The In-Flight Calibration System for the Airborne Imager GLORIA

The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) is an airborne imaging Fourier transform spectrometer developed for deployment on board different research aircraft such as the German research aircraft HALO or the high-flying Russian research plane M55-Geophysica. GLORIA uses a two-dimensional detector array for infrared limb observations (7–13 µm) providing detailed 2D/3D pictures of the Upper Troposphere/Lower Stratosphere (UTLS) region, which plays a crucial role in the climate system. The calibration of infrared sounding instruments is always a great challenge especially for air- and space-borne experiments when in-flight calibration becomes necessary. Due to the variable environmental conditions inside the belly-pod of HALO or inside the instrument bay of the M55-Geophysica, it is not sufficient to calibrate the instrument on the ground only. Since the instrument is exposed to the hostile environment of the UTLS (T ~ -50 °C, p \u3c 200 hPa), the in-flight calibration sources have to be carefully designed to cope with those adverse circumstances. Additional restrictions like weight limitations and power restraints are implied. GLORIA’s in-flight calibration system consists of two identical large-area (126mm x 126mm) high-precision blackbodies (GBB-C and GBB-H), which are independently controlled at two different temperatures. Thermo-Electric Coolers (TECs) are used to control the temperature of the calibration sources offering the advantage of avoiding cryogens and mechanical coolers. The achieved emissivity of the optical surface is 0.999(6) with a thermal homogeneity of about 0.1 K. The system has been comprehensively characterized for its spatially and spectrally resolved radiation properties in terms of radiation temperature traceable to the International Temperature Scale (ITS-90) at the Physikalisch-Technische Bundesanstalt (PTB), the national metrology institute of Germany. GLORIA’s in-flight calibration system was already successfully deployed during the ESSenCe campaign in 2011 and the TACTS/ESMVal campaign in 2012