DESIS Calibration: Status and Results after 4 Years of Operation

The DESIS Hyperspectral instrument is approaching four years in operation at the MUSES platform on the International Space Station. DESIS operates in the VNIR range (400-1000 nm) and has some unique properties among spaceborne hyperspectral instruments like a narrow spectral sampling distance (2.55 nm), the use of a pointing unit and a LED equipped calibration unit. During this time, the instrument has been calibrated using a combination of vicarious calibration and calibration unit data. The calibration unit is used for the spectral calibration of the sensor. The analysis of the LED data has shown two different operation modes depending on the temperature gradient inside the instrument. Moreover, the data shows that the spectral calibration is stable within 0.1 nm (RMS) for each temperature gradient mode, with a separation between them of around 0.5 nm. The vicarious calibration is used for the radiometric calibration and uses as input Earth images over homogeneous areas and reference data from RadCalNet stations. Data from the homogeneous areas are used to update individual pixel coefficients in order to provide a uniform spatial and spectral sensor response. Later, RadCalNet data are used for an adjustment of the absolute calibration scale. With this approach the DESIS instrument can be calibrated within 4% (RMS) and absolute mean bias within 2% at wavelengths above 500 nm. The long-term analysis of the DESIS calibration data also shows that the sensor experiences a mean degradation of around 3.4%/year above 500 nm. Below 500 nm the instrument shows a stronger degradation (up to 20%/year at the shortest wavelengths) between the start of operations and July 2021, being stable after this date. Finally, the orthorectification process achieves a RMSE of 20 m in North and East directions with the automatic extraction of ground control points and comparison with reference images