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Terrasar-x Sar Data Processing Results from Commissioning and Early Operational Phase
| Content Provider | Semantic Scholar |
|---|---|
| Author | Jürgens, Carsten |
| Copyright Year | 2008 |
| Abstract | TerraSAR-X, the first national German radar satellite, was launched June 15, 2007. SAR data taking and image generation started as early as four days after launch [i]. The commissioning phase was completed in December 2007 with the basic product release for StripMap and SpotLight mode, ScanSAR product release will follow in February 2008. Since beginning of 2008, basic products from all modes are provided to the scientific and commercial user community. Due to a comprehensive system validation testing performed pre-launch on ground including both, the ground and space segment, the TerraSAR-X mission experienced an extraordinary smooth transition into the commissioning phase. Thousands of data takes were acquired and provided to the verification team already in the first months. All level 1b user products are generated with the high-precision phase-preserving TerraSAR MultiMode SAR processor TMSP developed by the Remote Sensing Technology Institute of DLR (using a geocoding plug-in component provided by the German Remote Sensing Data Center). This does not hold for the ground segment itself, but also exclusively for the commercial direct access stations. Focus of its commissioning phase check-out was its fitness to deal with TerraSAR-X in-orbit SAR data characteristics which could not be assessed pre-launch on ground, namely the signal Doppler behaviour, calibration pulse processing influences and the resulting range and azimuth focusing quality in low and high contrast scenes. With the help of a detailed Doppler analysis, the correct working of the Total Zero Doppler Steering law applied during attitude control was shown. An observed beam mispointing with respect to the spacecraft attitude reference system was corrected based on Doppler deduced correction values. A major commissioning phase activity was the qualification and verification of the various SAR basic products for the science and commercial users, a challenging task considering their multitude in terms of complex and detected product types from the various imaging and polarization modes which span a wide range of incidence angles. Specific care was given to their geometrical accuracy and phase-preserving characteristics which promise to support new exciting applications using TerraSAR-X. The suitability of high-resolution TerraSAR-X data for interferometric processing was shown already in July 2007 as soon as the first repeat-pass data takes were available. Not only standard StripMap, but also SpotLight interferometry – a novelty in spaceborne SAR imaging was demonstrated using the GENESIS interferometry system developed by and operated at DLR’s Remote Sensing Technology Institute. An example over an urban area is presented. TERRASAR-X IMAGING MODES AND BASIC PRODUCT PORTFOLIO The unique flexibility in instrument commanding in combination with the azimuth steering capability of the phased array antenna allows the data taking in a variety of different imaging modes. Baseline for the standard SAR product generation are • StripMap configuration SM Carsten Jürgens (Ed.): Remote Sensing New Challenges of High Resolution, Bochum 2008 ISBN 978-3-925143-79-3 227 • four-beam ScanSAR configuration SC • two sliding SpotLight configurations SL and HS. The full performance incidence angle range covers 20o to 45o for SM and SC, 20o to 55o for SL and HS. Figure 1 : TerraSAR-X Imaging Modes Sketch In addition to single polarization, data from SM, SL and HS are offered as dual polarization variants, but at cost of a double-sized azimuth resolution and a halved swath with in SM. The basic products offered from the nominal imaging and polarization modes are specified in [ii] and consist of a • single-look phase-preserving complex image SSC and the derived detected multi-look variants • MGD in ground range projection • GEC in UTM (UPS) projection and ellipsoid corrected using an average scene height • EEC in UTM (UPS) projection and ellipsoid corrected using a digital elevation model The multi-look variants are offered in two flavours, the spatially enhanced SE and the radiometrically enhanced RE (SC products as RE variant only). Whereas the SE variants are essentially kept at their best quadratic resolution, a constant equivalent number of looks is chosen for the RE variants at the cost of reduced spatial but higher radiometric resolution and a considerable reduction in file size. Detected image data are presented in GeoTIFF format, complex data in the DLR proprietary CoSAR format, parameter annotation is structured XML and thus human readable. Specific care is taken to provide the user with appropriate information to convert between instrument (range and azimuth) times and geographic location on ground. In addition to basic products from the standard HS mode with its 150 MHz acquisition bandwidth, experimental products stemming from HS acquisitions with 300 MHz in single polarization mode were successfully qualified and characterized in the commissioning