Initial Analysis of Spectral Smile Calibration of Hyperspectral Imager Suite (HISUI) Using Atmospheric Absorption Bands

Abstract

This paper reports on the initial analysis of spectral smile calibration of the Hyperspectral Imager Suite (HISUI) onboard the International Space Station, which has been continuously acquiring data since September 4, 2020. HISUI is an optical hyperspectral imager consisting of two subsystems: VNIR covering 400 to 980nm at intervals of 10 nm, and SWIR covering 895 to 2481nm at 12.5nm intervals. Based on the atmospheric correction for actual observation images, we assessed cross-track dependences of the wavelength deviation (spectral smile) and the full-width at half-maximum (FWHM) of the HISUI response function. We found that significant spectral smile was observed, with maximum variations of 1.8nm in VNIR and 4.3–4.5nm in SWIR. In addition, the cross-track variation of FWHM was observed with maximum variations of 5.0nm for VNIR and 2.5–3.5nm for SWIR.We used the results to model the smile functions to update a smile correction table in the internal calibration system of HISUI. Then, we evaluated how the smile functions reduce the spectral smile in the data acquired after the update on September 27, 2021.We confirmed that VNIR showed a nearly flat profile within 0.25nm with a nearly constant FWHM. For SWIR, although a slight amount of spectral smile and a variation of FWHM were still observed partly due to wavelength dependence in the spectral smile, the spectral smile was reduced to <~2.2 nm. This study demonstrated that wavelength calibration using actual observation images for ground surfaces is important for the characterization of hyperspectral sensors.