The paper presents the current status of the operational calibration facility that can be used for radiometric, spectral and geometric on-ground characterisation and calibration of imaging spectrometers. The European Space Agency (ESA) co-funded this establishment at DLR Oberpfaffenhofen within the framework of the hyper-spectral imaging spectrometer Airborne Prism Experiment (APEX). It was designed to fulfil the requirements for calibration of APEX, but can also be used for other imaging spectrometers. A description of the hardware set-up of the optical bench will be given. Signals from two sides can alternatively be sent to the hyper-spectral sensor under investigation. Frome one side the spatial calibration will be done by using an off-axis collimator and six slits of different width and orientation to measure the line spread function (LSF) in flight direction as well as across flight direction. From the other side the spectral calibration will be performed. A monochromator provides radiation in a range from 380 nm to 13 μm with a bandwidth between 0.1 nm in the visible and 5 nm in the thermal infrared. For the relative radiometric calibration a large integrating sphere of 1.65 m diameter and exit port size of 55 cm × 40 cm is used. The absolute radiometric calibration will be done using a small integrating sphere with 50 cm diameter that is regularly calibrated according to national standards. This paper describes the hardware components and their accuracy, and it presents the software interface for automation of the measurements.
ESA currently builds the airborne hyper-spectral push broom imaging spectrometer APEX (Airborne Prism EXperiment) operating in the spectral range from 380 to 2500 nm. In the scope of the APEX project a large variety of characterization measurements will be performed, e.g., on-board characterization, frequent laboratory characterization, and vicarious calibration. The APEX instrument will only achieve its challenging measurement accuracy by regular calibration of the instrument between flight cycles. For that on-ground characterisation, a dedicated characterisation and calibration facility is necessary to enable a comprehensive and accurate calibration of the instrument. In view of the high relevance to scientific objectives, ESA is funding an external "Calibration Home Base" (CHB). It is located at DLR Oberpfaffenhofen and will be operational from 2006 on. The CHB provides all hard- and software tools required for radiometric, spectral and geometric on-ground characterisation and calibration of the instrument and its internal references and on-board attachments, and to perform measurements on polarisation- and straylight-sensitivity. This includes a test bed and the provision of the infrastructure. In this paper the calibration equipment and concept is outlined.