TSX (TerraSAR-X) Mission
TerraSAR-X1 (also referred to as TSX or TSX-1) is a German SAR satellite mission for scientific and commercial applications (national project). The project is supported by BMBF (German Ministry of Education and Science) and managed by DLR (German Aerospace Center). In 2002, EADS Astrium GmbH was awarded a contract to implement the X-band TerraSAR satellite (TerraSAR-X) on the basis of a public-private partnership agreement (PPP). In this arrangement, EADS Astrium funded part of the implementation cost of the TerraSAR-X system. In exchange, EADS Astrium/Infoterra received the exclusive commercial exploitation rights for the TerraSAR-X data. The satellite is owned and operated by DLR, and the scientific data rights remain with DLR. The satellite has a design life of at least five years. TerraSAR-X is of SIR-C/X-SAR (1994) and SRTM (2000) heritage - DLR SAR instruments flown on Shuttle missions. 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11)
The science objectives are to make multi-mode and high-resolution X-band data available for a wide spectrum of scientific applications in such fields as: hydrology, geology, climatology, oceanography, environmental and disaster monitoring, and cartography (DEM generation) making use of interferometry and stereometry. The science potential of the mission is given by:
• The high geometric and radiometric resolution (experimental 300 MHz Mode for very high range resolution)
• The single, dual and quad polarization mode capability
• The capability of multi-temporal imaging
• The capability of repeat-pass interferometry
• The capability of ATI (Along-Track Interferometry)
The business goal in this venture is to establish a commercial EO (Earth Observation) market by Infoterra on a sustainable service concept to its customer base. Infoterra, a subsidiary of EADS Astrium, is comprised of Infoterra Ltd. in Farnborough, UK, and Infoterra GmbH in Friedrichshafen, Germany (a subsidiary of EADS Astrium GmbH). Infoterra has established a global distribution network with a range of service options for its customers. A commercial goal is also to provide monitoring services for European initiative GMES (Global Monitoring for Environment and Security). 12) 13) 14)
Figure 1: Overview of the SAR program roadmap in 2012 (image credit: DLR, Astrium GEO-Information Services) 15)
Figure 2: Alternate view of the SAR development line — German radar missions (image credit: DLR) 16)
The TerraSAR X-band satellite, built by Airbus Defence and Space Geo-Intelligence/Infoterra GmbH (formerly EADS Astrium GmbH, Friedrichshafen, Germany) employs a mission-tailored AstroSat-1000 bus (successor of Flexbus and LEOSTAR due to industrial merger - initially AstroSat-1000 was referred to as AstroBus) concept with a heritage of CHAMP and GRACE missions. The hexagonal outer shape of the spacecraft, with a total height of about 5 m and a diameter of about 2.4 m, is mainly driven by the accommodation of the SAR instrument, the body mounted solar array, and the geometrical limitations given by the Dnepr-1 launcher fairing. The S/C bus design features a central hexagonal CFRP structure as the main load carrying element. The cross-sectional view of Figure 6 illustrates the mounting concept of radiator, solar array, and SAR antenna elements. Three sides of the hexagon are populated with electronics equipment, while the sun-facing side is additionally carrying the solar array. The SAR antenna is mounted on one of the hexagon sides, which in flight attitude points 33.8º off nadir. The other nadir looking side is reserved for the accommodation of an S-band TT&C antenna, a SAR data downlink antenna - carried by a deployable boom of 3.3 m length in order to avoid RF interferences during simultaneous radar imaging and data transmission to ground - and a Laser Retro Reflector to support precise orbit determination. The deep-space looking surface is used for the LCT (Laser Communication Terminal) and as thermal radiator. The total wet mass of the satellite is about 1230 kg.
Figure 3: Artist's view of the deployed TerraSAR-X spacecraft in orbit (image credit: DLR, EADS Astrium GmbH)
The solar array is of size 5.25 m2, triple-junction GaAs type solar cells are used providing an average orbital power of 800 W EOL. The attitude control system is based on reaction wheels for fine-pointing, with magnetorquers for desaturation, and a propulsion system also capable of attitude control in order to achieve rapid rate damping during initial acquisition.
Attitude measurement is performed with a GPS/Star Tracker system (MosaicGNSS) during nominal operation and a CESS (Coarse Earth and Sun Sensor) in safe mode situations, initial acquisition respectively (CESS is of CHAMP and GRACE heritage). A combination of IMU (Inertial measurement Unit) and a magnetometer serve to support rate measurements in all mission phases. In fine pointing mode, a pointing accuracy of 65 arcsec is achieved (3 σ). Nominal attitude control follows a novel "total zero Doppler steering" law developed by DLR. Precise orbit determination is performed with a dual-frequency GPS receiver and raw data post processing on ground, permitting for orbit restitution accuracies in the cm range. A set of high torque reaction wheels enables rapid rotation into the so-called SSL (Sun Side Looking) orientation which is used to acquire high priority imaging targets. To point the SAR antenna into the SSL direction, a roll movement of 67.6º is required which as achieved in < 180 s.
Figure 4: Flight unit box of the MosaicGNSS receiver (image credit: EADS Astrium)
The MosaicGNSS receiver of EADS Astrium represents a fully space qualified receiver that is specifically designed for high robustness and longterm use in a space environment. The receiver comprises a main electronic unit, a single L1 GPS patch antenna and an external low noise amplifier. The signal correlation is performed in software and up to eight satellites can be tracked simultaneously with the current hardware configuration. A navigation filter ensures a smooth and continuous navigation solution even under restricted GPS visibility.
The spacecraft is equipped with a monopropellant hydrazine blow-down mode propulsion system for orbit maintenance and safe mode attitude control. A propellant mass of 78 kg is considered sufficient for almost 10 years of orbit maintenance support.
Onboard data handling: The newly developed ICDE (Integrated Control and Data System Electronics) system is being used as the central component for all avionics services. The ICDE core consists of two redundant 32 bit processor modules, implementing the ATMEL ERC32SC (Embedded Real‐time computing Core ‐ 32 bit Single Chip) processor, giving it a processing performance of more than 18 MIPS and enough memory capacity to handle full AOCS and data handling software tasks, leaving sufficient margins in performance and memory capacity for future extensions and redundancy concepts. A dedicated, hot-redundant reconfiguration module provides all necessary surveillance, reconfiguration, command and telemetry functions. The ICDE modules are cross-coupled, providing a fully redundant unit.
The ICDE provides the spacecraft and payload interfaces with the following standard link protocols: MIL-1553 bus, HDLC and SpaceWire. An optional GPS receiver module with optional star sensor processing fits seamlessly into the architecture; it is capable of acquiring and independently tracking of up to eight GPS satellites and provides position, velocity and time. The ICDE uses full duplex UART (Universal Asynchronous Receiver/Transmitter) interfaces to all "intelligent" onboard equipment, except for the LCT experiment, where a MIL-STD-1553B bus is being used. The ICDE has a mass budget of 12-18 kg and a power demand of 15-30 W, depending on the configuration selected.
Figure 5: Illustration of the ICDE assembly (image credit: EADS Astrium)
The spacecraft design life is 5 years for operations with a goal of 6.5 years (de-orbiting is planned at the end of the useful life time).
Orbit: Sun-synchronous circular dawn-dusk orbit with a local time of ascending node at 18:00 hours (± 0.25 h) equatorial crossing, average altitude = 514.8 km (505-533 km), inclination = 97.44º, nominal revisit period of 11 days (167 orbits within revisit period, 15 2/11 orbits per day). The ground track repeatability is within ± 500 m per revisit period (repeat cycle). Due to its flexibility, TerraSAR-X can cover any point on Earth within a maximum of 4.5 days, 90% of the surface within 2 days.
See the TanDEM-X file for a more detailed description of the Helix orbit in tandem flight.
Spacecraft reentry (ESA requirement): At the end of its operational life the spacecraft orbit will be lowered to about 300 km (perigee) resulting eventually in enough air drag for a reentry (and a complete fragmentation and destruction of the S/C in the atmosphere).
RF communications: A standard S-band TT&C system with 360º coverage in uplink and downlink is used for satellite command reception and telemetry transmission. The uplink path is encrypted. Generated payload (SAR) data are stored onboard in a SSMM (Solid State Mass Memory) unit of 256 Gbit EOL capacity prior to transmission via the XDA (X-band Downlink Assembly) at a data rate of 300 Mbit/s. The X-band downlink is encrypted. The on-board SAR raw data are compressed using the BAQ (Block Adaptive Quantization) algorithm, a standard SAR procedure. The compression factor is selectable between 8/6, 8/4, 8/3 or 8/2 (more efficient techniques can only be applied to processed SAR imagery). Both communication links are designed according to the ESA CCSDS Packet Telemetry Standard. - The X-band antenna is mounted on a deployable boom 3.3 m in length (the only deployable item on the S/C) to prevent interference with the X-band SAR instrument. This arrangement enables for simultaneous SAR observations and X-band downlink.
In preparation of the TanDEM-X mission, where the TerraSAR-X satellite will fly in close constellation with TanDEM-X (an identical S/C) for interferometric observations, the TerraSAR-X instrument is furnished with all necessary features for PRF and synchronization between the two spacecraft. In particular, there are 6 sync horns for the omni-directional emission and reception of radar sync pulses.
Table 1: Overview of the TerraSAR-X1 spacecraft characteristics
Launch: The successful launch of TerraSAR-X took place on June 15, 2007 from the Russian Cosmodrome, Baikonur, Kazakhstan, on a Russian/Ukrainian Dnepr-1 launch vehicle with a 1.5 m long fairing extension. Launch provider: ISC Kosmotras, Moscow. - The launch, originally planned for Oct. 31, 2006, had to be shifted several times after an unsuccessful launch of a rocket of the same type in the summer of 2006. The single cause of this launch mishap was discovered and properly corrected.
