Ground Station Network for Payload Data Reception of German TanDEM-X Mission

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1 SpaceOps 2010 Conference<br><b><i>Delivering on the Dream</b></i><br><i>Hosted by NASA Mars April 2010, Huntsville, Alabama AIAA Ground Station Network for Payload Data Reception of German TanDEM-X Mission Erhard Diedrich 1, Norbert Bauer 2, Robert Metzig 3 and Max Schwinger 4 German Aerospace Center DLR, Oberpfaffenhofen, Wessling, 82334, Germany The TanDEM-X mission (TerraSAR-X add-on for Digital Elevation Measurement) comprises two nearly identical earth observation satellites, TerraSAR-X and TanDEM-X. Both are equipped with a Synthetic Aperture Radar (SAR) system. To cover the entire globe, three years of parallel operation will be required, with the satellites flying in very close formation at a distance of a few hundred meters. This mission concept leads to a challenge for the communications with respect to the payload data reception and the overall payload data handling. The main points are: due to the close distance the two satellites cannot be geometrically resolved with reasonable sized ground stations, both satellites transmit in the same frequency range in X-band, the transmission rate for the payload data is 300 Mbps being significantly lower than the instrument data rates. Furthermore the data takes have to be split in order to make full use of contact time per station and to free onboard memory. In addition multiple switch over of transmission from one to the other satellite during one pass will be necessary, while at the same time a near real time quality feed back of the radar data is necessary in order to assess and to keep the over all acquisition plan. This can only be solved with manageable effort by using a station network with a minimum of at least two polar stations. The result was to use the DLR ground stations GARS O Higgins in Antarctica, in Inuvik in Canada, in Chetumal in Mexico, in Neustrelitz in Germany and the partner ground station of Swedisch Space Corporation SSC in Kiruna. This paper describes in detail the overall concept of the cooperative ground station network and the data communication. Nomenclature TerraSAR-X = German radar mission in public-private partnership of German Aerospace Center DLR and EADS Astrium GmbH TanDEM-X = German radar mission comprising the TerraSAR-X satellite and a second nearly identical satellite for formation flight of both space craft (same public-private partnership as TerraSAR-X) TSX / TDX = Short acronyms of the two satellites I. Introduction HE TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) Mission has the primary objective of T generating a consistent, global DEM with an unprecedented accuracy according to the HRTI-3 (High Resolution Terrain Information) specifications. Beyond that, TanDEM-X provides a configurable SAR interferometry platform for demonstrating new SAR techniques and applications. The TanDEM-X formation is build by adding a second spacecraft (TDX) to TerraSAR-X (TSX) and flying the two satellites in a closely controlled HELIX formation. Together TSX and TDX build a combined space infrastructure serving both the TerraSAR-X and TanDEM-X missions simultaneously. 1 Head of Division International Ground Segment, DFD, Oberpfaffenhofen, Wessling, Germany. 2 Team Lead, Division International Ground Segment, DFD, Oberpfaffenhofen, Wessling, Germany 3 System Engineer, Division International Ground Segment, DFD, Oberpfaffenhofen, Wessling, Germany 4 System Engineer, International Ground Segment, DFD, Oberpfaffenhofen, Wessling, Germany 1 Copyright 2010 by German Aerospace Center DLR. Published by the, Inc., with permission.

