First Cross- and Inter-Band Calibrations of the Hyperspectral Imager Suite Using Off-Nadir Quasi-Simultaneous Overpass Counterparts

The hyperspectral imager suite (HISUI) is a recently commissioned spaceborne hyperspectral sensor that is expected to provide higher-quality information for terrestrial and aquatic monitoring at a global scale. To maximize the utility of the detailed spectra observed by HISUI, calibration and validation activities have been intensively conducted for the past several years. However, research on long-term monitoring of sensor radiometric characteristics and checking inter-band radiometric consistency is still limited. This study provided the first results of cross- and inter-band calibrations using five scenes acquired over a desert calibration site during 2021-2023, and calculated relative cross-calibration coefficients (RCCCs) to calibrate all the HISUI bands. To make full use of the limited amount of data archived over the calibration site, off-nadir scenes obtained by wide-swath counterpart sensors [i.e., the moderate resolution imaging spectroradiometer (MODIS)] were used for the cross-calibration, with correction by a bidirectional reflectance distribution function. The obtained RCCCs successfully mitigated the inter-band radiometric variability of HISUI data, which was consistent with the in situ surface reflectance data observed by the radiometric calibration network (RadCalNet). Time-series analysis further revealed a notable discrepancy between the visible to near-infrared and short-wave infrared (SWIR) spectral domains after the International Space Station (ISS) maneuvers in spring 2022. This article is also a prompt report of this phenomenon and will soon contribute to HISUI radiometric calibration, particularly after the maneuver events.