Figure 6: Cutaway illustration of the TerraSAR-X S/C (view from nadir direction)
Figure 7: Functional architecture of TerraSAR-X spacecraft (image credit: EADS Astrium GmbH)
Figure 8: The TerraSAR-X spacecraft bus in the manufacturing process at EADS Astrium (image credit: EADS Astrium)
TerraSAR-X mission status:
• June 2018: The missions TSX (TerraSAR-X) and TDX (TanDEM-X jointly share the same space segment consisting of two almost identical satellites orbiting in close formation. They are operated using a common ground segment, that was originally developed for TSX and that has been extended for the TDX mission. A key issue in operating both missions jointly is the combination of the different acquisition scenarios: TSX requests are typically single scenes for individual scientific and commercial customers, whereas the global DEM as well as science products require a global mapping strategy. Thus the TSX mission goal is retained and served by both satellites. 18)
1) On-Board Resources Status: TSX and TDX have reached their nominal lifetime at the end of 2012 and 2015, respectively. Therefore it is worth to have a look at the status of the irretrievable on-board resources. Especially the propellant and the battery status are crucial factors determining the future progress and remaining duration of the mission.
- Propellant: The consumption of propellant (Hydrazine) is deter-mined by the number of maneuvers, respectively factors as aerodynamic drag, solar activity, tidal forces, space debris avoidance maneuvers, etc. In addition, the adjustment of the formation between TSX and TDX for bistatic operations consumes a large portion of propellant on the TDX satellite. Currently the propellant filling level of TSX is about 46% and 43% for TDX. This ensures an extension of the mission of three years at least.
Also the filling level of the cold gas used by the additional propulsion system on TDX (only required for close formation flight) still amounts to 10%. This allows a fine orbit adjustment for further six month approximately, whereby strategies have been developed to reduce the cold gas consumption by an increased use of hydrazine thrusters. For this reason almost no cold gas was consumed since mid of 2016. Thus the overall propellant status is currently considered as uncritical for the next years and will not impair the continuation of the mission.
- Battery: The retained TSX battery capacity is about 67% and TDX battery capacity is about 77%. According to analysis, the batteries are in excellent health – exceeding original degradation predictions. However, measures have been taken to conserve the batteries, as for example a limitation of the datatake length during the polar eclipse when the satellites are in the shadow of the Earth.
2) Radar Instrument Status: The usability of the SAR products depends strongly on the absolute calibration and stability of the processed products. The SAR instrument plays a major role in the stability of the whole chain. To ensure the stability of the instrument and to detect weak components at an ear-ly stage, special test datatakes are evaluated on a regular base comprising:
- Health checks for transmit/receive modules
- Repeated acquisitions over corner reflectors and rain forest
- Ultra Stable Oscillator frequency measurements.
Dedicated long-term system monitoring activities make use of these measurements and still confirm the high performance and the excellent stability and radiometric calibration of the SAR system.
3) Global DEM Production: After the launch in June 2010, the TDX SAR system was calibrated and thereafter a comprehensive testing of the various safety measures, the close formation was achieved mid October 2010. The operation at typical distances between 120 m and 500 m is running remarkably smooth and stable since then. Final phase, delay and baseline calibration have reached such an accuracy level, that more than 90% of all Raw DEMs (long data takes are processed to scene based DEMs of 50 by 30 km extension, called Raw DEMs) are within ±10 m compared to SRTM/ICESat data already before the final calibration step using ICESat data as reference heights. More than 500,000 Raw DEMs have been generated in a fully automated process employing multi-baseline interferometric techniques. The first and second global coverages (except Antarctica) were completed in January 2012 and March 2013, respectively. Difficult terrain (e.g. mountains, deserts) have been mapped up to 6 times under special viewing geometries. Antarctica was also mapped twice during local winter conditions. The primary data acquisition program was concluded in 2015.
The final calibration and mosaicking chain was fully operational since the end of 2013 and as of September 2016 the production of the global TanDEM-X DEM was finished. The final global DEM consisting of more than 19,000 1° by 1° tiles is well within specification. A comprehensive system has been established for continuous performance monitoring and verification, including feedback to the TDX acquisition planning for additional acquisitions. The cumulative absolute height error is with 0.9 m outstanding (excluding ice and forested areas) and one order of magnitude below the 10 m requirement.
Beyond the generation of a global TDX DEM as the primary mission goal, a dedicated science phase from 2014 to mid of 2016 aimed at demonstrating the generation of even more accurate DEMs on local scales and applications based on along-track interferometry and new SAR techniques, with focus on multistatic SAR, polarimetric SAR interferometry, digital beamforming and super resolution.
4) Global DEM Update: During the production of the DEM, it turned out that there are height differences from different acquisition periods. In particular, repeated acquisitions for scientific purposes clearly show that the Earth's surface is a very dynamic system when analyzed at this level of accuracy. Not only height changes in glaciers, permafrost regions and forests but also agricultural activities and changes in infrastructure leave clear signals in the X-band DEM.
Therefore, in 2017 the mission decided to acquire an additional complete coverage of the Earth's landmass and to provide an independent unique DEM-dataset from a well-defined time span (September 2017 until the end of 2019) to be used specifically for the assessment of temporal changes in comparison to the TDX DEM on global scales. Hence the name of the resulting product is "Change DEM".
The Change DEM will allow monitoring topographic changes on a global scale. In addition, data to provide updates for dedicated areas and for gap-filling in the global DEM will be collected as well. The data takes for this product are still conducted in bistatic operation in close formation, started in September 2017 and are expected to last until end of 2019.
The acquisition planning for the global Change DEM is based on the experience and lessons learned from the TanDEM-X global DEM acquisition. All landmasses of the Earth were separated in dedicated acquisition areas as shown in Figure 9. Each acquisition area is furthermore constrained by certain acquisition requirements in terms of season, number of coverages and desired baselines:
Figure 9: Areas to be acquired for the Change DEM with dedicated parameters (image credit: DLR)
- Glaciers (light blue) will be acquired twice during local winter in order to avoid low coherence of melted ice and snow.
- Mountains with forest (red) will be acquired twice in local summer time in order to acquire additional information for phase unwrapping where this information is too sparse in the present DEM.
- Temperate & boreal forest (dark green) will be acquired in local summer ("leaf on").
- Tropical forest (light green) will be acquired all year round.
- Deserts (yellow) will be acquired with steep incidence angles to ensure a sufficiently high signal-to-noise ratio.
- Deserts with mountains (orange) will also be acquired twice.
- Most of the polar regions (grey) were already acquired in the respective local winter seasons of 2016/2017.
- The rest of the world (brown) will be acquired once independent of the season, permafrost areas (also brown, north of 60 degree latitude) in the local winter season.
Besides that, several constraints of operational nature are considered as well. The memory on board is shared between the TSX and the TDX mission. In addition, the data take length is limited due to the degradation of the battery after ten years operation. -Also, only a reduced number of ground stations com-pared to the first global DEM acquisition are available for data downlink.
5) Global Change DEM: This Change DEM benefits from improvements in the acquisition planning process and the data processing which enables to achieve reliable DEM data of high accuracy with fewer acquisitions. For this goal, the use of an edited TanDEM-X DEM as "starting point" for the processing is mandatory.
Since the limited satellite resources and time do not allow several coverages for the majority of the landmass, the Change DEM is processed on the basis of the final TanDEM-X DEM product by a newly developed so-called "delta-phase" approach instead of the Dual-(or Multi-)Baseline-Phase-Unwrapping algorithm developed for the mission. The phase unwrapping is now based on an edited version of the global DEM to reduce the density and number of the interferometric fringes. This approach has been tested with demanding acquisition data of very low height-of-ambiguity, yielding a nearly error-free data set. It is important to note, that - although the process starts with the first global DEM - the new phase (height) values are independent of the old ones.
Areas which show no significant changes are used to pre-calibrate the individual DEM scenes (Raw-DEMs) prior to geocoding. This further reduces possible offsets and horizontal shifts in the data – facilitating final calibration and mosaicking. The absolute height accuracy which is driving the use for temporal height change detection, will be in the same order as the first Global DEM, respectively, well below 10 meters.
The lack of several coverages will affect the relative height error performance. As detailed before, a sophisticated acquisition scenario has been developed to maximize the performance. Yet, the random errors will vary slightly over the swathes (in range) since a clapboard pattern as applied in the 1st global DEM is no longer available. Nevertheless, the Change DEM will be processed to same pixel spacing as the Global DEM but with slightly more filtering (more interferometric looks and different filters) applied at the benefit of a lower random height error. Most data will have approximately the height of ambiguity values as the first global coverage (locally even better), thus the relative height error performance is expected to be comparable to the intermediate TDX DEM product (IDEM) tiles which were generated from selected 1st global coverage data only. Unlike the IDEM, the coverage of the Change DEM will be nearly complete and unaffected by larger phase-unwrapping errors. The relative height error depends on the geographical regions outlined in the previous chapter. This means the expected value is in the order of 1 to 2 meters for the majority of the mapped area and increases to about 4 meters over difficult terrain as mountains or deserts.
In summary, in June 2018 TSX will have exceeded its nominal lifetime by 5.5 years and TDX by 2.5 years. Fuel and battery status are considerably better than predicted. The radar performance and calibration of the individual satellites is still within specification or better and no indication of any degradation is noticeable at the moment.
As both satellites are still working very well and have plenty of resources left, it is planned to continue the mission beyond 2020 with the focus on selectively updating and improving the global TanDEM-X DEM and generating a global Change DEM as a self-contained product.
Figure 10: TanDEM-X Raw-DEM of an open pit mining in Wyoming, USA (left) and a three-dimensional change map with 6 m x 6 m resolution from 2016 (right), image credit: DLR
• February 9, 2018: The satellite duo, TerraSAR-X and TanDEM-X, continue to orbit in close formation to make bistatic observations for scientific applications. In addition, observations are being made to fill small areas in the DEM and to improve the quality of the data. Furthermore, observations are being conducted to capture topographic changes. 19)
- Both satellites have used their consumables frugally; each spacecraft spent so far somewhat less than half a tank volume of hydrazine; the batteries are in good operational conditions, and the quality of the radar imagery remains excellent since the start of the missions. The project expects a continuation of operational services of the two missions to at least 2020, subject to unforeseen events.