2 The SAR instruments have capability of different imaging modes like Spotlight for high resolution, Stripmap or ScanSAR for covering large areas. The two satellites may be operated in different modes: As completely separated space craft in constellation or in the so called bi-static operations mode requiring the close formation flight. In the latter one, one of the two satellites will emit radar signals while the backscatter from the earth's surface will be received by both. To cover the entire globe, a minimum of two years with certain extensions and acquisitions for processing pilot areas better than HRTI-3 quality, three years of this bi-static parallel operation will be required, with the satellites flying in very close formation at a distance of a few hundred meters. The instrument imaging mode in use for producing the digital elevation model is the Stripmap mode. The following characteristics of the mission provides the scope of the payload data downlink strategy: A. Impact on payload data handling from data acquisition On top of the underlying imaging modes and the mode of cooperative operation of the two satellites also the imaging geometry and the dimensions of the HELIX formation have to be adjusted for the TanDEM-X specific applications. For DEM generation the HELIX dimensions defining the baselines for interferometric processing have to be increased with increasing incidence angle of the illuminated swath. Because of fuel limitations, the baselines can only be adjusted in a monotonically increasing manner over a year of global acquisition period. Therefore, in certain periods throughout this year only those swath having adequate HELIX geometry will be acquired and the global acquisition is slowly filling up over the year. On the other hand, if one acquisition fails it has to be re-planned as quickly as possible (typically within two to three 11-day repeat cycles) to ensure still adequate HELIX geometry. It is therefore crucial to receive timely feedback of the acquisition status from any ground station and in addition a near real time quality feed back of the radar data is necessary in order to assess and to keep the over all acquisition plan. B. Constraint of downlink technology The data reception concept has to be compatible to the technical and operational constraints given by the TanDEM- X mission and the technical design of the spacecrafts. The technical design of the communication channel for the transmission of the SAR data is identical at both spacecrafts. The relevant properties of the data downlink properties of the satellites are: The TanDEM-X mission was designed at the time when the TerraSAR-X satellite was already built, thus no changes were possible and the TanDEM-X downlink technology had to follow the TerraSAR-X satellite for reasons of compatibility Transmission frequency and bandwidth of payload data downlink are in the X-band channel with a data rate of 300 Mbps Low-gain hemispherical transmission antenna design with low cross polarization isolation Structure of the data transmission protocol is based on the CCSDS standard. However it is assumed that both s/c provide different s/c-identifiers Limited SSM on-board storage capacity with 256(TSX)/512(TDX) Gbit capacity Furthermore both spacecrafts fly in a very close formation flight with only some 100m distance Because of the summary of these facts the satellites cannot transmit at the same time, because they would disturb the signal of each other. Even more than one antenna at the same location are not resolving this constraint. Thus the main facts for the payload data reception are summarized to be: due to the close distance the two satellites cannot be geometrically resolved with reasonable sized ground stations, both satellites are transmitting in the same frequency range in X-band, the transmission rate for the payload data is 300 Mbps being significantly lower than the instrument data rates. Also a separation on basis of the polarization is not possible due to the properties of the downlink antenna at the space crafts. C. Data volume and corresponding requirement for payload data downlink The requirements analysis for the downlink for the TanDEM-X mission has to consider the data rate of the instrument, the possible orbit usage and of course the overall acquisition strategy. The result shows up with a rather high overall data volume to be transmitted from the space crafts to ground segment of about 300 up to 350 GByte per day for the TanDEM-X mission. This is resulting in a necessary contact time of ground stations of about s per day. It is important to note here, that the two space crafts have also to serve further the goals of the TerraSAR-X mission for commercial and scientific purposes by performing data acquisition according to acquisition orders of the commercial and scientific users. This might also comprise high priority orders or orders for disaster management 2

3 support. The overall data volume from those orders may also sum up to about 100 to 150 GByte per day in addition to the amount mentioned for the TanDEM-X mission. For the TanDEM-X mission the it is important to take the unequal distribution of the land mass of the earth into account, which leads to the fact, that the onboard memory of the space craft is not filled equably in time. Therefore the downlink concept has to foresee the handling of peak load. Furthermore the data takes have to be split in order to make full use of contact time per station and to free onboard memory. In addition for balancing the usage of the different sized onboard memories of the two satellites multiple switch over of transmission from one to the other satellite during one pass will be necessary. II. Design of The Ground Station Network for TanDEM-X For the reception of TanDEM-X relevant SAR data of the TSX satellite it is inevitable to design a network of ground-stations in order to fulfill the down-link requirements in contact time and data volume. Due to operational and financial reasons as first step a concentration of this network onto the TanDEM-X mission related SAR data reception of both satellites was undertaken and at the same status the DLR ground station capabilities were used as a starting point (see figure 1). Figure 1. DFD Ground Station Network. Station locations foreseen for possible reuse for TanDEM-X mission. 3

