Detector-Based Prelaunch Absolute Radiometric Calibration for CLARREO Pathfinder HySICS Instrument

The HyperSpectral Imager for Climate Science (HySICS) is the sole instrument of the CLARREO Pathfinder (CPF) mission, which was previously planned for launch to the International Space Station (ISS) in 2026. It is an Offner-Chrisp imaging spectrometer designed to achieve an exceptional radiometric uncertainty of 0.3%-0.6% (k=1) for Earth reflectance measurements across a spectral range of 350-2260 nm. This represents a significant improvement over existing spaceborne spectrometers, which typically achieve uncertainties greater than 2%. To validate this performance, an Independent Calibration (IndCal) strategy has been developed, relying on a pre-launch, detector-based absolute radiometric calibration (RadCal) approach using the Goddard Laser for Absolute Measurement of Radiance (GLAMR), a tunable laser source developed at NASA's Goddard Space Flight Center. The RadCal test, conducted in September 2023, systematically characterized the radiometric and spectral performances of HySICS through full-field, full-aperture, and full-spectrum evaluations. The test measured the absolute spectral responses (ASRs), band-integrated responsivities and other radiometric parameters of HySICS' detector at the pixel level, focusing on minimizing uncertainties at every stage of the measurement process. This paper details the methodology, configuration and key results of the test, which notably marks the first application of detector-based absolute RadCal to an operational imaging spectrometer. The demonstrated performance is expected to establish a new benchmark for high-accuracy absolute RadCal of remote sensing instruments in Earth observation.