- A very recent paper has been published providing forest maps on the basis of interferometric TanDEM-X data. 20)
Figure 11: Global TanDEM-X Forest/Non-forest Map (image credit: DLR)
- Project Forest/Non-Forest Map: 21) The TanDEM-X Forest/Non-Forest Map is a project developed by DLR/MRI (Microwaves and Radar Institute), within the activities of the TanDEM-X mission. The goal is the derivation of a global forest/non-forest classification mosaic from TanDEM-X (i.e. TerraSAR-X and TanDEM-X) bistatic InSAR (Interferometric Synthetic Aperture Radar) data, acquired for the generation of the global DEM (Digital Elevation Model) between 2011 and 2015 in stripmap single polarization (HH) mode.
- In this work, the global data set of quicklook images was used, characterized by a ground resolution of 50 m x 50 m, in order to limit the computational burden. For classification purposes several observables, systematically provided by the TanDEM-X system, can be exploited, such as the calibrated amplitude, the bistatic coherence, and the DEM height information. In particular, the volume correlation factor quantifies the amount of decorrelation due to multiple scattering within a volume, which typically occurs in presence of vegetation.
- This quantity is directly derived from the interferometric coherence and used as main indicator for the identification of vegetated areas. For this purpose, a fuzzy multi-clustering classification approach, which takes into account the geometric acquisition configuration for the definition of the cluster centers, is individually applied to each acquired scene.
Figure 12: TanDEM-X Forest/Non-Forest Map example over the Alps (image credit: DLR)
Figure 13: TanDEM-X Forest/Non-Forest Map example over the Amazon Rainforest (image credit: DLR)
Figure 14: TanDEM-X Forest/Non-Forest Map example, a zoom-in over the Amazon Rainforest in the state of Rondonia, Brazil (image credit: DLR)
• On June 15, 2017, the TerraSAR-X spacecraft and its payload were 10 years on orbit. Designed to return unique images of the Earth for five years, the German radar satellite TerraSAR-X has outdone itself. The satellite has been in operation for twice that time – and there is still no end in sight to its service. Since its picture-perfect launch on 15 June 2007 from the Russian cosmodrome in Baikonur, the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) TerraSAR-X mission has exceeded all expectations. 22)
- "TerraSAR-X has stood for outstanding research and development performance and top-level satellite operation for 10 years. To this day, the mission continues to set standards in precision and image resolution. Thanks to its globally unique radar technology, TerraSAR-X has opened up a new era in remote sensing and paved the way for the equally successful follow-up mission, TanDEM-X. I am pleased that both satellites have been fully functional and efficient," emphasizes Pascale Ehrenfreund, Chair of the DLR Executive Board.
- TerraSAR-X and its twin TanDEM-X, which was launched three years later, have been flying in formation since 2010. Together, they generate the highest resolution three-dimensional images of the Earth's surface. To this day, the special mission concept of TanDEM-X, the first bistatic SAR interferometer in space, developed at the DLR Microwaves and Radar Institute, is one-of-a-kind.
- "With the TerraSAR-X mission and its successor mission TanDEM-X, we have also entered new territory in industrial policy: TerraSAR-X was the first space project undertaken between DLR and the aerospace industry as a public-private partnership (PPP) on the initiative of the DLR Space Administration with funds from the German Federal Ministry of Economic Affairs and Energy," added Gerd Gruppe, DLR Executive Board Member responsible for the Space Administration.
- Developed and constructed by Airbus Defence and Space teams from Friedrichshafen for the DLR (German Aerospace Center), the satellite orbits at a height of 514 km and provides radar imagery to a wide variety of scientific and commercial users. 23)
- "TerraSAR-X has not only achieved double its service life, having orbited the Earth 55,459 times and travelled 2.4 billion km, all while boasting 99.9 percent availability, it has also delivered an outstanding performance", said Eckard Settelmeyer, Head of Earth Observation, Navigation and Science at Airbus in Germany. "TerraSAR-X is in such a good condition that a current assessment indicates it can be operated for a few more years in space until a follow-on system is in place."
- "TerraSAR-X features a unique geometric accuracy," said François Lombard, Head of the Intelligence Business Cluster at Airbus Defence and Space. "With six imaging modes, it offers flexible coverage and resolutions ranging from 0.25m to 40m, and answers the needs of a wide range of domains, like engineering companies to ensure the safe operation of large construction projects, oil and gas enterprises to monitor their production, or Intelligence and Security agencies for targeted surveillance and detailed change detection."
Figure 15: TerraSAR-X image of Las Vegas (Winchester), Nevada, USA (image credit: Airbus DS) 24)
Big Data for Earth observation:
- The satellite has already delivered 303,714 images. The data is received via a global network of ground stations and processed and evaluated by experts at the DLR Earth Observation Center (EOC). Even the first analyses document indisputable details of climate change, including the retreat of glaciers across the globe. Approximately 1000 scientists from more than 50 countries are now using the data for their research – and demand is on the rise. The global radar images are of particular value to environmental and climate research. DLR ensures access to the images in the long term in the German Satellite Data Archive in Oberpfaffenhofen.
- In this time, GSOC (German Space Operations Center) has sent more than 1.85 million commands to TerraSAR-X, and an additional 1.4 million commands to control the orbiting TanDEM-X satellite. A particular challenge, both during the development and in operation, was and is the 'double-helix dance' of the two radar satellites. The tightest flight formation between TerraSAR-X and TanDEM-X was at a distance of 120 m distance perpendicular to the direction of flight – at an average speed of 7.6 km/s. The exceptional performance and success of the mission is not least down to the close interdisciplinary collaboration within DLR. In Oberpfaffenhofen, almost 100 staff from four DLR institutes have combined their expertise such that they have mastered the entire process chain of the TerraSAR-X and TanDEM-X mission for 10 years now.
- The exceptional lifetime of the satellite has been possible thanks to careful operation and robust construction. Only about half of the fuel supply has been consumed and the performance level of the batteries is approximately 72 percent, so the experts expect TerraSAR-X will continue to operate for another five years. The twin satellite TanDEM-X is also showing no signs of fatigue, meaning that more high resolution elevation images will be generated and the global data set enhanced by autumn 2017. The focus is on areas undergoing strong processes of change, and are therefore of particular scientific interest. These include the coastal regions of the Antarctic, Greenland and the permafrost regions, and the Amazon rainforest.
- "The new images are also being used for the demonstration and preparation of the Tandem-L mission. With Tandem-L, DLR has designed a new satellite mission to observe how Earth is changing – for 10 years, on a weekly basis, at high resolution and in three dimensions. Such data will be of inestimable value for science and politics," explains Hans Jörg Dittus, DLR Executive Board Member for Space Research and Technology. With regard to the extent and effect of climate change, Tandem-L could provide important information that is still lacking – for improved scientific forecasts and the social and political recommendations for action that are based on this. The concept builds on the experience and exceptional success of the TerraSAR-X and TanDEM-X missions. If the mission proposal gets the 'green light', Tandem-L will take radar remote sensing into the next era of technology and applications in 2022.
- With the X-band SAR family, Germany has developed a globally recognized expertise and a unique selling point for decades. In order to ensure this leadership role in the future, the continuation of the X-Band family is being carried out at the DLR Space Administration. The future lies in an even higher resolution with wider observation swaths. This is intended to continuously provide the scientific, governmental and commercial stakeholders with data.
• January 13, 2017: The satellite duo, TerraSAR-X and TanDEM-X, continue to orbit in close formation to make bistatic observations for scientific applications. Both satellites have used their consumables frugally; each spacecraft spent so far about half a tank volume of hydrazine; the batteries are in good operational conditions, and the quality of the radar imagery remains excellent since the start of the missions. The project expects a continuation of operational services of the two missions to about 2020 — these predictions depend of course on the assumption that no unanticipated events occur. After all, in June 2017, TerraSAR-X will be 10 years on orbit. 25)
Legend to Figure 16: The measurement of the polar regions by TSX/TDX is so precise that glacier movements can be seen in the centimeter range, and changes in elevation caused by ice melting measured in the meter range. Initial studies have shown that in some regions, glaciers are losing up to 30 m thickness per year in the area of the glacier tongues. The color-coded terrain model shows a region in the Northeast Greenland National Park, the largest national park in the world. The Elephant Foot Glacier, which is about 5 km in diameter, stands out at the center of the image. It flows from the mountains to Romer Lake.
Figure 17: TSX/TDX image of rainforest clearing in Bolivia: At first glance, nature seems untouched - forest areas crossed by rivers and small mountain chains, Upon close inspection, one can see strip-like structure - wetland surfaces cleared for plantations (image credit: DLR, Ref. 26)
• October 17, 2016: The German satellite duo TerraSAR-X and TanDEM-X have consistently delivered one-of-a-kind Earth observation data since 2007 and 2010, hence shaping the international research landscape. Now, scientific users from across the globe have gathered for the TerraSAR-X and TanDEM-X Science Meeting at DLR (German Aerospace Center) in Oberpfaffenhofen, where they will discuss the results obtained from the data and define requirements for future remote sensing technology. 26)
• July 2016: Since the launch of the TDX (TanDEM-X) mission in 2010, the two missions TSX (TerraSAR-X) and TDX share a joint space segment consisting of both TSX and TDX and a common ground segment which was first developed for TSX and had been later extended for TDX. While TanDEM-X uses both satellites (nominally one in receive-only mode) for an acquisition, TerraSAR-X data are acquired by either one of the two satellites. It is this sharing of one ground segment which necessitated on-going updates of the TerraSAR-X ground segment in accordance with the TanDEM-X mission constraints. 27)
- Table 2 summarizes the acquisition mode portfolio. In response to a growing demand for larger coverages at moderate resolution and for higher resolution with more radiometric looks, the original portfolio was extended by the new Wide ScanSAR and Staring Spotlight mode in 2013.