4 For the full analysis it is clear that two satellites serving two missions needs a harmonized solution for the overall downlink concept. Therefore the TerraSAR-X downlink concept is recalled shortly: the main station for downlink of TerraSAR-X payload data is located in Neustrelitz in Germany as part of the ground stations of the German Remote Sensing Data Center DFD. Then there are several Direct Access Stations in use as part of the commercial infrastructure. For additional support of scientific purposes or a background mission the German Antarctic Receiving Station GARS at O Higgins (station of DLR / DFD) is also in use. Especially the Neustrelitz station but also partly the Direct Access Station may not be blocked out by TanDEM-X downlinks. Due to these operational constraints for the network of ground-stations, the main guidelines for the design have to be The TanDEM-X station network must not constrain TerraSAR-X user order related data downlink Sufficient geographical separation to maximize the use of the contact times (avoid overlaps) Allow the completion of the sensing of wide areas of landmass before transmitting to ground and allow for handling of peak load situations caused by overall data acquisition plan Provide good technical and communication infrastructure and operational availability. Allow for a mean of minimum s contact time per day Avoid blind orbits completely and try to achieve approximately 2 contacts per orbit. Reuse existing stations of German Remote Sensing Data Center as far as possible (see figure 1). In order to achieve this, the ground station network can be designed as Polar network with high spacing in latitude (north and south polar locations) and longitude separation (at least 120 latitudinal spacing). The advantage of a polar network for this mission is given by its orbital characteristics and a high number of visible passes and long contact period, as well as the completion of sensing over the majority of the landmasses. At least 2 to 3 ground-stations are necessary to fulfill the requirements, depending on the location. Equatorial network with a distribution of at least 6 or 7 stations at lower latitudes. Advantages of this type of network on the TanDEM-X mission are very limited and can only be found in the low daily workload which can normally be easily integrated into existing station schedules. A configuration of 6 to 7 equally spaced ground stations around the equator is presently not available Or by a combination of several higher and lower latitude stations. For further analysis the advantages of the polar network are so evident that only this configuration was pursued. An analysis of existing facilities was performed. At the beginning of this analysis the Inuvik Satellite Station Facility, as a result of an international cooperation, was not yet existing and of course also not a DLR station at this place. A location in Alaska was considered, but the overall analysis including operational and financial aspects triggered the development of Inuvik very much and now the station is available and is used for the further analysis. There were a number of model computations performed evaluating the contact times and there distribution in time. Beyond this classical contact time calculation also analysis was performed inkling a pre-calculated data acquisition plan. The result is a compromise of optimal placed downlink stations and existing infrastructure: this is, to use the DLR ground stations GARS O Higgins in Antarctica, in Inuvik in Canada, in Chetumal in Mexico, the latter one covers one type of the peak load situations during the TSX/TDX orbit cycle, and in addition the partner ground station of Swedish Space Corporation SSC in Kiruna is also used whereas the DLR station in Neustrelitz is always used with priority for the TerraSAR-X mission data (see figure 2). 4

