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The Environmental Mapping and Analysis Program (EnMAP) is a German hyperspectral satellite mission to monitor and characterize the Earth’s environment. The EnMAP payload, the Hyper Spectral Imager (HSI) features an on-board calibration assembly (OBCA) which is designated to provide the optical radiation to monitor the instrument radiometric and spectral stability during the mission lifetime. The assembly comprises two integrating spheres in twin configuration equipped with several different optical radiation sources. The large sphere made of white diffuse reflecting material is dedicated for radiometric stability measurements, while the small sphere, made of rare-earth doped diffuse reflecting material, is dedicated for spectral stability checks. The OBCA utilizes two types of optical radiation sources: tungsten halogen lamps and white light LEDs.
Here we report on the spectral and radiometric calibration of the OBCA qualification and flight model in the Reduced Background Calibration Facility 2 (RBCF2) of Physikalisch-Technische Bundesanstalt (PTB) [1].
The demanding requirements were to perform a calibration in air and in vacuum with an uncertainty of less than 2% with a spectral resolution of 0.1 nm over a wavelength range from 400 nm to 2500 nm not exceeding an operating time of 40 h for the halogen lamps and 100 h for the LEDs. Furthermore, a precise mapping of the OBCA exit aperture of size 2 mm by 24 mm with 1 mm sampling diameter had to be performed. For that purposes PTB developed a calibration procedure based on spectral comparisons of the OBCA with respect to dedicated vacuum radiance standards with an FTS in three wavelength ranges which were covered by three beamsplitter detector combinations. A dedicated imaging optics was designed transforming the F:3 opening of the OBCA to the F:8 opening ratio of the FTS and providing also the required small sampling area.
Before and after their application, the dedicated vacuum qualified radiance standards were calibrated against the primary standards of PTB and corrected for the transition from air to vacuum and back to account for possible drifts of the sources. By this procedure a spectral and radiometric calibration of the OBCA traceable to the SI was achieved with the aspired uncertainties.
[1] C. Monte et al, The new Reduced Background Calibration Facility 2 for Detectors, Cameras and Sources at the Physikalisch-Technische Bundesanstalt, Sensors, Systems, and Next-Generation Satellites SPIE 2018
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