On-orbit Calibration of the Carruthers GCI: Radiometric Sensitivity

The Carruthers Geocorona Observatory is NASA's first mission dedicated to investigating the fundamental nature of Earth's exosphere. Its primary payload, the GeoCoronal Imager, consists of two co-aligned broadband photometric imagers that support simultaneous, common-volume sensing of ultraviolet emission by exospheric hydrogen atoms. However, accurate parameter retrieval requires precise knowledge of the instrument's wavelength dependent responsivity. To that end, the mission aims to perform photometric measurement of stars and invert the observed fluxes to constrain the final passband. An objective, algorithm-driven ranking criterion identifies the best subset of stars to observe from a refined UV stellar spectral library tied to CALSPEC standards. The entire workflow - including the spectral library, passband inversion, and selection criterion - is validated using synthetically generated stellar measurements, which show that the proposed retrieval algorithm has high recovery fidelity, achieving passband error rates of <7% for both primary Lyman-alpha science channels.