VALUE ADDED GEOCODED ENVISAT-ASAR PRODUCT SERVICE

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1 VALUE ADDED GEOCODED ENVISAT-ASAR PRODUCT SERVICE Detlev Kosmann (1), Martin Huber (1), Manfred Bollner (1), Hannes Raggam (2), Dave Small (3) ¹ DLR, German Remote Sensing Data Center, Oberpfaffenhofen, D Wessling, Germany (Deltev.Kosmann, Martin.Huber, ² Joanneum Research Graz, Wastiangasse 6, A-8010 Graz, Austria, ³ Remote Sensing Laboratories, University of Zürich, Winterthurerstr. 190, CH-8057 Zürich, Switzerland, ABSTRACT/RESUME The German Remote Sensing Data Center (DFD) of the German Aerospace Center (DLR) is an element in the network of the Multimission Facilities of ESA and responsible among other tasks for SAR processing. The DLR has implemented a service for value added products over land. This service can support the user community with preceise geocoded products in addtion to the standard products of ESA [3] Envisat GEOcoding system (EGEO) supports high resolution images, the medium resolution and also the low resolution mode from ENVISAT, product examples will be presented. The user service includes also the integration of national geodetic maping parameter to the products. Different datumshift parameter sets are implemeted. New parameter sets can be easily integrated, if the user can deliver the required geodetic parameters. Two major types of geocoded products are available: Enhanced Ellipsoid Corretced (EEC) Enhanced Geocoded Terrain Corrected (ETC) The EEC generation is a fully automated process. The process has access to a Digital Elavation Model (DEM) data base, because the GLOBE DEM (1km resolution) is used to reduce height induced errors in a global quality. The ETC uses the best available DEM to eliminate geometric mislocation. SRTM DEMs with 100m and 25m resolution and other high resolution DEMs, e.g. InSAR DEMs or from other sources can be utilized. The system was built up in co-operation with the Remote Sensing Laboratory (RSL) of the University Zürich (CH) and Joanneum Research, Graz (A). ESA and the German Ministry of Education and Research (BMBF) support this project. UNIX system environment It is implemented on different SUN workstation, desktop and server architectures. The PF-ASAR processor produces for the different modes of the ASAR sensor a large variety of products at different levels [1]. The highest standard processing level 1b supported by ESA are ellipsoid geocoded products. They are important data sets for the value adding industry. But only the consideration of topographic height information allows to generate precise geocoded ENVISAT products. 2 GEOCODED PRODUCTS - IMAGE MODE The availability of Digital Elevation Models is the most important factor for the geocoding. Therefore the products are divided by the height information used into EEC and ETC. 2.1 Enhanced Ellipsoid Corrected (EEC) The EEC is the basic product service for ENVISAT. It is comparable to the GEC from the ESA Processing and Archiving Facilities/Centers. Both the standard products single look complex (IMS) and precision (PRI) can be used as input. A GLOBE DEM with a mesh size of 1kmx1km is used as standard height source. Regarding this mesh size a geometric height correction is done to correct small-scale geometric distortion. As a special Service also higher resolution DEMs can be used if available. No ground control points are necessary for this total automatic process. But therefore a precise orbit product is evident, because orbit errors have a direct impact to the geometry and can only be eliminated by control points. Tab. 1: EEC productparameter 1 INTRODUCTION The geocoding service continues the SAR product service beginning with the geocoding of ERS-1 in 1991 and integrates the new features of ENVISAT. EGEO is the basic software of this systems. The package is coded in CC-C++ and is running in the SOLARIS Input Product Type Pixel Spacing Product Format Cartograhy Envisat Modes Control Points Availabity ASA_IMS and ASA_IMP 12.5 m Sun Raster plus Parameter Pool National System/WGS84 IM, AP,WS, GM no worldwide Proc. of the 2004 Envisat & ERS Symposium, Salzburg, Austria 6-10 September 2004 (ESA SP-572, April 2005)

