New Fixed Point for an in-Orbit Calibration Scale Developed on the Basis of In–Bi Eutectic Alloy for Use in New-Generation Highly Stable Onboard Reference Sources

The paper discusses a problem associated with ensuring the stability of Earth-observing satellite instruments performing long-term temperature measurements within the thermal infrared spectral range. Reliable monitoring of in-flight characteristics exhibited by infrared satellite instruments requires improved stability of onboard reference sources. The function of these highly stable sources is to be performed by new-generation onboard blackbodies currently under development, which are based on the melting↔freezing phase transition (PT) phenomenon. This development process requires the creation of an in-orbit calibration scale within the dynamic temperature range of Earth observation systems ranging from 210 to 350 K, which incorporates fixed-point PT temperatures of several potentially suitable substances. The fixed points of the in-orbit calibration scale will serve as the basis for a corresponding series of onboard reference blackbodies. It is shown that the task at hand requires several space experiments to be performed with selected fixed points and prototypes of the proposed PT-based onboard blackbodies. A new In–Bi eutectic melting fixed point (~345 K) is recommended as a significant point for the future in-orbit calibration scale. The results of preliminary laboratory studies on the new fixed point are analyzed. The obtained results were used to inform the preparation of space experiments aimed at investigating a In–Bi alloy for use in new-generation onboard reference sources.