Zurich Open Repository and Archive. Supporting facilities of the airborne imaging spectrometer APEX

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1 University of Zurich Zurich Open Repository and Archive Winterthurerstr. 190 CH-8057 Zurich Year: 2008 Supporting facilities of the airborne imaging spectrometer APEX Nieke, J; Itten, K I; Meuleman, K; Gege, P; Dell'Endice, F; Hueni, A; Alberti, E; Ulbrich, G; Meynart, R Nieke, J; Itten, K I; Meuleman, K; Gege, P; Dell'Endice, F; Hueni, A; Alberti, E; Ulbrich, G; Meynart, R. Supporting facilities of the airborne imaging spectrometer APEX. In: 2008 IEEE International Geoscience & Remote Sensing Symposium, Boston, US, 06 July July 2008, Postprint available at: Posted at the Zurich Open Repository and Archive, University of Zurich. Originally published at: 2008 IEEE International Geoscience & Remote Sensing Symposium, Boston, US, 06 July July 2008,

2 Supporting Facilities of the Airborne Imaging Spectrometer APEX J. Nieke 3, K.I. Itten 1, K. Meuleman 2, P. Gege 4, F. Dell Endice 1, A, Hueni 1, E. Alberti 1, G. Ulbrich 3, R. Meynart 3 and the APEX team 1 U. of Zurich, Dept. of Geography, Remote Sensing Laboratories, RSL, Zurich (CH) 2 VITO, Mol (BE) 3 ESA-ESTEC, Noordwjik (NL) 4 DLR, Oberpfaffenhofen (DE) Corresponding author: jens.nieke@esa.int Abstract The facilities to support the ESA s airborne APEX hyperspectral mission simulator are described. These facilities include calibration tools, such as specific processing in a dedicated Processing and Archiving Facility (PAF), operational calibration and characterization using the Calibration Home Base (CHB), the In-Flight Characterization facility (IFC) and the Calibration Test Master (CTM). Further on, a preview on major applications and the corresponding development efforts to provide scientific data products up to level 2/3 to the user are outlined. Products dedicated for the retrieval of limnology, vegetation, atmospheric parameters, as well as general classification routines and rapid mapping tasks are currently under development and prepared for dissemination by the APEX Science Center (ASC) and the APEX Operations Center (AOC). I. INTRODUCTION The imaging spectrometer APEX (Airborne Prism EXperiment) is a project of the European Space Agency ESA focusing on (1) the preparation, calibration, validation and simulation of future hyperspectral imaging space instruments and (2) the understanding of associated atmospheric, water and land/vegetation processes at local and regional scale in support of global applications. The APEX project started in 1997 by performing a feasibility study on the design of an imaging spectrometer /1/, which resulted in a first performance definition /2/, and a subsequent design phase /3/. Currently, APEX is being finalized being in the testing and characterisation phase /4/5/6/, i.e., the subsequent final testing of the instrument started. The instrument is planned to be final by the end of the year 2008 available for first campaigns in In the scope of the project various facilities were set-up in order to support the project during the APEX exploitation phase (Phase E), i.e., the APEX Science Center (ASC), the APEX Operations Center (AOC), the Processing and Archiving Facility (PAF) and the calibration support facilities, i.e., Calibration Home Base (CHB), In-Flight Characterization facility (IFC), Calibration Test Master (CTM). II. APEX APEX is a flexible airborne hyperspectral mission simulator and calibrator for existing and upcoming or future space missions. It is operating between 380 and 2500 nm in 313 freely configurable bands, up to 534 bands in full spectral mode. Besides general applications development and research, the system is foreseen, /08/$ IEEE V IGARSS 2008

