A Quantum Cascade Laser–Based Reflectometer for On-Orbit Blackbody Cavity Monitoring

Abstract

Satellite measurements pinned to international standards are needed to monitor the earth?s climate, quantify human influence thereon, and test forecasts of future climate change. Credible observations require that measurement uncertainties be evaluated on orbit during a mission?s operational lifetime. The most accurate spaceborne measurements of thermal infrared radiance are achieved with blackbody calibration. The physical properties of blackbody cavity surface coatings are known to change upon extended exposure to the low earth orbit environment. Any such drift must be quantified to continue correctly calibrating observed radiance on orbit. A method is presented to diagnose the effective emissivity of a blackbody cavity in situ using a quantum cascade laser (QCL)-based reflectometer. QCLs provide high-power single-mode output in the thermal infrared and have small mechanical footprints that facilitate integration into existing optical systems. The laser reflectivity in a test blackbody cavity was measured to be 9.22 ? 10?4 with an uncertainty of 8.9 ? 10?5, which is equivalent to a detection limit of 3 mK in the error in radiance temperature for a calibration blackbody (at 330 K and 1000 cm?1) resulting from cavity emissivity drift. These results provide the experimental foundation for this technology to be implemented on satellite instruments and thus eliminate a key time-dependent systematic error from future measurements on orbit.