A new German Hyperspectral Mission EnMAP: Image Products

Kurzfassung

The upcoming Environmental Mapping and Analysis Program (EnMAP) - German imaging spectroscopy mission - is a joint response of German Earth observation research institutions, value-added resellers and space industry to the increasing demand for accurate, quantitative information about the status and evolution of terrestrial ecosystems. EnMAP is currently in the construction phase with a launch planned in 2020. The project management is led by the Space Agency of DLR. The space segment consisting of instrument and bus will be established by OHB System AG. The EnMAP satellite will be operated on a sun-synchronous orbit at 643 km altitude with a local time of descending node 11:00 to observe any location on the globe under defined illumination conditions featuring a global revisit capability of 21 days under a quasi-nadir observation. The satellite has an across-track tilt capability of ± 30° enabling a revisit time of four days. The hyperspectral instrument will be realized as a pushbroom imaging spectrometer. Its data acquisition over the broad spectral range from 420 nm to 2450 nm will be performed by a CMOS (Complementary Metal Oxide Semiconductor) detector array for VNIR (visible and near infrared) with 95 spectral channels, i.e. 6.5 nm spectral resolution, and by a MCT (Mercury Cadmium Telluride) detector array for SWIR (shortwave infrared) with 135 spectral channels, i.e. 10 nm spectral resolution. The ground pixel size is 30 m × 30 m at nadir at 48° northern latitude. In this context a pointing accuracy of better than 500 m is expected. The pointing knowledge and therefore the accuracy of image products will be better than 100 m and can be improved by ground processing, if a reference image is available, to approximately 30 m (i.e. 1 pixel) w.r.t. the used reference image. The sensors’ 1000 pixels in spatial direction result in a swath width of 30 km. Regular on-board calibration measurements are performed to update the calibration tables for the processors. EnMAP level 0 (L0) image products (raw data) will be long-term archived while L1B products (systematically and radiometrically corrected data), L1C products (geometrically corrected data) and L2A products (atmospherically corrected data) will be processed on demand. The L1B processor corrects the hyperspectral image cube for systematic effects of the focal plane detector array, e.g. radiometric non-uniformities, and converts the system corrected data to physical at-sensor radiance values based on the currently valid calibration tables. The L1C processor creates ortho-images based on direct geo-referencing techniques implementing a line-of-sight model, which uses on-board measurements for orbit and attitude determinations as well as the sensor look direction vectors based on the currently valid calibration values. Furthermore, it is foreseen to automatically extract ground control points from existing reference data sets by image matching techniques to improve the geometric accuracy better than one pixel size. The L2A processor performs atmospheric correction and haze detection of the images by estimating the aerosol optical thickness and the columnar water vapor. Output products will be the ground reflectance cube and masks of land, water, cloud, cloud shadow, haze, cirrus and snow.