Table 2: TerraSAR-X acquisition modes
Future data takes from all modes may be directly ordered by users. Once a data take is archived, catalog ordering, i.e. the ordering of products to be newly generated based on the archived L0 products, is possible. By using the redundant instrument receiver chain, StripMap data are also available as fully polarized (SM quad) or as ATI (Along-Track Interferometric) data for a limited number of acquisitions. These are taken during specific campaigns since the DRA (Dual-Receive Antenna) configuration not only uses redundant on-board parts, but also influences the acquisition and downlink timeline (data replay not possible in parallel to data acquisition). To overcome the timely limited availability of ATI products, data from the so-called ATIS (Aperture Switching Mode) with antenna attenuation switches on a pulse-by-pulse basis and thus relying on the standard single-antenna receive configuration only are also made available on an experimental basis. Data acquired in all these experimental modes are provided via catalog ordering to users. -The total number of TerraSAR-X data takes acquired in these modes is steadily increasing over the last years as shown in Figure 18.
Figure 18: Total number of basic and experimental product acquisitions per year (image credit: DLR)
Figure 19: Distribution of basic and experimental product acquisitions between the different TerraSAR-X acquisition modes in 2015 (image credit: DLR)
- Implications of TanDEM-X science phase flight configurations: After completion of the global DEM data acquisition in 2014, the TanDEM-X mission entered the so-called Science Phase to focus on the secondary mission goals, namely the provision of radar data products for a number of new science and technology related applications. Specific flight configurations realizing various along-track and across-track baseline conditions were applied.
From September 2014 until March 2015, TSX and TDX were flown in a pursuit monostatic (PSM) configuration with an along-track separation of about 76 km (10 sec). This separation is large enough to avoid any mutual interaction between the two SAR missions, i.e. the instruments may be operated in active mode simultaneously. The PSM was followed by a bistatic flight configuration again, however this time with varying large horizontal baselines showing a maximum of 3.6 km at the equator.
From December 2014 until January 2016 the DRA configuration was operated allowing the acquisition of fully polarized and along-track interferometric TanDEM-X data.
These TanDEM-X specific flight configuration and operation modes have various direct impacts on the TerraSAR-X system. Appropriate countermeasures were implemented to mitigate the constraints and thus make them as transparent as possible for the TerraSAR-X mission. New opportunities were exploited to even better serve the TerraSAR-X users.
- Preferred satellite concept: Data takes taken by the TDX satellite shall show the same quality and performance as those taken by the TSX satellite in the reference orbit. Appropriate roll angles and time delays are considered during instrument commanding. Still, TDX images may show slightly different characteristics, specifically when taken outside the 250 m reference orbit tube. As a countermeasure, the TerraSAR-X preferred satellite concept was implemented inside mission planning. Whenever the TDX pre-calculated perpendicular baseline exceeds a (configurable) threshold and sufficient TSX resources are available, an acquisition is performed by the TSX satellite. In return for that and if possible, the TDX satellite is chosen as active one for a TanDEM-X acquisition to preserve TSX resources (e.g. battery). The option to fix selected acquisitions, e.g. those for a time series, to a given satellite furthermore exists.
- Ground station antenna and downlink constraints: During the DEM acquisition phase, the satellites TSX and TDX were flown so close to each other that X-band data reception using one antenna only was possible and downlinks were performed sequentially due to the identical downlink frequency. Tracking of the TSX satellite only (reference orbit) was sufficient. This "close" formation assumption is no longer true for the science phase flight configurations, which involve so-called "near" and "far" constellations between TSX and TDX. In a "far" constellation, the two satellites are too far apart to be tracked as one during a contact (but still too close to be tracked one after the other), simultaneous X-band downlinks from both satellites are possible. In a "near" constellation, the two satellites are still too far apart to be tracked as one, but already so close that downlinks can only be executed sequentially (one after the other).
The PSM with its 10 second separation is a typical "far" scenario and thus has a major impact on all ground stations using one single antenna only. X-band support can only be given for one satellite during a given contact. Contacts are therefore assigned by mission planning in an alternating manner to a given satellite and this constraint is considered accordingly in the acquisition and downlink planning. Only a possibly increased downlink latency of a given data take may be observed. NRT data takes may be (e.g.) fixed to the TSX satellite.
On the other hand, the TerraSAR-X main station Neustrelitz equipped with several antenna and independent receiving chains receives the PSM SAR data from both satellites simultaneously and thus nearly doubles the downlink capacity for this flight configuration.
Much more complex is the situation in case of the bistatic with large horizontal baseline flight configuration resulting in a mixture between "close" and "near" scenarios within the same orbit. Whether tracking of both satellites with one antenna is possible depends on the geographic location of the station and the current elevation angle w.r.t. the tracked satellite. Whenever the angular separation between TSX and TDX exceeds the antenna critical threshold which depends on the antenna beam characteristics, i.e. the antenna size, a "near" scenario has to be assumed, otherwise "close". As a result of the analysis performed by the flight dynamics team based on the planned flight configuration, the polar ground station Svalbard was well within the "close" margins for all orbits whereas Neustrelitz was pre-classified as "near" for a number of orbits. Therefore a two antenna Neustrelitz operation was chosen at first but could be reduced to one based on the observed data quality. This indicates that the critical threshold value chosen in the analysis might have been too conservative.
- Increased acquisition opportunities in pursuit of the monostatic phase: Due to the simultaneous operation of the TSX and TDX instruments in active mode, the imaging opportunities over a given scene are increased and thus the conflict potential among acquisition hot spots is reduced. Furthermore, acquisitions may directly follow each other when taken sequentially by both satellites. Neighboring or overlapping (either along-track or across-track) data takes are possible consistently enlarging the imaged area.
- TerraSAR-X Like Products from pursuit of the monostatic phase: A PSM TanDEM-X acquisition actually consists of two single independent data takes taken over the same scene or in other words, it just consists of two TerraSAR-X acquisitions. Consequently, each satellite channel can be processed independent from its counterpart into a SAR product. Therefore, the TerraSAR-X product portfolio (SSC, MGD, GEC, EEC) is offered accordingly for these data takes. The products are named TerraSAR-X Like to express their commonality with the TerraSAR-X basic and experimental products w.r.t. to product format and content, but to stress in parallel that there might be differences in product performance and / or characterization parameters.
- NRT (Near Real-Time) capabilities: Since NRT support had not been a TerraSAR-X stake holder requirement, the NRT functionality had to be included "as best as possible" within the given system constraints – both technical and financial. Nevertheless, NRT flavors (with a product latency between 10 and 20 min after downlink) of all basic products are offered to the user community since mission beginning. The NRT capabilities were further extended over the last years.
The Svalbard and Neustrelitz station are grouped into a NRT receiving station pool and mission planning ensures that a NRT data take is downlinked to the next possible contact. Data are transferred online from Svalbard to Neustrelitz for NRT processing and dissemination. Mission timelines are generated and uploaded twice per day with the order deadline a few hours before. This results in a latency of about 6 to worst case 18 hours between the latest possible order input and an NRT acquisition.
NRT processing systems instances are installed at the Inuvik and O'Higgins ground stations for which a transfer of raw data to Neustrelitz is not feasible due to the limited network performance. Geocoded quicklooks with sizes smaller than 5 MB are derived from the NRT level 1b product at the stations and are sent by e-mail as png and/or kmz files to the user, e.g. ships. For this station NRT support, the former centralized production system had been extended into a distributed one which allows the automated routing of NRT production requests into the appropriate NRT processing system (either the main one in Neustrelitz or the station ones).
The NRT product portfolio supported by the main NRT processing system in Neustrelitz furthermore supports maritime applications like ship and oil detection and is currently extended for wind and wave charts.
The extended NRT features, specifically the delivery of quicklook information out of the stations, were recently tested in the frame of two research projects. In October 2015, NRT support was given to the ONR (Office of Naval Research) Arctic Sea State Campaign in the Beaufort Sea using the main NRT processing system in Neustrelitz. This included the delivery of wind and wave maps in addition to the NRT products and quicklooks. In January 2016, NRT support was given through the German Antarctic Receiving Station O'Higgins to the Weddell Sea expedition PS96 of the Polarstern (Ref. 27).
• On-Board Resources Status in June 2016: TSX (TerraSAR-.X) had reached its nominal lifetime at the end of 2012 and TDX (TanDEM-X) at the end of 2015. Hence, it is worth to have a look at the status of the irretrievable on-board resources. Especially the propellant and the battery status are crucial factors determining the future progress and remaining duration of the mission. 28)
- Propellant: The consumption of propellant (Hydrazine) is determined by the number of maneuvers, respectively factors as aerodynamic drag, solar activity, tidal forces, space debris avoidance maneuvers, etc. Currently the filling level of TSX propellant is about. 55%, of TDX about 56% (status March 2016). This ensures an extension of the mission of three years at least. — Also the filling level of the cold gas used by the additional propulsion system on TDX (only required for close formation flight) still amounts to 13% allowing a fine orbit adjustment for further six month approximately, whereby strategies have been developed to reduce the cold gas consumption by an increased use of hydrazine thrusters. Thus the overall propellant status is currently considered as uncritical for the next years and will not impair the continuation of the mission.
- Battery: The retained TSX battery capacity is about 74% and the TDX battery capacity is about 82% (status March 2016). According to analysis, the batteries are in excellent health – exceeding original degradation predictions.
- SAR Instrument Status: The usability of the SAR products  depends strongly on the absolute calibration and stability of the processed products. The SAR instrument plays a major role in the stability of the whole chain. To ensure the stability of the instrument and to detect weak components at an early stage, special test datatakes are evaluated on a regular base comprising
a) Health checks for transmit/receive modules
b) Repeated acquisitions over corner reflectors and rain forest
c) Ultra Stable Oscillator frequency measurements.
Dedicated long-term system monitoring activities make use of these measurements and still confirm the high performance and the excellent stability and radiometric calibration of the SAR system.
- Key Features of the TanDEM-X System: The TDX satellite is a rebuild of TSX with only minor modifications. This offers the possibility for a flexible share of operational functions for both the TSX and TDX missions among the two satellites. The TSX and TDX satellites were designed for a nominal lifetime of 5.5 years. Predictions based on the current status of system resources indicate several extra years of lifetime for both satellites and joint operation. An orbit configuration based on a Helix geometry has been selected for safe formation flying. The Helix like relative movement of the satellites along the orbit is achieved by a combination of an out-of-plane (horizontal) orbital displacement imposed by different ascending nodes with a radial (vertical) separation imposed by different eccentricities and arguments of perigee. Cross-track and along-track baselines ranging from 120 m to 10 km and from 0 to several 100 km, respectively, can be accurately adjusted depending on the measurement requirement.
