to the CLARREO mission

NASA's anticipated plan for a mission dedicated to climate (CLARREO) will hinge upon the ability to fly absolute standards that can provide the basis to meet stringent requirements on measurement accuracy For example, instrumentation designed to measure spectrally resolved infrared radiances will require high-emissivity calibration blackbodies having absolute temperature uncertainties of better than 0.045 K (3 sigma). A novel scheme to provide absolute calibration of temperature sensors, suitable for CLARREO on-orbit operation, has been demonstrated in the laboratory at the University of Wisconsin. The scheme uses the transient temperature signature obtained during the phase change of different reference materials, imbedded in the same thermally conductive medium as the temperature sensors - in this case the aluminum blackbody cavity. Three or more reference materials can be used to assign an absolute scale to the thermistor sensors over a large temperature range. Using very small quantities of phase change material (<1/250th the mass of the cavity), melt temperature accuracies of better than 10 mK have been demonstrated for Hg, H2O, and Ga, providing calibration from 233K to 303K. The flight implementation of this new scheme will involve special considerations for packaging the phase change materials to ensure long-term compatibility with the containment system, and design features that help ensure that the on-orbit melt behavior in a microgravity environment is unchanged from pre-flight full gravitational conditions under which the system is characterized.