Calibration of the DESIS Instrument

Abstract

The DLR Earth Sensing Spectrometer (DESIS) on board the International Space Station (ISS) has been providing high quality hyperspectral data to the scientific community and commercial users since the start of operations in September 2018. Now 5~years in orbit, the DESIS instrument continues to operate correctly and provide hyperspectral data products for a wide variety of applications. This successful activity is supported by the calibration team that regularly analyzes the instrument data and provides calibration updates. DESIS calibration uses the on-board LED calibration unit to monitor the spectral response of the instrument. Over the years the spectral performance of DESIS shows little variation over time, what makes unnecessary spectral calibration updates in most occasions. However, there are as well variations in spectral calibration from one measurement to another. Part of this variability is related to temperature gradients inside the instrument that can be corrected during data processing. However, there are other variations that seem to have a random nature (RMS ~0.1 nm) and cannot be corrected. Radiometric calibration, on the other hand is based on vicarious calibration using RadCalNet sites as reference. We also use a harmonization of the radiometric response of all sensor elements, where small spectral adjustments can also be incorporated. This pixel harmonization is performed using different algorithms applied over a large selection of uniform scenes in several different steps. Comparison with RalCalNet reference data shows that the radiometric calibration of DESIS can change a 3.4% over one year above 500 nm. However, below 500 nm there is a larger variation that increases with decreasing wavelengths up to a maximum of 25% per year. This higher variation in the first 100 nm has been reducing over time, until July 2021, when seems to have stopped. Typical DESIS calibration updates are performed every 7 months, resulting in average variation of up to 2% above 500 nm. Finally, the geometric calibration of the sensor is performed by comparison of ground control points (GCPs) automatically extracted from the DESIS images and from reference images of higher geometric accuracy. When enough GCPs can be found on an image, DESIS images have a similar RMSE of ~21 m, which is similar in both North and East directions. RMSE when no GCPs can be obtained is typically 300 m in the across-track direction and 500 m in the along-track direction. In this contribution we present the concept used for the DESIS calibrations and the results obtained in these 5 years of operation. We offer here an overview of the uncertainties in the DESIS originating at the sensor calibration and how they vary with time.