- Global DEM Production Status: After the launch of TDX in June 2010 and the subsequent commissioning phase, global DEM acquisitions started in December 2010. Parallel to the first month of operational data acquisition the team concentrated its efforts on the calibration of the bistatic interferometer. Correction of differential delays between TSX and TDX was necessary to facilitate the utilization of radargrammetry for resolving the 2π-ambiguity band. Phase, delay and baseline calibration have reached such an accuracy level , that more than 90% of all so-called Raw DEMs (long data takes are processed to scene-based DEMs of 50 km by 30 km extension) are within ±10 m of DEMs derived from SRTM/ICESat data already before the final calibration step using ICESat data as reference heights. More than 500,000 Raw DEMs have been generated in a fully automated process employing multi-baseline interferometric techniques.
The first and second global coverages (except Antarctica) were completed in January 2012 and March 2013, respectively. After some gap-filling, Antarctica was mapped for the first time under local winter conditions. In early August 2013 the helix formation was changed to allow imaging of mountainous areas from the opposite viewing geometry. Due to lack of SNR, desert areas had to be re-acquired as well, but at steeper incidence angles. Afterwards the satellites were maneuvered back to the original formation and Antarctica was covered again at larger baselines. The primary data acquisition program was concluded mid-2014.
Since the end of 2013 the final calibration and mosaicking chain is fully operational and as of March 2016 final DEM products for more than 80% of the total land surfaces are already available. It is expected to complete the global DEM consisting of more than 19,000 tiles of 1º x 1º in size by mid-2016. A comprehensive system has been established for continuous performance monitoring and verification including feedback to the TanDEM-X acquisition planning for additional acquisitions. Figure 20 shows as an example the absolute height accuracy (90% linear error) per tile derived from the comparison of the TanDEM-X heights against ICESat validation points (the majority of ICESat points not being used for DEM calibration). The accumulated absolute height error is with 1.3 m outstanding and one order of magnitude below the 10 m requirement. Compared to SRTM the TanDEM-X DEM features a much lower percentage of void areas, especially in desert areas, a result of the re-acquisition at lower incidence angles and hence better SNR. Further details on the global DEM quality can be found in reference 29) .
Beyond the generation of a global TanDEM-X DEM as the primary mission goal, a dedicated science phase was aiming at demonstrating the generation of even more accurate DEMs on local scales and applications based on along-track interferometry and new SAR techniques, with focus on multistatic SAR, polarimetric SAR interferometry, digital beamforming and super resolution.
As both satellites are still working very well and have plenty of resources left, an agreement to continue the mission beyond 2015 was concluded between DLR and Airbus Defence & Space. Acquisition of interferometric data for and generation of local DEMs of even higher accuracy level (posting of 6 m and relative vertical accuracy below 1 m) is the key objective for this new mission phase. If the baseline geometries are suitable further scientific experiments will be included in the timeline as well.
Figure 20: TanDEM-X DEM production status as of March 2016: the absolute height accuracy (90% linear error) is shown per 1º x 1º DEM tile; the accumulated absolute height error is with 1.3 m one order of magnitude below the 10 m requirement (image credit: DLR, Ref. 28)
• Nov. 30, 2015: Based on the trusted collaboration in space, CSA (Canadian Space Agency) and DLR (German Aerospace Center) have announced the funding of six major research projects in the domain of Emergency Response and Safety of Operations. DLR has awarded Airbus Defence and Space with two of them. 30)
1) In collaboration with MDA (MacDonald Dettwiler and Associates, Ltd.), the first project will examine man-made changes on land using multi-frequency SAR satellite data. The methods developed throughout this project will monitor the changes' impact on the environment including new buildings, roads, forests, and surface movements due to industrial activities such as mining.
2) For the second project, Airbus Defence and Space will work with C-CORE to investigate the synergistic use of X-and C-band SAR data for tactical ship route planning in Arctic waters, its objective being to monitor the sea ice situation along shipping routes in the north. Both Canadian partners receive funds from CSA.
Taking benefit from combining both missions - the German TerraSAR-X and the Canadian RADARSAT-2 satellites - these projects aim to support safe transportation, exploration, and monitoring. Enfotec Technical Services Inc., one of the end users, believes that "satellite imagery plays a vital role in ensuring the safety and efficiency of navigation in ice covered waters. This project addresses how to best use different satellites concurrently in order to increase the overall quality of the ice information provided to ships." -"With our experience in natural disasters and maritime monitoring, we are confident to support Canada in improving its emergency capacity readiness in the High North" said Simon Jacques, President of Airbus Defence and Space Canada Inc.
• July 30, 2015: Airbus Defence and Space, owner of the commercial distribution rights for TerraSAR-X data, and ESA have signed a contract securing the continued supply of TerraSAR-X data for the Copernicus Data Warehouse. The agreement is valid until the end of 2020, thus continuing the successful cooperation between Airbus Defence and Space and ESA for the provision of TerraSAR-X data to public institutions across Europe in place since 2008.
- TerraSAR-X has been a key data source particularly for activities addressing emergency and security related issues, reliable monitoring needs, and land cover change, both in Europe and beyond. Its unique reliability and high accuracy make it an ideal data source within ESA's multi-mission approach.
- The agreement includes the provision of archived and newly acquired TerraSAR-X data for the Copernicus core datasets in the area of maritime monitoring, as well as additional datasets for further maritime applications particularly sea ice monitoring. The contract also includes the maintenance for the Copernicus-specific interfaces for the ordering, delivery and reporting process that have been established during the previous phases.
• In February 2015, the TanDEM (TanDEM-X and TerraSAR-X) missions are operating nominally, supporting the science mission. The new science phase, started in Sept./October 2014, applies to both missions of the formation, TerraSAR-X and TanDEM-X, and is planned to last until the end of 2015. 31)
- The first part of the science mission, from Sept. 20, 2014 to the end March 2015, is the so-called the "Pursuit Monostatic Phase", in which the TanDEM-X spacecraft is flying at a distance of 76 km behind its twin, TerraSAR-X. Each spacecraft acquires monostatic data of the same area which are then interferometrically ground-processed .
- At the end of March, the formation is changed again; the "Bistatic Phase" is started in which bistatic acquisitions with a large horizontal baseline (up to 3600 m) are taken up to September. After a drift phase of about a month, the bistatic phase is continued in October with a small baseline of ~250 m, until the end of the year. Note: The horizontal baseline is here the cross-track component to measure the surface topography.
- Independently from the science mission, the TerraSAR-X mission (generation of non-interferometric SAR products), is being served in parallel from both spacecraft.
Note: The TerraSAR-X has the primary objective to acquire 2D SAR observations in the operational modes Stripmap, ScanSAR and Spotlight. These objectives are being pursued in parallel to the TanDEM mission; these functions are actually supported by both spacecraft of the formation. — The TanDEM mission has the primary goal to generate a global DEM, while the secondary objective is to support also the goals of the current science mission.
• October 10, 2014: After four years of successful data acquisition for the new global topographical map of Earth, the Science Phase is beginning. The radar satellite TerraSAR-X has been orbiting the Earth since June 2007; in June 2010 its twin, TanDEM-X, followed it into space. For almost four years, the two satellites have been operated in a close flight formation by DLR (German Aerospace Center). During this time, the satellites have been acquiring data to generate a new global topographical map of the Earth. 32) 33)
- The goal of the TanDEM-X mission is to produce a highly precise, three-dimensional image of the Earth with uniform quality and unprecedented accuracy. For large parts of the Earth, there currently exist only approximate, inconsistent or incomplete elevation models derived from different data sources and collection methods. TanDEM-X is filling in these gaps and providing a homogeneous elevation model to be used as an indispensable basis for numerous commercial applications and scientific investigations.
- With the start of the TanDEM science phase, another significant milestone in the mission has been reached. "In the coming 15 month mission phase, the orbit and imaging mode will be configured and optimized so that new radar techniques and innovative applications can be tested and demonstrated. The expectations of the scientific user community for the science phase are very high, and more than 100 science proposals have already been submitted," explains Alberto Moreira, Director of the DLR Microwaves and Radar Institute and Principal Investigator for the TanDEM mission.
- The initial preparations for the science phase began with the transition to the new formation on 17 September 2014. TanDEM-X moved away from TerraSAR-X and has been flying at a distance of 76 km behind its twin since 20 September 2014. This has resulted in a time delay of 10 seconds. Since the Earth rotates at approximately 500 m/s at the equator, the orbit of TanDEM-X has also had to be displaced laterally by 5 km so that both satellites are imaging the same area (footprint) on the surface. TanDEM-X continues to follow a helical orbit. Unlike the imaging for the DEM (Digital Elevation Model) of the Earth, the helix will not be adhered to for weeks at a time; instead, significantly greater variations will be permitted. The distance between TanDEM-X and the nominal orbit of TerraSAR-X will vary between 0 and 1000 m over the next five months.
- The aim is to continue to operate both satellites using interferometry, to enable three-dimensional imaging of the surface of the Earth to continue. After changing the orbit of TanDEM-X in recent weeks, the two satellites are being operated independently of one another, in what is known as 'Pursuit Monostatic Mode'. The advantage of this new orbital configuration is that the distance between the satellites – the baseline – can be made substantially more flexible. In the new orbital configuration, data for elevation models can be generated with an elevation accuracy of a few tens of centimeters, for example. This opens up new applications in the areas of the geosphere, cryosphere and hydrosphere. This data is unique and will be used in the investigation of volcanic eruptions, the melting of ice as well as, for example, tomographic imaging of cities. The orbital configuration will be changed again in the spring of 2015 to enable other applications and demonstrations.