3 to carry out experiments for e.g. ESA Sentinels 2/3, the ESA Explorers FLEX/TRAQ currently under evaluation, the German national initiative ENMAP among others. The APEX instrument consists of several sub-units. The optical sub-unit (OSU) is the core element of the instrument including the sensitive optics, properly interfaced with customized front-end electronic (FEE) boards. The OSU is operated on a stabilized platform (STP) in order to dampen all the externally induced vibrations and ensure stable vertical measurements. The platform is controlled by the navigation system, which receives orientation information from an inertial measurement unit (IMU) implemented on the OSU and position signals from a GPS receiver. The orientation and position information are then synchronized with the image data by the control and storage unit (CSU). III. ASC AND AOC During the APEX exploitation phase (Phase E), the APEX team is organized in an APEX Science Center (ASC) and an APEX Operations Center (AOC). The ASC is hosted at RSL in Zurich (Switzerland) and the AOC is located in Mol (Belgium), hosted by VITO. One objective of the ASC is to foster the use of imaging spectrometer data and the development of new scientific algorithms in close cooperation with scientific users, experts and algorithm developers. Another objective is to monitor APEX calibration, validation and long-term performance. Also calls for airborne/field experiments will be announced via the ASC. In this center the new interface between PAF and algorithm developers will be established. A documentation of all APEX related algorithms is provided in form of algorithm theoretical basis documents. The AOC will interact with all user requests, such as flight requests, archived data search, flight planning, user support, etc. ASC and AOC closely collaborate, demonstrated recently in the successful setup of the Processing and Archiving Facility (PAF) /4/ which allows besides the generation of calibrated at-senor radiances/reflectances the retrieval of various parameters making use of state-of-the-art L2/3 processors /7/. Figure 1: Dedicated APEX data Processors for Level 2/3 products currently under development at ASC/AOC. IV. CALIBRATION FACILITIES System calibrations of the APEX instrument are carried out on a regular basis. The collected calibration data sets provide means for long-term system performance analysis. Figure 2: The APEX CHB enabling spectral, spatial and radiometric calibration A detailed characterization of the APEX instrument must be carried out to achieve the required data quality. The needed system parameters can be gathered by specific measurements performed in the Calibration Home Base (CHB) /8/. The CHB with dedicated spectral, radiometric and geometric calibration facilities allows full laboratory characterization and calibration of APEX. The CHB is located at DLR near Munich (Germany). V - 503

4 Short term changes of a limited set of instrument characteristics can also be observed by using the Inflight characterisation facility (IFC) /9/. Recording IFC data at the start and end of each flight strip will be used to assess the stability of the instrument over shorter periods of time. related parameters, i.e. the APEX settings and calibration facility settings (e.g. monochromator wavelength selection) for a particular calibration procedure, (b) the storage unit, which is partly embedded in APEX and partly located on an external desktop computer, and (c) the processor, whose function is to process the measured raw data in combination with the relevant settings of APEX and CHB in order to generate the calibration parameters necessary to calibrate the image data in the PAF. Figure 3: Opto/Mechanical Unit (spectrometer hermetic sealed) with Inflight characterisation facility IFC. Following the APEX calibration strategy /9/, repetition of various calibration and characterization measurements making use of the above facilities is needed. In order to allow automatic generation of related parameters, a calibration test master (CTM) was developed /10/. Figure 4: The logical working flow of the Calibration Test Master (CTM) interfacing the instrument APEX, the calibration home base (CHB), and the Processing and Archiving Facility (PAF). The CTM consists of three main elements: (a) The controller physically located in the APEX processor enables to set up all relevant APEX (e.g. frame rate, integration time) and CHB V. PROCESSING FACILITY A Processing and Archiving Facility (PAF) must thus be able to deal with the above-mentioned facilities and the high volume of data typically produced by hyperspectral imagers. It furthermore acts as data source for the user, offering products at several processing levels via online order pages and on demand processing facilities. The PAF is defined as the combination of all hardware and software components and their interfaces required for handling and processing APEX imagery and its related data /4/. The typical data size of hyperspectral imagery necessitates a computing architecture capable of delivering the needed processing power. The APEX PAF relies on the Master/Worker and Task/Data decomposition patterns implemented as a workflow framework /11/. Major design requirements are on-demand, user configurable product generation, and full reproducibility of user orders and re-processing capability of any data product level. This is all made possible by the product and processing database (PPDB), which forms the heart of the processing system. The PPDB keeps track of (a) all imagery data, (b) related metadata such as calibration or housekeeping data and (c) subsequent products in the archive and stores the processing settings for on demand generation of higher-level products. The PPDB is the single source for the dynamic building of the product order web pages. V - 504

