The hyperspectral Mission DESIS - Entering the operational Phase

Abstract

In 2014, the German Aerospace Center (DLR) and the US company Teledyne Brown Engineering (TBE) decided to collaborate and develop the hyperspectral instrument "DLR Earth Sensing Imaging Spectrometer" (DESIS) for integration into the "Multi-User System for Earth Sensing" (MUSES) installed on the International Space Station (ISS). Four years later, on 29 June 2018, the instrument was launched from Cape Canaveral with a SpaceX Dragon rocket. One month later, the instrument was robot-assisted integrated into MUSES and the commissioning phase started. First light images were available after 48 hours. The commissioning phase includes checkout for functional testing, End-to-End tests, processing parameter tuning, in-flight calibration tests and product validation. The operational phase of the mission has been entered several months later. From this point on, hyperspectral data will be available to the scientific and commercial user community. In this paper a summary of results from the commissioning phase will be presented including aspects of data quality, operational performance and first data evaluation results. The MUSES platform provides accommodations for two large and two small hosted payloads. MUSES is attached at the ELC-4 (EXPRESS Logistics Carriers) starboard of the ISS. It is a space-based, Earth-pointing platform providing position and attitude sensing, data downlink, and other core services common for each payload. DESIS has a mass of ~88 kg and is integrated in one of the large containers. Two gimbals allow a rotation of the whole MUSES platform around two axes resulting in ±25° forward / backward view, 45° backboard (port) view and 5° starboard view. The platform is equipped with a star tracker (sampling rate 10 Hz) and a MIMU (Miniature Inertial Measurement Unit) (sampling rate 50 Hz) providing a 10 Hz attitude measurement after filtering. ISS GPS data provide position and velocity vectors and time tags (sampling rate 1 Hz). The predicted viewing capability of MUSES, when operating at the ISS orbit inclination of 51.6°, will enable the DESIS instrument to scan greater than 90% of the populated Earth with an average cadence of about one week. The DESIS hyperspectral instrument is realized as a pushbroom imaging spectrometer and features 235 bands with 3.5-4.0 nm spectral resolution, covering a range from 400 nm to 1000 nm with 30 m spatial resolution employing a 2-dimensional back illuminated CMOS (Complementary Metal Oxide Semiconductor) detector array. The optical design is based on the Offner-type grating spectrometer widely used in hyperspectral imaging. DESIS is equipped with a Pointing Unit (POI) consisting of a rotating mirror in front of the entrance slit allowing a forward and backward viewing change up to ±15° w.r.t. the nominal (e.g. nadir) view. The POI can be operated in a static mode with 3° angle steps for the viewing direction and in a dynamic mode with up to 1.5° change in viewing direction per seconds. This allows, besides standard Earth data products, acquisition of Bidirectional Reflectance Distribution Function (BRDF) products and continuous observations of the same targets on ground (using forward motion compensation mode). DESIS has been characterized in laboratory and will be re-calibrated after launch employing measurements of internal light sources (bank of white and colored LED lamps) and globally distributed reference sites aiming to assess radiometric, spectrometric and geometric characteristics of the DESIS hyperspectral instrument in orbit. Vicarious validation activities will be performed at regular interval to ensure that a suitable data quality is met. This includes validation over well-known calibration sites, cross-comparisons with other simultaneous acquired data sets and analysis of homogenous areas. The Ground Segment at DLR has developed the operational processing chain to derive different types of DESIS products from tiled data takes of size 1024 x 1024 pixels (~30 x 30 km²). An identical processing chain is licensed to TBE running on a cloud based system (© Amazon Cloud) for commercial product generation. DESIS level 1A products (Earth image scenes, on-board calibration measurements, dark current measurements and experimental products) will be long-term archived together with the corresponding metadata, while level 1B products (systematically and radiometrically corrected data), level 1C products (geometrically corrected data employing global references) and level 2A products (atmospherically compensated data) will be processed on demand before being delivered to the user for further value-add product generation. For scientific purposes only, DLR can share DESIS scientific data with other scientific organizations. TBE distributes data on a commercial basis.