- One topical subject is the thawing of permafrost soils, which is being caused by global warming. It is causing massive damage to roads and houses and is leading to landslides. At present, it is known that huge areas are involved, although the precise extent is still unclear. "In the new phase of the mission, one of the things TanDEM-X will do is map these areas with a very high spatial resolution and contribute valuable insights into climate change," explains Irena Hajnsek, the Scientific Coordinator for the TanDEM-X mission.
Global elevation model of the Earth is being created: On 17 September 2014, the imaging for the DEM was completed, with the exception of a few images. A data set of over 2500 TB forms the basis for the new topographic map of the Earth. The quality of the elevation models generated to date exceeds all requirements. Final DEM tiles for more than a quarter of the land area – for example, for the flat areas of Australia, North America, Siberia, South and West Africa and South America – have already been processed. The new 3D map should be available in its entirety by the end of 2015. "Both satellites are functioning well, and the propellant supplies are certain to last until 2020," adds Manfred Zink, DLR scientist and Project Manager for the TanDEM-X ground segment. "We are already thinking beyond the science phase. The operation of two SAR satellites in close flight formation is a unique achievement and demonstrates Germany's leading position in radar technology. We can generate even more accurate elevation models or more precise coastline maps."
The success of TanDEM-X forms the basis for the development of innovative radar technologies. Researchers at DLR are already working on a new mission proposal with a digital radar antenna – Tandem-L. The aim is to achieve a significantly higher imaging capability, which will exceed that of TanDEM-X by a factor of 100. While TanDEM-X only enables one global image of the Earth to be acquired per year, Tandem-L will image the entire landmass of the Earth at a higher resolution twice a week. Hence, Tandem-L will be able to capture dynamic changes on the surface of the Earth with the required imaging repetition frequency and provide urgently needed information for solving topical scientific questions involving the areas of the biosphere, geosphere, cryosphere and hydrosphere. Such a mission could be launched in 2020.
• Sept. 16, 2014: The lava outflow on the Holuhraun field northeast of Iceland's Bardarbunga volcano continues unabated. The lava field has grown to cover an area greater than 25 km2 (Figure 21). In this satellite image, the extent of the lava field is revealed using different colors. 34)
Researchers from DLR/IMF (Institut für Methodik der Fernerkundung), Oberpfaffenhofen are continuing to monitor the area. Radar images can be used to analyze changes to Earth's surface throughout the entire process.
Figure 21: Superimposed images of 3 TerraSAR-X observations to show the lava flow of the Bardarbunga volcano (image credit: DLR)
Legend to Figure 21: To create this image, three sets of data were acquired at different times, but from the same viewpoint, and then superimposed. They date from 13 August, 4 September and 15 September 2014 and were acquired by the German radar satellite TerraSAR-X. Yellow shows the growth of the lava field between 13 August and 4 September; red shows the expansion between 4 and 15 September. It is obvious that the area has doubled in the shorter second period. A second eruption area can also be seen as a small red spot in the lower right corner of the image.
• Sept. 10, 2014: Bardarbunga, (Bárðarbunga) in Iceland, one of the largest volcanoes in Europe and located beneath the biggest glacier in Europe, became active again in mid-August. For several years now, DLR researchers have been keeping a close eye on Bardarbunga and the system of volcanoes associated with it - an enormous network of subterranean magma channels, vents and craters. TerraSAR-X has now provided important data on the volcano's latest activity. 35)
Activity in the Bardarbunga volcano system began with earthquakes having magnitudes of up to of 5.7 on the Richter scale, indicating that magma beneath the surface was moving and rising. On 27 August, volcanologists discovered several new depressions in the ice to the south of the caldera of Bardarbunga, with depths of up to 15 m. This is another indication that a heat source lies beneath the ice sheet of the glacier. On 29 August, a lava flow escaped from a breach in the Holuhraun lava field to the north of the glacier - an ice-free area. On 31 August, a second eruption occurred there. The Holuhraun lava field has now grown to cover an area of over 19 km2. If the lava had escaped directly beneath the ice and forced its way to the surface, there would have been a large steam explosion, reducing the lava to tiny particles of ash and forming an ash cloud. This is exactly what happened in 2010 with the eruption of Eyjafjallajökull, another subglacial volcano in this region, which lead to significant disruption for air traffic.
Figure 22: Image of the Bardarbunga crater area as observed by TerraSAR-X (image credit: DLR)
Legend to Figure 22: The image shown here covers an area of approximately 30 km x 50 km, and the recently ejected lava covers an area of roughly 10 km2. The brighter areas, which are also highlighted in red for better visibility, indicate changes in amplitude (the intensity of the radar signals returned to the satellite). Since the rough surface of freshly-cooled lava reflects the radar signals back very strongly, it appears bright and is easily visible on the lava flow at the lower right in the image or on the two arcs at the right edge of the image (the northern edge of Vatnajökull). Smooth surfaces, such as water, reflect the incoming radar signals away from the satellite and so appear dark on the images.
In the bottom half of the image, the lake in the caldera of the Askja volcano is visible as a black area. A landslide occurred recently on this volcano, triggering a tsunami in the lake - with a wave height of up to 30 m. From this image, it is clear that the area adjacent to the water is darker in color than the more elevated regions. This is very probably due to flooding associated with the tsunami.
• On June 15, 2014, TerraSAR-X was 7 years on orbit, operating nominally. Nominal mission operations applies also to the TanDEM-X mission. Meanwhile, TerraSAR-X has reached its nominal lifetime by the end of 2012, the TanDEM-X mission will reach it by the end of 2015. The current status of the TerraSAR-X spacecraft resources allow at least three years extra lifetime until end of 2015, providing five years of joint operation with its twin satellite in order to accomplish the TanDEM-X mission. The radar performance and calibration of the individual satellites is still within specification or better. 36)
- Propellant: The consumption of propellant (hydrazine) is determined by the number of maneuvers, respectively factors as aerodynamic drag, sun activity, tidal forces, space debris avoidance maneuvers, etc. Currently the filling level of TerraSAR-X propellant is ca. 60%, of TanDEM-X ca. 80%. This ensures an extension of the mission of three years at least.
- Batteries: The retained TerraSAR-X battery capacity is ca. 80 % and TanDEM-X battery capacity is ca. 90 %. According to analysis, the batteries are in excellent health – exceeding original fade predictions.
- Radar instrument status: The condition of the radar instrument, especially the T/R modules decisively determines the usability of the SAR products and therefore needs a close monitoring and accurate calibration. The radiometric stability, measured over a period of six years, amounts to 0.15 dB for both satellites. Thus almost no drift was observed. Also the PN-gating method, which is used to measure transmit and receive gain and phase of each TR module, shows no anomalies. Due to the excellent calibration, it is hardly distinguishable if the final image products have been acquired by TerraSAR-X or TanDEM-X.
Table 3: Comparison of TerraSAR-X versus TanDEM-X calibration status
- New imaging modes (introduced in July 2013): To meet an increasing demand for higher resolution and more detail, as well as increased coverage, the capabilities of the TerraSAR-X mission were extended by implementing two new modes. The flexible instrument design allows such upgrades even with the satellites in orbit. A Wide ScanSAR product with an extended swath width of up to 260 km and a Staring Spotlight product with a with an intrinsic azimuth resolution of 24 cm, that is being traded for a considerably improved radiometric resolution ,have been added to the product portfolio.
- Staring Spotlight mode: The staring Spotlight mode even further increases the high geometric resolution of the standard High Resolution Spotlight product. By widening the azimuth beam steering angle range, thereby extending the synthetic aperture, an azimuth resolution of 0.24 m can be achieved. It is available as single polarization 300 MHz range bandwidth variant. The scene extent varies between 2.5 km and 2.8 km in azimuth and 4.6 km to 7.5 km in range. A staring Spotlight image with a resolution of 1 m x 1 m is depicted in Figure 23, showing the Burning Man Festival 2013, a 'Tent City' arising in a semicircle around its central point. A further Staring Spotlight image is shown in Figure 25.
Table 4: Comparison of High Resolution Sliding Spotlight (HS) and Staring Spotlight (ST) product parameters
Figure 23: Staring Spotlight scene of 3 km x 3.5 km with a resolution of 1m x 1m, showing the Burning Man Festival 2013, a 'Tent City' arising in a semicircle around its central point in Black Rock City, Nevada, USA. The original azimuth resolution of 24 cm was reduced to 1 m, due to multi-looking, in order to improve the radiometric resolution (image credit: DLR)
- Wide ScanSAR Mode: The standard ScanSAR covers a 100 km wide swath. In order to meet an increasing demand for wider swath coverage, the new Wide ScanSAR mode was introduced. This new ScanSAR variant offers a 195 km to 266 km wide swath by employing 6 specific beams with a reduced azimuth resolution of 40 m and a variable range bandwidth of 50 MHz to 100 MHz. Figure 24 depicts a Wide ScanSAR scene of the German Bight with a scene size of 220 km x 500 km.
Table 5: Comparison of nominal (four beam) and wide ScanSAR (six beam) product parameters
The new operational TerraSAR-X 6 beam Wide ScanSAR mode with cross track coverage of up to 260 km shows excellent results in NESZ, ambiguity ratios and resolution. Taking into account, that during the design of the satellite, only swath widths of 100 km were required, the challenging task was achieved with an iterative optimization approach. The outcome is a very impressive achievement for the mission. 37)
Figure 24: Wide ScanSAR image of the German Bight with a scene size of 220 km x 500 km and a resolution of 40 m x 40 m. A gusty low-pressure area approaches from north roughening the sea surface, which is characterized by a higher radar backscatter coefficient, respectively brighter image areas (image credit: DLR)
Both GEOINT (Geospatial Intelligence) and IMINT (Image Intelligence) increasingly make use of high-resolution SAR data. The new TerraSAR-X Staring Spotlight mode provides the highest spatial resolution presently available on a commercial spaceborne SAR system. The TerraSAR-X Staring Spotlight mode provides a means to assess man-made objects more precisely. Image measurements of size, shape and positions are more accurate, target interpretation is more reliable. 38)
• June 2014: LTSM (Long-Term System Monitoring) of the TerraSAR-X and TanDEM-X missions.