5 The workflow automates the archiving of the raw input and its processing up to level 1C, thus generating a spectrally, geometrically and radiometrically calibrated, uniform data cube. This sensor model inversion is parameterized by calibration cubes generated by the CTM. Level 1C and higher-level products are ordered by user input via dynamic web interfaces. These orders are entered into the PPDB and trigger the processing by the workflow. The final data products are downloadable via FTP accounts. Figure 5: The logical working flow of the Processing and Archiving Facility (PAF). VI. CONCLUSION Currently full end-to-end testing of the airborne ESA- APEX hyperspectral APEX and its supporting facilities has been started. The APEX Science and Operations Centers (ASC and AOC) directly support the testing activities making use of newly developed processing and calibration tools, such the dedicated Processing and Archiving Facility (PAF), the Calibration Home Base (CHB), the In-Flight Characterization facility (IFC) and the Calibration Test Master (CTM). By the end of the year 2008 the testing of APEX and its facilities should be finalized resulting in the subsequent start of the exploitation phase. All members of the team are looking forward to this phase in order to allow making use of the dedicated instruments and its research facilities. REFERENCES 1 Itten, K.I., Schaepman, M., De Vos, L., Hermans, L., Schläpfer, D., and Droz, F., APEX Airborne PRISM Experiment: A new concept for an airborne imaging spectrometer, Proc. ERIM, Vol. 1, , 3rd Intl. Airborne Remote Sensing Conference and Exhibition, Schaepman, M., De Vos, L., and Itten, K.I., APEX Airborne PRISM Experiment: Hyperspectral radiometric performance analysis for the simulation of the future ESA Land Surface Processes Earth Explorer Mission, Proc. SPIE, Vol. 3438, , Schaepman, M., Schläpfer, D., and Itten, K., APEX A New Pushbroom Imaging Spectrometer for Imaging Spectroscopy Applications: Current Design and Status, Proc. IGARSS, , Hawaii, Hüni, A., J. Biesemans, K. Meuleman, F. Dell Endice, D. Schläpfer, D. Odermatt, M. Kneubühler, J. Nieke, Structure, components and interfaces of the APEX processing and archiving facility, submitted to IEEE- TGARS, Ulbrich, G., R. Meynart, J. Nieke, APEX Airborne Prism Experiment: The Realization Phase of an Airborne Hyperspectral Imager, SPIE Europe, Maspalomas, Gran Canaria, Spain, Sep. 2004, SPIE Vol. 5570, Nieke, J., K.I. Itten, W. Debryun, The Airborne Imaging Spectrometer APEX: From Concept to Realisation,Proc. 4th EARSeL Workshop on Imaging Spectroscopy, Warsaw, April 2005, CD-ROM., Schläpfer, D., Nieke, J., Dell'Endice, F., Hueni, A., Biesemans, J., Meuleman, K. and Itten, K., Optimized Workflow for APEX level 2/3 Processing in Proc. 5th EARSeL Workshop on Imaging Spectroscopy, Bruges (B), April CD-ROM, Gege, G. J. Fries, P. Haschberger, P. Schötz, H. Schwarzer, P. Strobl, B. Suhr, G. Ulbrich, W.J. Vreeling, Calibration facility for airborne imaging spectrometers accepted for publication in ISPRS Journal of Photogrammetry & Remote Sensing, Nieke, J., J. Kaiser, D. Schläpfer, J. Brazile, K.I. Itten, P. Strobl, M. Schaepman, G. Ulbrich, Calibration Methodology for the Airborne Dispersive Pushbroom Imaging Spectrometer (APEX), SPIE Vol. 5570, Dell'Endice, F., J. Nieke, J. Brazile, D. Schläpfer, A. Hüni and K. Itten, Automatic Calibration and Correction Scheme for APEX, 5th EARSeL Workshop on Imaging Spectroscopy, Bruges, Belgium, Biesemans, J., Sterckx, S., Knaeps, E., Vreys, K., Adriaensen, S., Hooyberghs, J., Meuleman, K., Kempeneers, P., Deronde, B., Everaerts, J., Schlaepfer, D., Nieke, J., Image processing working flows for airborne remote sensing, in 5th EARSeL Workshop on Imaging Spectroscopy, Bruges, Belgium, April V - 505