phase and are made available as well to the end user (who may use a specific order option). PAYLOAD GROUND SEGMENT OVERVIEW A central element in the TerraSAR-X ground segment is the payload ground segment PGS [iii] performing the reception of the SAR payload data, their processing and archiving and the distribution of the generated SAR basic products to users. Carsten Jürgens (Ed.): Remote Sensing New Challenges of High Resolution, Bochum 2008 ISBN 978-3-925143-79-3 228 Figure 2 : Schematic Payload Ground Segment Workflow Overview (courtesy: W. Balzer) The user places a TerraSAR-X basic product order via EOWEB into the system. This order is further processed by the PGS ordering and production control and submitted to the mission planning entity from the mission operations segment MOS. Following successful planning, commanding and acquisition of the data take, the data are down-linked to DLR’s ground station in Neustrelitz where they are decrypted and processed into L0 archival products by the Processing System TMSP. This process is data-driven and thus stimulated by data arrival only, i.e. the downlink followed later by orbit and attitude data provided off-line from MOS Flight Dynamics. The request for the L1b product to be generated is sent by the production control to the Processing TMSP as soon as commanding of a data take is confirmed by mission planning and thus well in advance of the downlink itself. The L1b product is generated by the TMSP, transferred into the product library (short-term archiving only) and delivered from there to the user. Both L0 and L1b processing use SAR instrument and calibration information from the auxiliary product provided by the instrument and calibration segment IOCS. TERRASAR-X MULTI-MODE SAR PROCESSOR TMSP Heart of the PGS SAR Data Workflow is the TerraSAR-X Multi-Mode SAR processor TMSP [iv], an in-house development of DLR’s Remote Sensing Technology Institute (geocoding plug-in by German Remote Sensing Data Center). It’s most prominent challenge is the consistent generation of phase-preserving products from all different imaging and polarization modes at varying incidence angles with a considerable throughput. Specifically the operational space-borne SpotLight constellation represents a novelty in that sense. Whereas many conventional SAR processors are built and optimized for one of the imaging modes StripMap, ScanSAR and SpotLight, the TMSP uses a “one fits all” approach, i.e. the core SAR correlator module provides phase-preserving single-look slant-range complex data sets for all imaging modes. This is achieved through a hybrid focusing kernel based on the chirp-scaling algorithm variants as developed at DLR [v], [vi]. Derivation of multi-look detected products is consistently based on SSCs as an interim production stage. In a “one fits all” sense, the TMSP is also used exclusively at the direct access stations of the commercial service segment. This will ensure that the same high basic product quality standards as set by the ground segment are met throughout the mission life time. PRE-LAUNCH ON-GROUND TESTING AND TRANSITION INTO COMMISSIONING PHASE Pre-launch integration testing of the PGS SAR data workflow was a challenging task, since no TerraSAR-X like data were available from other space-borne sensors, particularly due to the SpotLight technique with its high spatial resolution. Furthermore, the system had to cope with a number of Carsten Jürgens (Ed.): Remote Sensing New Challenges of High Resolution, Bochum 2008 ISBN 978-3-925143-79-3 229 TerraSAR-X specific downlinks scenarios like partial replays and data encryption. In collaboration with the space segment supplier Astrium and the ground segment instrument operations team, test data from the TerraSAR-X instrument itself in various presentations (instrument source packets, transfer frames and raw binary stream) were recorded and used both for subsystem integration and verification as well as validation tests spanning the whole PGS SAR data workflow up to the delivery of L1b products [vii]. In the end, a ground validation test starting with user orders and ending with the delivery of the generated L1b product was conducted. As far as we know, this type of end-to-end validation testing including not only real-time spacecraft operation from the mission control room, but also the real-time payload data acquisition, recording and specifically further processing by the payload ground segment (interrupted only by the time needed to physically transport the data on portable disk from spacecraft to the Neustrelitz ground station) is a novel approach in ground segment validation testing. The vast exercising of the operational workflows needed in the commissioning phase enabled the successful switch-on of the ground segment directly following the launch with the activation of the first thousand order requests from the verification team. The first imaging data take was acquired on mission day 5 at 15:03:19 UTC and down-linked |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://www.earsel.org/workshops/HighRes2008/FullPapers.pdf |
| Alternate Webpage(s) | https://elib.dlr.de/66026/1/29_Schaettler.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