- Instrument operations: Continuous monitoring of both SAR instruments in orbit is required to detect degradations of the satellite hardware and to compensate them by adapting the respective parameters. Therefore, the instrument status of TSX-1 and TDX-1 is checked regularly. Main source is the telemetry data of the satellites, downlinked via S-band. In addition, complementary ground segment data is evaluated in order to derive regular statistics on instrument load (commanded and executed acquisitions) etc.
- Monitoring of TRMs (Transmit/Receive Modules): For characterizing individual TRMs simultaneously a method based on orthogonal codes is applied. Regular antenna health checks, based on the automated acquisition of special system datatakes at regular intervals, monitor the TRM transmit and receive gain, as well as transmit and receive phase for both instruments in-flight. Possible degradation or drifts can be found by depicting gain and phase trends over time. As an example, one of these 8 parameters, the phase deviation with respect to a reference value on receive on TSX-1 is plotted in Figure 26 versus datatake execution time for 383 of the 384 TRMs. The remaining TRM was deactivated prior to launch and is not monitored.
All active TRMs work within the established limits and no trend can be observed, indicating the stability of the TSX-1 instrument and the TRM settings, respectively. The amplitude deviation (1σ) stays below 0.1 dB and the phase deviation under 1° for all TRMs.
Figure 26: PN-gating results of TSX-1 TRMs: Deviation of Rx phase of each individual TRM from the reference value (image credit: DLR)
- Monitoring of front-end temperatures: This activity shows that the instrument is operated in its space qualified thermal conditions. Figure 27 shows the daily maximum temperatures of the 12 front-end antenna panels of each instrument. The measured panel temperatures are far from the limit temperature of 30°C for nominal performance. The temperature peak observed in both plots shows the instrument thermal behavior in extreme conditions during a Hot-Cold Test executed during the TDX-1 Commissioning Phase. Furthermore, the temperature plots clearly reflect the start of the operational TanDEM-X DEM acquisition phase in December 2010.
Figure 27: Maximum daily front-end panel temperatures (hot-spot) over time. Left plot: TSX-1, right plot: TDX-1 (image credit: DLR)
- Instrument performance: SAR performance parameters are monitored continuously in order to provide long-term statistics and to prove the quality of the SAR data products. In a first step, each nominal datatake is checked immediately after screening of the raw data. In this screening process a number of limit checks is performed on selected statistical performance parameters. Limit violations will trigger immediate notifications.
Datatake verification combines information from both the datatake ordering as well as the datatake reception and processing chain. It comprises a verification of: a) Completeness of raw data and correctness of source packet sequence; b) Completeness of scene coverage; c) Raw data saturation/clipping level; d) Raw data statistics of in-phase and quadrature channel data; e) Doppler Centroid estimation.
The LTSM system is accessing the results of datatake verification and provides overall statistics and trend analyses. Similar to the handling of instrument operations data, occurred events are summarized and visualized at regular intervals. Two examples shall be shown in the following sections: The monitoring of Doppler Centroid and raw data statistics.
- Doppler centroid: SAR image quality is affected by the Doppler centroid. The main contribution, an effective squint angle due to Earth rotation, is compensated by Total Zero Doppler Steering. The residual Doppler centroid is estimated for each data take by the operational SAR processors.
As the Doppler centroid frequency of the SAR signal is related to the location of the azimuth beam center, the evaluation of Doppler estimations over a number of datatakes can reveal antenna mispointing in flight direction. This sensitivity of the SAR system has been exploited in the monostatic commissioning phase of the satellites.
A long-term measurement of the Doppler centroid is shown in Figure 28. The mean Doppler values are concentrated around 0 Hz mainly (95 % of the total acquisitions) in a tube of ±120 Hz, providing a stable image quality over the mission time. The data collected over the operational mission time does not show any trends. Outliers could be identified as non-nominal satellite conditions (e.g. GPS anomalies).
Figure 28: Maximum and minimum Doppler centroid for each datatake / polarization channel since 2011 (image credit: DLR)
- Raw data statistics: The complex data collected in in-phase (I) and quadrature-phase (Q) channels is not completely free of biases or cross-coupling (non-orthogonality) between the I and Q channels, introduced by the receiver electronics. This can be estimated by collecting statistics of the SAR raw data. Figure 29 shows an evaluation of the bias in the I- and Q-channel of the TDX-1 SAR raw data. These results show once again the accuracy of the TSX-1 and TDX-1 SAR systems, as well as their high stability over the mission lifetime.
Figure 29: I and Q channel bias of TDX-1 in 2013 (image credit: DLR)
The measurements and the extended analyses performed for long-term system monitoring of both SAR systems TerraSAR-X and TanDEM-X show a very high stability of the instrument performance. Since launch of the respective satellite, no degradation in the performance of both instruments has been observed. All parameters show a constant behavior. Hence, by all these measurements performed for LTSM it can be concluded that TerraSAR-X and TanDEM-X could be characterized and adjusted precisely, achieving at the end a highly accurate and stable SAR System (Ref. 39).
• January 09, 2014: For ten days, 74 scientists and tourists were trapped in the Antarctic on board the Russian Akademik Shokalskiy research vessel. Strong winds had driven ice floes into a bay, blocking the ship's advancement. High-resolution satellite data of TerraSAR-X provided by DLR (German Aerospace Center) helped to assess the ice conditions at the location. 40)
In pack ice, the situation can change quickly when the wind shifts. This is why researchers from the DLR Earth Observation Center (EOC) use up-to-date, high-resolution images from the Earth observation satellite TerraSAR-X to provide the crew of the research vessel with up-to-date information regarding the ice conditions. The German radar satellite operates in a variety of modes to permit imaging with varying swath widths, resolutions and polarizations.
Seeing through clouds and darkness, the satellite is able to observe the ocean and frozen waters from an altitude of around 500 km, providing a swath width of 30 km. To do this, it emits microwaves that are reflected back to the satellite in a way that depends on the characteristics of the reflecting surface. The technology provides an extremely high resolution image of down to 3 m. This is crucial, as the ice structure may change greatly over just a few hundred meters.
Faced with the situation of the Akademik Shokalskiy, the DLR ground station processed the satellite images in near real time and transmitted them to the rescue center in Australia just one hour after acquisition of the Antarctic scenes. Scientists from the DLR Microwaves and Radar Institute (IMF) used TerraSAR–X to acquire images of the trapped research ship on 1 January 2014. Software at the DLR Research Center for Maritime Safety in Bremen was used to track the ships, by utilizing the contrast and differing textures of the vessel and sea ice to detect the vessels amongst the frozen masses. Assessing the ice can yield a wealth of information on its thickness and properties, for instance whether two floes have collided to form a ridge. Even icebreakers have a tough job making their way through heavier layers such as these.
The Chinese icebreaker Xue Long finally arrived to assist the Akademik Shokalskiy. But the ship could only get to within sight of the trapped research vessel before the icebreaker itself was penned in by the ice masses. On 3 January 2014, a helicopter was dispatched from the Xue Long to transport the passengers on board the Russian research vessel to the Australian icebreaker Aurora Australis, waiting out in open waters. Both icebreakers have since succeeded in breaking free from the ice under their own power.
Figure 30: The pack ice zone enclosing the two ships (zoomed in) Akademik Shokalskiy and Xue Long (image credit: DLR)
• October 2013: To comply with increased requirements on data freshness, especially from the MERS (Maritime and Emergency Response Services) segments, Astrium Geo-Information Services / Infoterra GmbH has constantly been upgrading TerraSAR's ground station network access. As a benefit, especially through improved polar station access and processing capabilities, NRT (Near-Real-Time) delivery requirements can be served since early 2012. 41)
- ST (Staring Spotlight) mode,available since October 15, 2013, with resolution: 0.25 m (azimuth) x 0.8 m to 1.77 m (range); scene size: 2.1 to 2.7 km (azimuth), 7.5 to 4.6 km (range); single polarization: (HH, VV).
- SCW (ScanSAR Wide) mode, available since July 14, 2013, with resolution: 40 m (azimuth) x 6 to 10 m (range); scene size: 200 km (azimuth x 194 to 266 km (range); single polarization (HH, VV, HV/VH). The new TerraSAR-X Wide ScanSAR mode (SCW) provides an overview of an area of up to 400,000 km2 within a single acquisition - anywhere and independent of weather conditions. Wide ScanSAR data is thus ideally suited for monitoring of ship traffic, detection of oil spills, monitoring of maritime assets and sea ice, contributing to the security, safety and efficiency of maritime activities around the globe.
Table 6: TerraSAR-X High Resolution SpotLight Modes
Figure 31: Principle of Staring Spotlight, Sliding Spotlight and Stripmap Mode (image credit: DLR)
Figure 32: Staring (left) vs. Sliding (right) Spotlight, extension of image example 380 m (Az) x 350 m (Rg), incidence angle 41º (image credit: DLR, Astrium)
Astrium GEO-Information Services is now also working with Hisdesat, the Spanish government satellite service operator of the PAZ radar satellite to establish a constellation approach with TerraSAR-X and PAZ which will be operational in 2014. Operating the two virtually identical satellites as a constellation will enhance a wide range of time-critical and data-intensive applications through shorter revisit times and increased data acquisition capacities. 45)
Figure 33: Schematic view of the TerraSAR-X / TanDEM-X / PAZ constellation (image credit: Astrium, Hisdesat, Ref. 41)
bull; August 30, 2013: With a spacecraft design life of 5 years, TerraSAR-X should have been out of service for over a year and a half now (launch on June 15, 2007). However, engineers at DLR have switched the satellite to yet another mode: TerraSAR-X can now record image strips over 200 km wide (in ScanSAR Wide mode, also referred to as SCW). The satellite does so by sweeping this large area in multiple stages, very quickly pivoting the radar beam numerous times across the direction of flight. For example, the image of the German Bight shows the Frisian Islands from Borkum to Wangerooge and cities such as Wilhelmshaven and Bremen (Figure 34). This new ‘wide-angle' mode is of particular interest to oceanographers, who will be able to use it to investigate the tidal range, changes to mudflats, shipping movements, wave patterns, ice floes and wind levels. 46)
- TerraSAR-X has already delivered more than 120,000 images since being launched. However, the image strips from the TerraSAR-X satellite have been limited to a width of 100 km so far. For the first time, DLR is able to acquire an image of the entire German Bight from east to west, at a single point in time and in high resolution. The wide-swath radar imagery is providing the oceanographer with a great deal of information on the tidal flat and associated inlets between individual islands and the coast, as well as on the high water level in the Elbe estuary and near the island of Sylt. Further to the north, the satellite shows Sylt and numerous wind farms, where wind turbines appear as geometrically arranged bright points in the black and white image (Figure 35). Individual ships can also be made out in the radar images, which means that, with a resolution of 40 m, the Wide-ScanSAR mode can also be used for monitoring shipping routes.
