Status and planning of EnMAP operations

Kurzfassung

The launch of the spaceborne imaging spectroscopy mission EnMAP (Environmental Mapping and Analysis Program; www.enmap.org) is scheduled for April 2022. The presentation will detail the status and planning of EnMAP operations. The status covers on the one hand the realized system to perform operations and on the other hand the results of the Launch and Early Orbit Phase (LEOP) (0.5 months) and, in particular, the first insights of the Commissioning Phase (CP). The planning covers the complete activities of the CP (5.5 months) and the subsequent routine phase (54 months) with the provision of quantitative imaging spectroscopic measurements substantially improving remote sensing standard products and allowing advantageous user-driven information products to be established. The objective of EnMAP is to measure, derive, and analyze quantitative diagnostic parameters describing key processes on the Earth’s surface focusing on issues related to soil and geology, agriculture, forestry, urban areas, aquatic systems, ecosystem transitions and associated science. The spectral range of EnMAP covers 420 nm to 2450 nm based on a prism-based dual-spectrometer with a spectral sampling distance between 4.8 nm and 8.2 nm for the VNIR (Visible and Near Infrared; 450 nm to 1000 nm) and between 7.4 nm and 12.0 nm for the SWIR (Shortwave Infrared; 900 nm to 2450 nm). An on-board doped Spectralon sphere enables a spectral accuracy of better than 0.5 nm in VNIR and 1.0 nm in SWIR. The target signal-to-noise ratio (SNR) is 500:1 at 495 nm and 150:1 at 2200 nm (at reference radiance level representing 30% surface albedo, 30° Sun zenith angle, ground at sea level, and 40 km visibility with rural atmosphere). The signal is fed into two parallel amplifiers with different gains for each of the two detectors to have a large dynamic range. Sun calibration measurements with an on-board full-aperture diffuser enable a radiometric accuracy of better than 5%. Additional measurements, e.g. for non-linearity and closed shutter measurements for subtraction of dark signal, complement the calibration. Each detector array has 1000 valid pixels in spatial direction and, with a geometric resolution 30 m x 30 m, a swath width (across-track) of 30 km is realized. A swath length (along-track) of 5000 km, split to several observations, is reached per day. The repeat cycle of 398 revolutions in 27 days combined with an across-track tilt capability of 30° enables a target revisit time of less than 4 days. And each region is viewable under an out-of-nadir angle of at most 5°. The local time of descending node is 11:00. The satellite, which is realized by OHB System AG, will be operated by the ground segment. DLR’s Earth Observation Center (EOC) together with the German Space Operations Center (GSOC) are responsible for operations. Mission management is covered by DLR’s Space Agency. Control and command of the satellite based on flight operations procedures using real-time and dumped data is performed via S-band ground stations for telemetry and telecommand data in Weilheim (Neustrelitz as backup) and in addition Inuvik, O’Higgins, and Svalbard for non-nominal operations. Proposals, observations, and associated research are presented by an interactive map supporting the establishment of a world-wide user network. In case of tasking conflicts, issued observations are prioritized not only according to static information like the underlying priority of the request, but also based on historical and predicted cloud coverage information taking satellite constraints such as power and storage into account. All information is incorporated into the mission timeline immediately on reception and feedback to users on planned observations is provided whenever the planning state of an observation changes. Required orbit maneuvers, for orbit maintenance and collision avoidance, and contacts with X-band ground stations for instrument data reception in Neustrelitz (Inuvik as backup) are also considered in the mission timeline and as such transmitted to the satellite during S-band passes. Together with orbit and attitude data of the satellite, image products from received instrument data during X-band passes are fully-automatically generated at three processing levels, and long-term archived in tiles of 30 km x 30 km. A catalogue allows users to search and browse all products based on the standardized protocols CSW (catalogue service for the web) and WMS (web mapping service). Because of the necessary various processing options, each product is specifically generated for each order and delivered using SFTP (secure file transfer protocol) to the scientists. Level 1B products are corrected to Top-of-Atmosphere (TOA) radiances including defective pixel flagging, non-linearity correction, dark signal (and digital offset) correction, gain matching, straylight correction, radiometric/spectral referencing, radiometric calibration, and defective pixel interpolation. Level 1C products are orthorectified to a user selected map projection and resampling model. The physical sensor model is applied by the method of direct georeferencing with a correction of sensor interior orientation, satellite motion, light aberration and refraction, and terrain related distortions from raw imagery. The products have a geolocation accuracy of 30 m with respect to a reference image based on selected Sentinel-2 Level 1C products having an absolute geolocation accuracy of 12.5 m. Level 2A products are compensated for atmospheric effects with separate algorithms for land and water applications. For the land case the units are expressed as remote sensing, namely Bottom-of-Atmosphere (BOA), reflectances. For the water case the units are normalized water leaving remote sensing reflectance or subsurface irradiance reflectance based on user selection. A pixel classification (e.g. land-water-background, cloud) is performed and aerosol optical thickness, columnar water vapor, and adjacency corrections are treated accordingly. At all processing levels per-pixel quality information and rich metadata are appended online. Offline quality control, e.g. based on pseudo invariant calibration sites (PICS), maintenance and fine tuning of the image processing chain, and calibrations of the instrument during operations complete the ground segment. The independent validation of products, e.g. based on already established calibration/validation procedures, sites, networks, and products of other missions, is performed by the science segment led by the GFZ German Research Centre for Geosciences. All elements of the mission are characterized and calibrated, technically verified and operationally validated until launch. EnMAP will be launched by a Falcon 9 of SpaceX from Florida, USA. The subsequent LEOP covers the first contact with satellite after separation, setting up telemetry and telecommand communications, continuous monitoring of health status, checkout and configuration of all platform functions, e.g. achievement of nominal power and thermal configuration, activation and calibration of sensors and actuators, e.g. for attitude and orbit determination and control, and acquisition of required orbital parameters. The subsequent CP covers the activation of the instrument data storage and all payload functions including the first image acquisition, downlink, and processing. The focus is first on radiometric, second on spectral in-flight calibration using all on-board equipment and taking pre-flight characterization and calibration into account, and in parallel on geometric characterization using Earth observations. The results lead to the optimization of the radiometric and geometric processors at first and then to the atmospheric processors. These activities are iterated and complemented by continuous instrument monitoring and quality control. Finally, based on end-to-end experiences the user interfaces from observation planning to product delivery and the complete processing chains are fine-tuned. The objective of the CP is to put space and ground segment into nominal routine operations with detailed in-orbit performance analyses of on-board and on-ground functionalities resulting in product approval for users which is expected for October 2022. The subsequent routine phase keeps the mission working in nominal operations based on ground procedures and by appropriately handling non-nominal operations, if required. All elements are supervised, the satellite is kept in the required orbit, data are acquired and dumped according to the requests and following the planned mission timeline, and products are processed and delivered to users. The offered operational services are complemented by a service team offering expert advice on the exploitation of EnMAP. EnMAP operations are planned to be continued until April 2027.