Calibration Insights from the DESIS Instrument: Lessons for Future Imaging Spectroscopy Missions

Abstract

German Aerospace Center, Earth Observation Center, Münchener Str. 20, 82234 Weßling, Germany The DLR Earth Sensing Spectrometer (DESIS) aboard the International Space Station (ISS) has been delivering high-quality hyperspectral data to both the scientific community and commercial users since it began operations in September 2018. With an increasing number of spaceborne hyperspectral instruments in use, DESIS data remain highly relevant due to their superior spectral resolution (2.55 nm) and the acquisition of over 300,000 product tiles over more than five years of operation. This extensive dataset makes DESIS data particularly valuable for sensor inter-comparison, inter-calibration, and integration into time series with other instruments. In this contribution, we present key insights from the DESIS calibration and derive useful lessons for future hyperspectral missions. DESIS calibration uses the on-board LED calibration unit to monitor the spectral response of the instrument. Over the years, the average spectral performance of DESIS shows little variability over long periods of time. However, it also shows high measurement-to-measurement variability. Part of this variability is related to temperature gradients between the optical elements and can be corrected during data processing. Other variations seem to have a random nature and are not corrected (RMS ~0.1 nm), contributing to the measurement uncertainties. Radiometric calibration, on the other hand, is based on vicarious calibration using RadCalNet sites as reference and flat-fielding of the radiometric response of all sensor elements with uniform scenes. Our results show that the radiometric calibration of DESIS can change by 3.4% over one year above 500 nm. Below 500 nm, a significantly larger variability is observed, increasing as the wavelengths become shorter. During the first 3 years of operations, we observed a fast decrease in sensitivity below 500 nm, followed by a short period of stability and then a rapid increase that has recovered an important part of the sensitivity lost during the first years. Finally, the geometric calibration is performed by comparing ground control points (GCPs) automatically extracted from many DESIS images and from reference images with higher geometric accuracy. This geometric calibration becomes important when an on-the-fly geometric improvement per scene is not possible because no GCPs can be found. In this case, the RMSE is typically 300 metres in the transverse direction and 500 metres in the longitudinal direction. By default, however, if enough GCPs are found per image, an improvement of the geometric sensor model is estimated, whereby a RMSE of ~21 m is achieved in the north and east directions.