2 2.2 Enhanced Geocoded Terrain Corrected (ETC) The ETC presents the highest-level product type for ENVISAT that is generated automatically. This product can be used to combine historical ERS SAR Geocoded Terrain Corrected (GTC) products from the D-PAF with ENVISAT products. One improvement to this product is due to the new availability of Digital Elevation Models (DEM). For ERS-era processing only DEMs from Germany were available, so that the usage of the GTC was limited. The ERS Tandem Mission and the Shuttle Radar Topographic Mission offers a new dimension of DEM availability. DEMs all over the world are now available in a resolution of 1 x 1, fulfilling the accuracy requirements for ETC products. For the greatest height accuracy the X-Band data processed at DLR can be used. Since May 2004 the complete worldwide dataset is integrated in DLR s DEM database. The well-known gaps of the X-band can be filed up with SRTM C-band DEMs with a resolution of 3 x3 [7]. If these DEMs do not comply with the service requirements of the user, the DFD can use the GEMOS interferometric processing systems to use former ERS and ENVISAT data to generate the required DEMs [5]. Additionally DEMs from the ASTER sensor system can be processed as a special user service. All these single DEMs sources can be combined by the processing algorithm to a Best of DEM for the geocoding., a homogeneous DEM without gaps or discontinuities [6]. The improved geocoding system of the DFD has the new capability to produce a high quality product with and without Ground Control Points. The availability of a precise orbit is recommended for the ETC processing. The time-consuming interactive tiepoint measurement is not in any case necessary. The output format is no longer the well-known CEOS. Two files are delivered to the customer: Raster image file Parameter pool dataset The very easy raster format from SUN is used for the image information. The product starts with 32 Bytes of header information, the pixel information follows line by line. In the sun raster file format a specific footer can be append. EGEO uses this footer information to store the basic cartographic and statistic information as shown in the following example. Geo_Reference: projection : ESA TM UT35 coord units : ME top-left n/e: bottom-right: max, min : mean, std : resno, resea inv val + nr: All important processing parameter and detailed product parameter are archived in a separate product called the Parameter Pool. This ASCII-file is very easy to read and the parameters are self-explanatory. It is part of the delivered product. Tab.2: ETC product parameters Input Product Type ASA_IMS and ASA_IMP Pixel Spacing 12.5 m Radiometric depth 16 bit Product Format Sun Raster plus Parameter Pool Cartography National System/WGS84 Envisat Modes Imaging, Medium Res.,Wide swath Control Points With/without Availability Worldwide The geometric accuracy of the input products is very important. The DEM and the IMS/IMP from the PF- ASAR are directly effected the accuracy the ETC processing [10]. The quality of both products is well known. The geometric error of the pixel location is in the order of 25m Auxiliary Products The EGEO processing produces some more additional products: Geocoded Incidence Angle Mask (GIM) Height Error Map (HEM Coverage Plots The GIM contains the local incidence angle between the radar beam and a line perpendicular to the slope at the point of incidence. A detailed description is in [9]. The height reference is the most critical data source. The accuracy is direct linked to the DEM accuracy. Therefore the customer will get a height error map. For each pixel of the ETC the RMS Error of the DEM is annotated and with the coverage plots it is possible to retrieve the local DEM resolution that was used for an selected area. All auxiliary products can be used of quality estimation in selected region of the scene. Fig. 1 shows an example of these products. The upper left shows the used geocoded DEM and the Height Error Map for the region. The customer can derive from this HEM local quality information for his product, lower left is the final output product in this example combined with the amplitude of SRTM and the lower right images represents the number of available DEMs for individual areas.

