Analysis of Long-Term Variation in Spectral Smile of HISUI Using Atmospheric Absorption Bands

This article reports on the long-term variation of the spectral smile and the full-width at half-maximum (FWHM) of the sensor response function for the Hyperspectral Imager Suite (HISUI) onboard the International Space Station (ISS). HISUI is an optical hyperspectral imager comprising two subsystems: a visible-and-near-infrared (VNIR) system covering 400–980 nm and a short-wave infrared (SWIR) system covering 895–2481 nm. On the basis of atmospheric corrections for actual observation images, we found that the characteristics of the spectral smile and the FWHM can be classified into five major periods. The results show that the first two transitions of the periods are due to updates of the smile correction table used in HISUI and that the last one is related to the occurrence of vertical stripes in the HISUI images. However, two unexpected changes are observed. One is a sudden increase in the spectral smile in the SWIR data to 2.0–2.1 nm on September 30, 2021, which was just three days after the second update of the smile correction table. The other is a sudden improvement on January 5, 2022, showing almost zero spectral smile and a nearly constant FWHM. Because neither sudden change is associated with HISUI or ISS operations/events, the optical and electrical systems of HISUI might exhibit instability, leading to sporadic changes of the spectral smile and FWHM. The findings of this study therefore suggest that, although the postlaunch spectral smile correction has been completed, continued long-term evaluation of the spectral stability is important.