5 Figure 2. The TanDEM-X Payload Ground Station Network. This network utilizes the stations GARS O Higgins, DLR station in Inuvik, Kiruna, DLR Chetumal station and Neustrelitz as far as the TerraSAR-X priority in Neustrelitz allows for. III. Data Handling at the Ground Stations and Security Requirements Two main constrains, besides security thoughts, led the design of ground station data handling: The first one evolves from the lengthy data takes, which are needed for the TanDEM-X mission (better for creating a consistent DEM). To downlink these huge data takes they have to be cut to smaller data packages. To support a as-fast-as-possible downlink concept one data take will be dumped to several ground stations. As there are two data takes for a cooperative TanDEM-X acquisition the data for one acquisition can be split over several ground stations. This leads to a quite complex data assembly scenario in the processing center located at Oberpfaffenhofen / Germany, which is implemented in using the inventory functionality of the archive, i.e. of the so called product library. The second constraint is the need for a fast quality feedback. As the HELIX formation of the two satellites TanDEM-X and TerraSAR-X constantly changes the baseline between the both of them during the mission time and every baseline is useable for different latitudes a failed acquisition has to be repeated as fast as possible. To trigger a reacquisition a fast quality feedback is necessary. To create a quality evaluation information about a whole acquisition is needed. These two constraints and the fact, that not all the ground stations have a fast enough network connection for immediate transfer of the full data to our processing facility in Oberpfaffenhofen / Germany lead to the following solution: Figure 3. Action Diagram: Data handling at ground station This diagram doesn t contain the actions which have to be performed if a data take is security sensitive. 5

6 Each segment of every data take has to be decrypted and quality information has to be extracted. This process is called parameter screening and the result screening annotation. The quality annotations are small enough to be transferred even with the lowest bandwidth connection in our ground station network. The raw data is transferred to the ingestion system and archive location Oberpfaffenhofen separately on magnetic tape. The quality annotations are transferred online to Oberpfaffenhofen. An Interferrometric Quality PreCheck (IQPC) is performed after all quality annotations of all segments of all data takes of a TanDEM-X acquisition are available at Oberpfaffenhofen. The IQPC gives rough quality information. This information is enough to trigger a reacquisition if needed. Figure 4. Sensitive data handling. Data workflow form groundstation to archive with focus on data sensitivity This workflow would respond to all needs except for the security requirements created by the SatDSiG, the German satellite data security law. Satellite data of a certain quality must not be transferred decrypted or be decrypted without special permission. The TanDEM-X mission creates SAR-data which can sometimes reach a quality, where a clearance for decryption is needed. The decision, weather a data take can be decrypted or not, has to be made during specification of the data takeorder. Together with other downlink-related information the clearance for decryption is delivered for each downlink to the concerned ground station. Also every security sensitive data take is encrypted with a different key, which is not distributed to the ground station. If a data take is dumped to a ground station and that data take is marked as security sensitive the data take will not be decrypted and as a result no IQPC will be performed for the acquisition. IV. The overall Data Workflow To get a rough understanding of the TanDEM-X mission s data workflow figure 5 provides an overview of the main steps performed during the mission. As described in the previous paragraph the actions performed at the ground stations are the reception of data, frame synchronization the data, if allowed decrypting and screening the data and finally delivering the quality annotation data online via sftp (quality annotations) as well as offline encrypted primary data on a magnetic tape (SAR Raw data) to the processing center located in Oberpfaffenhofen. The quality annotation data delivered online to the processing center are already used to create the product. This means that the later L0 product is logically ingested in the so called Product Library (DLR name for the unification of inventory and physical archive). The Product Library than already has the principal product structure for each data take and all necessary meta data even if the primary SAR data are still on their way. 6