3 a b c d Fig 1: a - DEM, b - Height Error Map, c - Geocoded product, d - DEM coverage map 2.3 Mosaic Based on the single EEC and ETC datasets EGEO has the capability to produce large area mosaics. The mosaicing postprocessor uses the information derived during the geocoding steps and the information from the auxiliary datasets and generates a mosaic from the selected scenes. To guarantee a reproducible dataset not special interactions like definition of cut lines, feathering or edge smoothing is carried out. The quality depends on the quality of the process input products and especially from the used orbit products. To receive a location accuracy in the order of one pixel the precise DORIS orbit must be available for EGEO. Other orbit products will entail severe location errors. Fig. 2 shows an actual example of a mosaic from the Darfur region in Sudan. Three standard EEC products were mosaiked in an operational way. No manual interactions were carried out to generate this image. The mosaicing procedure uses the annotations in the sunraster footer of the Geocoded image. Fig. 2: Mosaic of EEC products from Sudan Alternating Polarization Mode The alternating polarization mode is geocoded with the same approach, which is used for the IMS datasets [4]. In this case, the procedure starts with the setup of input, DEM and geocoding parameters, as the second polarization is geocoded. Using the identical geocoding parameter the second polarization is corrected. Both polarization exist as individual sun raster file, but have an identical geometric model and co-register to each other. Fig. 3 shows an example in the northern alps. Both products of the AP-Mode differs only in the radiometry (Lake Starnberg top border), the geometry is identical. 2.4 Specific Product Service for ENVISAT One important feature and also a challenge for the processing of ENIVISAT are the number of modes. Each mode needs some specific processing modules. In general each swath type of ENVISAT can be handled from EGEO. Fig. 3: Left Geocoded Alternating mode products Sensor Combination Service EGEO is capable of geocoding in addition to ENVISAT s multiple modes also SRTM, ERS and RADARSAT data. Only a few sensor specific adaptations were implemented. Each SAR product has the same geometric model and parameter and the product format is identical. As a result a combination of various

4 data sources is possible. Fig. 2 shows the information from SRTM (red) and two different ENVISAT (green, blue) datasets. Both sensors deliver different information dependent on wavelength and acquisition time. The Global Monitoring mode can process in the same quality as the input GM is processed. A standard EEC can be processed. Fig. 5 represents a geocoded product from this mode. The data were acquired over Algeria and Libya. Fig. 4: SRTM and ENVISAT geocoded IM product from Flevopolder 3 GEOCODED PRODUCTS LOW RESOLUTION In addition to the high-resolution imaging mode, the ASAR can operate in a Wide Swath mode. One wide swath scene can cover an area of 400 km x km. The scene size is limited to an area of 400 x 4000 km. This very different systems compared to the imaging mode requires also a very special SAR processing and geocoding approach, e.g.. The ground-to slant range conversion polynomial is not represented by one parameter set. It is updated depending on the track length several times. This effect is not taken into account in standard geocoding systems [2]. They use only one range polynomial update; during the standard geocoding a mis- location of several km appears. The new updated EGEO implementation reduces this effect and produces precise geocoded images, which have already been used in the SIBIRIA project [8], [11]. Based upon the standard EEC, a calibration can be carried out by the customer. Fig. 3 is an example from the SIBIRIA project in the area of Krasnojarsk. Fig. 3: IMM product for SIBIRIA project Fig. 5: Geocoded product Global Mode 4 SERVICE WORKFLOW HOW TO ORDER The DFD offers a service for geocoding of ENVISAT ASAR data. The customer interface is the web portal of the DFD : EOWEB. This is very similar to ESA webportal EOLI. Also a direct contact with the author is possible. The customer has to identify the product type and area. In the second step he has to contact the helpdesk of the DFD. Depending on the project requirements DFD will generate and offer. The DLR has not the right to use ENVISAT data fort free. The customer has to buy the input products and to make them available. But it is possible, that DFD will assist the customer, e.g. data inventory, product selection or ordering. A few days with much interaction are necessary for this preparation. After completing this preparation phase the processing can start. This and the input product delivery will be is the most time consuming work. But in special cases like products for the International Charter "Space and Major Disasters" products can

