Radiometric Characterization, Calibration, and Correction for the Imaging Spectroscopy Mission EnMAP

Abstract

The high-resolution imaging spectroscopy remote sensing satellite mission EnMAP (Environmental Mapping and Analysis Program, enmap.org) will cover the spectral range from 420nm to 2450nm with a spectral sampling distance varying between 4.8nm and 12.0nm comprising 262 spectral bands. The expected signal-to-noise ratio at reference radiance level is 500:1 at 495nm and 150:1 at 2200nm. The radiometric resolution is 14bits and an absolute radiometric accuracy of better than 5% is achieved. Each of the two 2-dimensional detector arrays of the prism-based pushbroom dual-spectrometer works in a dual-gain configuration to cover the complete dynamic range. EnMAP will acquire 30km in the across-track direction with a ground sampling distance of 30m and the across-track tilt capability of 30° will enable a target revisit time of less than 4 days. The launch is scheduled for 2020. In-flight radiometric characterization and calibration are based on Sun calibration measurements with a full-aperture diffusor for absolute calibration. In addition, a weekly relative calibration monitors the instrument during the complete mission lifetime based on an integrated sphere (on the satellite) coated with Spectralon and illuminated with a Tungsten halogen lamp and a white LED (light emitting diode). These calibration measurements also allow for the regular update of the dead pixel mask containing e.g. hot, cold, and flickering pixels. The frequency of the absolute calibration is scheduled based on a model using previous calibration measurements to minimize the aging of the diffusor. The calibration is complemented with closed shutter measurements before and after each observation for dark signal subtraction and additional deep space measurements every four months for shutter thermal emission monitoring. Due to air-vacuum transition and gravity release, most significant effects are expected during and shortly after launch and therefore, a high frequency of calibration measurements is planned during the commissioning phase. Aging effects within the operational phase can be studied with longer calibration intervals. Pre-flight radiometric characterization and calibration also consider effects which are not measureable during operations, e.g. spectral and spatial straylight, and verify temperature and radiometric stability assumptions. Furthermore, this provides the first reference and characterization measurements of the calibration equipment on the satellite. Based on the pre- and in-flight calibration activities, the on-ground fully-automatic processing chain generates standardized Earth observation products with TOA (top-of-atmosphere) radiances and annotates spectral and geometric characterization as well as pixel classification and information for later processing, e.g. geometric and atmospheric corrections. For radiometric correction the following steps are performed: non-linearity correction; dark signal correction; gain matching; straylight correction; and the radiometric calibration as well as data quality control routines including defective pixel flagging, where dead and abnormal pixels are considered, generation of raw and calibrated detector maps, and analysis for striping, banding and other artefacts. The online data quality control and monitoring routines for all acquisitions are complemented by offline data quality activities for selected acquisitions. The high-quality products will be freely available to international scientific users for measuring and analyzing diagnostic parameters which describe vital processes on the Earth's surface.