7 At the processing center the TanDEM-X Ingestion System collects the arriving primary data, decrypts it and creates complete products from the segments and triggers processing of these complete products. Data, which was found security sensitive and has not been transcribed and screened at the ground station is handled in Oberpfaffenhofen as it would have been at the ground station, if the data weren t sensitive. After this procedure it is handed over to the Ingestion System and afterwards handled the same way, as the insensitive data. Space Segment TSX1 and TDX1 Satellite TanDEM Rx Station (n) TanDEM Rx Station (2) TanDEM Rx Station (1) TanDEM PGS - Processing Center Transcription System Decryptor SAR Screener Online/Offline Data Ingestion Decrypted, formatted DataTakes RAW DEMs Global HRTI 3 DEM Data Reception Direct Archive System Encrypted RAW DataTakes or DataTake segments InSAR processor Phase Unwrapper Geocoder DEM generator Figure 5. Overall Workflow. Data workflow throughout the TanDEM-X mission Mosaicking Processor Block Adjustment Mosaicking DEM Editing The quality annotations are used for a fast quality pre-check. After all data takes of a TanDEM-X acquisition are completely available the data is processed by the Integrated TanDEM-X Processor, which generates several so called Raw-DEMs from each acquisition. After enough RawDEMs of an area are available, this area is processed by the Mosaicking processor which adjusts the RawDEMs and generates a mosaic from them. V. Ground Station Network Operations The overall procedure includes the Mission Operations Segment (MOS), in case of TerraSAR-X / Tandem-X satellites implemented by the German Space Operations Center in Oberpfaffenhofen as well as the Payload Ground Segment responsible for payload data reception and the payload data handling. The procedure starts with the registration of a station within MOS. Then, via the ground station capability info, a station informs about the location and the characteristics of the X-Band receiving antennas to be used for the TSX/TDX payload data reception. The next exchange of information concerning individual downlinks is organized by the station availability info through which a station informs about its readiness to support specific TSX/TDX payload data dumps. In turn the MOS issues then by the downlink info through which a station is informed about the scheduled TSX and / or TDX downlinks including replay and data take information. The TSX / TDX orbit data message, the two-line element set, provides the satellite orbit information which enables a correct pointing of the antenna in the receiving station. The key info transports the decryption key information through which a station is enabled to perform the payload data decryption. At the same time the ground station network is centrally coordinated from Oberpfaffenhofen. This is performed of course from management point of view, but also on the technical level. The technical instance is realized with the so called Station Monitoring, Control and Scheduling System SMCS. This system is located centrally in Oberpfaffenhofen but clones of the system are installed at the stations (of course in reduced functionality for the partner station Kiruna). The interfaces are designed such, that a operational redundancy for communicating the interface items is realized. A schematic overview is given in figure 6. The overall workflow for exchanging the interface information is operationally automated and incorporated in the Station Monitoring, Control and Scheduling System SMCS. The system has a number of modules grouping the different functionalities for scheduling the station consistent with the needs of other missions according to priority rules, for remote monitoring of station status and accepting station reception reports, for organizing the above described planning workflow and as far as applicable for remote control of the station. 7

8 Figure 6. Station Monitoring, Control and Scheduling System. Station operations scenario for the TanDEM- X mission The ground station network and the interaction with the SMCS are shown schematically in figure 7. Figure 7. Station Monitoring, Control and Scheduling System. Overall station Network Scheme and SMCS Interfaces. 8

9 VI. Conclusion This following conclusion may be drawn from the analysis and the station network set up: Sufficient downlink capacity is ensured with baseline station GARS O Higgins DLR, ESRANGE Kiruna SSC, Inuvik DLR and ERIS Chetumal DLR Robustness is achieved with flexibility and backup Technical status at time of SpaceOps 2010 is readiness for launch of the satellite, all stations are in operational use Upon launch the centrally coordinated station network will be operational for the TanDEM-X mission. Acknowledgments The authors like to thank the whole TanDEM-X mission design and ground segment team of DLR and EADS Astrium GmbH for the team spirit, the numerous iterations and reviews of the concept and the overall support and patience with the work package Ground Station Network of the project. References Proceedings The TanDEM-X Mission Design and Data Acquisition Plan.; Fiedler, Hauke; Krieger, Gerhard; Werner, Marian; Reiniger, Klaus; Eineder, Michael; D'Amico, Simone; Diedrich, Erhard; Wickler, Martin (2006): In: VDE [Hrsg.]: Proceedings of European Conference on Synthetic Aperture Radar (EUSAR), S. 4, European Conference on Synthetic Aperture Radar (EUSAR), Dresden, Germany, A Description of the Data-Driven SAR Data Workflow in the TerraSAR-X Payload Ground Segment. ; Schättler, B.; Wolfmüller, M.; Reissig R.; Damerow, H.; Breit, H.; Diedrich, E. (2004): In: Proceedings of IGARSS'04 IGARSS, Anchorage, Alaska, USA, September