5 be generated in less than one day from the inventory the delivery. This was done during the Charta Call 68 Dafur-Sudan. Using standard products from Landsat TM and ENVISAT EGEO processing, a combination with the image processing software ERDAS IMAGINE was produced. Fig. 6 is a direct snapshot of an interactive overlay from an Landsat TM and an EEC product from Darfur, Sudan. Only the cartographic reference information from the Landsat geocoding and the sun raster file footer information are used with the ERDAS IMAGINE software. Inside the viewer a swipe operation was applied. The datasets co-register well to each other. Starting with these products, all following interpretation results are in the same geometry and can be used e.g. for generating maps. SIBIRIA II and the Charter Call shows the quality and very fast access to this service. EGEO is being updated and adapted to produce geocoded products from RADARSAT. Also the products are compatible to amplitude images from SRTM. Therefore an easy combination of all these different information sources can support the customer in his projects. 6 REFERENCES 1. ESA, ENVISAT PAYLOAD DATA SEGMENT, PO- RS-MDA-GS-2009, Rev. C, Huber M., Hummelbrunner W., Raggam J., Small D., Kosmann D., Technical Aspects of Envisat ASAR Geocoding Capability at DLR, ESA, Salzburg Kosmann D., Bollner M., Roth A., Benjes A.,Value Added Geocoded SAR Products from the German ENVISAT PAC, Proceedings ESA ENVISAT Symposium, Gothenburg, Sweden, Pasquali, P., Small D., Holecz F., Meier E., Nüesch D., First results of the ESA project on the ENVISAT ASAR Alternate Polarization Mode, CEOS SAR Workshop, Nordwijk 1998, pp Fig. 6: Geocoding Service for the International Charter "Space and Major Disasters" 5 CONCLUSION The Envisat GEOcoding system EGEO is capable of using ENVISAT ASAR modes to generate geocoded products. As input standard level 1b from the various Processing and Archiving Centers can be used. Two major types of quality products are available: Enhanced Ellipsoid Corrected EEC Enhanced Terrain Corrected ETC Precise products (IM and APP), medium resolution (IMM and APM) from the WS-mode (WSM) and overview images from the GM-mode(GM1) are available. This service is offered by the German Remote Sensing Data Center. A limitation to only small parts of the globe as for the well-known Geocoded Terrain Corrected (GTC) products from ERS no longer exists. Using DEMs from the SRTM data set, a worldwide availability in different quality standards is guaranteed. Projects like 5. Rabus B., Eineder M., Roth A., Bamler R., The shuttle radar topography mission - a new class of digital elevation models acquired by spaceborne radar. In: ISPRS Journal of Photogrammetry & Remote Sensing, 57 (4), pp , Roth A., et al. GeMoS A System for the Geocoding and Mosaicking of Interferometric Digital Elevation Models, Proceedings IGARSS 99, Hamburg, Roth A., Knöpfle W., Strunz G., Lehner M., Reinartz P., Towards a Global Elevation Product: Combination of Multi-Source Digital Elevation Models. In: Proc. of Joint International Symposium on Geospatial Theory, Processing and Applications, Ottawa, Canada, Santoro M., Schmullius C., et al., The SIBERIA -II Project as seen by Envisat ASAR. In: Proc. CEOS SAR Calibration Workshop, Ulm, Germany, May 27-28, (in press), Schreier, G., Geometrical Properties of SAR Images. In: Schreier G. (Ed.): SAR Geocoding and Systems, Wichmann, Karlsruhe, Small D., Rosich B., Meier E., Nüesch D., Geometric Calibration and Validation of ASAR Im-

6 agery. In Proc. CEOS SAR Calibration Workshop, Ulm, Germany, May 27-28, (in press), Voigt S., Petrocchi A., Huber M., Roth A., Processing of ENVISAT ASAR Wide Swath Imagery for the derivation of Biophysical parameters of Sibirian forest a contribution of DLR to the SIBIRIA II project, ESA, Salzburg 2004