Calibration Insights from the DESIS Insturment: lessons from over six years in orbit

Kurzfassung

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 more than 300,000 product tiles over more than six years of operation. Calibration is a fundamental aspect of any instrument in operation and even more when the operations extend for several years. The approached followed in the DESIS mission for calibration is based on the use of the on-board LED calibration unit for spectral calibration and vicarious calibration for radiometric calibration. The data obtained from the calibration unit shows that the average spectral performance of DESIS is very stable over long periods of time. However, it also shows 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 after flat-fielding of the radiometric response using 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 in operation. Geometric calibration is conducted by comparing ground control points (GCPs) automatically extracted from DESIS images with those from reference images of higher geometric accuracy. When a large enough number of matching points are identified between the reference and DESIS images, an improved geometric sensor model is derived, achieving a root mean square error (RMSE) of approximately 21 meters in both the north and east directions. However, when matching is unsuccessful, the RMSE typically increases to around 300 meters in the transverse direction and 500 meters in the longitudinal direction. This contribution summarizes the calibration results obtained over more than six years of the DESIS hyperspectral mission in orbit. Additionally, we provide updates on calibration activities and guidelines for users interested in the accuracy and reliability of DESIS data calibration.