- The operational condition of TerraSAR-X spacecraft and its payload is still very good and the fuel reserves should enable the mission to continue operating until at least 2015.
Figure 34: TerraSAR-X image of the German Bight in the Wide Scan mode (image credit: DLR)
Figure 35: Radar images of wind parks in the German Bight (image credit: DLR)
• June 2013: Following severe flooding in northern India and Nepal, the Indian government activated the 'International Charter Space and Major Disasters on 19 June 2013. DLR tasked its radar satellite TerraSAR-X with acquiring images of the affected areas and made these available to the Indian civil protection authorities. 47)
In India, the situation is far worse than initially thought. The heavy rains surprised the people in the disaster areas. So far, the floods are known to have killed more than 680 people and thousands are still missing; about ten thousand military personnel have been deployed. The biggest rescue operation in the history of the Indian military is underway. The effects are especially bad in the mountainous state of Uttarakhand, where the Ganges River and its tributaries have flooded. TerraSAR-X has imaged this region over the last few days.
Figure 36: Observation of the June 2013 floods with TerraSAR-X in the North Indian states of Uttarakhand and Himachal Pradesh (image credit: DLR)
• The TerraSAR-X spacecraft and its payload are operation nominally in 2013.
• November 2012: Since January 2011, the Earth under the Santorini volcano has been stirring. Most of the time, it is barely noticeable, but every now and then the inhabitants notice small tremors jolting the volcanic archipelago. Nearly circular, and seemingly carved from stone, the submerged caldera is located in the Aegean Sea (Mediterranean, Greece).48)
Funded by the UK National Environment Research Council, radar specialist Juliet Biggs, Parks and volcanologist David Pyle (University of Oxford) began to study the Santorini volcano closely. Using GPS receivers, they determined precise locations with millimetric accuracy on a daily basis. The TerraSAR-X radar satellite also observed the archipelago from orbit, at an altitude of 514 km, recording its uplift and expansion from one orbit to the next. The results showed that the Kameni islands had risen 8 to 14 cm in many places. The breadth of the caldera as a whole has increased by about 14 cm since early 2011. In the analysis of the radar data (Figure 37), the red and yellow shading shows the areas where the ground has risen the most. The main island of Thira is unaffected by the deformation, thus appearing blue.
The researchers believe that a magma chamber has formed at a depth of 4 km. They must now combine the information on the volcano's present behavior with the knowledge of previous eruptions. The TerraSAR-X radar satellite has provided important information in this regard.
Figure 37: TerraSAR-X image of the month (Nov. 2012) – the Santorini volcano expands (image credit: DLR)
Figure 38: TerraSAR-X image of the Bonneville Salt Flats, located to the west of the Great Salt Lake in Utah, USA (image credit: DLR)
Legend to Figure 38: The image was acquired on June 23, 2009. The scene measures 50 km x 30 km. TerraSAR-X also cast a penetrating eye on the around 2000 m high mountains and the salt lake, which lies at an altitude of some 1270 m. The image shows rough surfaces in orange and smooth ones in grey/black. - The large, black surface bordering the industrial area in the middle of the image is the Wendover Facility. Large-scale industrial extraction of brine takes place here, which is needed for manufacturing potash. Next to the city of Wendover is the airport, which was an air force base until 1965. Even from an altitude of over 500 km, TerraSAR-X can detect the fine, parallel orange lines extending from the airport, as well as other transportation routes such as Highway I-80 and the parallel railway running from east to west (east is at the bottom is the image, the I-80 intersects the image in the middle).
• On June 15, 2012, the TerraSAR-X mission completed its 5th year on orbit. Designed to operate for five years, the satellite has now completed its nominal service life but it remains in excellent condition; it is expected to continue functioning for several more years.50)
Over the past five years, the German TerraSAR-X satellite mission has successfully supported or enabled a wide range of relief efforts and projects. Since June 2010, the satellite has been in good company; TerraSAR-X has been orbiting the Earth in close formation with its almost identical twin, TanDEM-X. Together, they are creating a highly accurate digital elevation model of Earth. With its own unchanged mission targets still in focus, TerraSAR-X has been meeting all expectations here as well (Ref. 50).
• In January 2012, the TerraSAR-X satellite is fully operational and continuous its close formation flight with the TanDEM-X spacecraft. After a year of formation flight of TerraSAR-X, with TanDEM-X, the twin satellites have completely mapped the entire land surface of Earth for the first time. The data is being used to create the world's first single-source, high-precision, 3D digital elevation model of Earth. DLR controls both radar satellites, generates the elevation model, and is responsible for the scientific use of TanDEM-X data. 51)
Figure 39: TerraSAR-X radar image of the Mackenzie River in the Northwest Territories of Canada, acquired on January 31, 2012 (image credit: DLR)
Legend to Figure 39: Ice and snow can be colorful - when observed by TerraSAR-X. The radar signals are able to penetrate the snow cover to a depth of one ~ 1 meter – and the subsurface reflects the pulse in different ways. This makes the frozen delta of the Mackenzie River in Canada appear multi-colored in an image revealing the various structures in the landscape underneath the snow. 52)
From the Great Slave Lake to the Arctic Ocean, the Mackenzie River snakes its way for 1900 km through the Northwest Territories of Canada. During the few ice-free months of the year, the river flows gently through the flat landscape. But when the Arctic winter arrives, everything comes to a standstill – on the surface at least. The polar night would make it impossible for an optical satellite to image the ice world of the Mackenzie River; even when the scant daylight permits a view of it, the entire landscape appears uniformly white in optical images. But the TerraSAR-X satellite image from 31 January 2012 depicts the landscape in violet, blue and green.
Appearing as a grey-green surface in the image, the structure of the vegetation in the barren tundra, which consists of grass and dwarf shrubs, contrasts clearly with the frozen river. River islands, which are only sparsely vegetated, also show signs of sand and gravel bars or dunes – the bright violet coloring in the radar image makes this subsurface clear. From its orbit 514 km above the Earth, TerraSAR-X can even observe variations in the roughness of the ice – surfaces that are actually white display numerous shades of color in the radar image. Deep-frozen lakes and pools to the left and right of the river look like shimmering violet mirrors, because ice on standing water is especially flat and reflects the majority of the radar signals back to the satellite's receiver. In contrast, the frozen Mackenzie River, with its irregular ice layer, reflects radiation back quite differently and appears blue in the radar image. This landscape contains what is referred to as hummock ice and smallish ice floes that have been pushed onto and against one another. The numerous corners and edges reflect the radar signals back to the satellite particularly well. Ice surfaces that were particularly heavily distorted during their formation are visible in shades of yellow. The depth of the lakes and thickness of the ice layer also play a part in this colorful winter landscape – shallow lakes that are frozen to the bottom are colored differently to lakes that still have liquid water under their ice (Ref. 52).
The colorful winter landscape serves a particular purpose for researchers, as they can use various images of the same region to track movement – when the river landscape freezes, when the ice sheet begins to break up again and when the thaw begins. The duration and intensity of this icy period are important indicators for climate research.
• DInSAR (Differential SAR Interferometry) study in 2011. TOPS (Terrain Observation with Progressive Scan) data was used in the processing chain to measure ground displacement movements by means of DInSAR. The investigation analyzed a stack of 8 TOPS and 8 stripmap images in terms of time-series performance for subsidence estimation. The estimated deformation used the SBAS (Small BAseline Subset) technique, which takes into account possible DEM errors and the APS (Atmospheric Phase Screen), have shown a good agreement between the TOPS and stripmap results. The investigation of the deformation using TOPS-stripmap cross-interferograms has also been performed and successfully exploited by means of CS (Coherent Scatterers). The results yield that, as expected, the phase is preserved for CS even if there is no spectral overlap.53)
• In January 2011, TerraSAR-X is fully operational and in close formation flight the TanDEM-X spacecraft. The two spacecraft provide a single-pass interferometric configuration, which was declared operational in December 2010. The collection of data for a global homogeneous DEM started - as planned - in early 2011.
Figure 40 is the image of the month of July 2011 of TerraSAR-X. The Puyehue volcano erupted on June 4, 2011 in the southern Andes mountains. A field of lava, appearing as a uniform, light blue surface, is currently forming there. Radar images acquired by TerraSAR-X have been providing valuable information to the staff of the Chilean Volcano Risk Program since the eruption began, helping them to assess the situation and predict its future development. 54)
Figure 40: TerraSAR-X image of the Puyehue volcano in Chile on July 6, 2011, one month after its eruption (image credit: DLR)
Legend to Figure 41: The Bosphorus (or Bosporus), also known as the Istanbul Strait, forms part of the boundary between Europe and Asia. It is one of the Turkish Straits, along with the Dardanelles. The world's narrowest strait used for international navigation, it connects the Black Sea (on top of the image) with the Sea of Marmara (which is connected by the Dardanelles to the Aegean Sea, and thereby to the Mediterranean Sea).
• The TerraSAR-X spacecraft and its payload are operating nominally as of 2010.
• In January 2010, imagery of the TerraSAR-X spacecraft as well as optical imagery from other spacecraft was being used by a DLR/DFD analysis team to support the disaster relief activities of the devastating earthquake that hit Haiti on Jan. 12, 2010. Satellite-based maps were generated of the stricken region and provided to the relief organizations via internet. In the absence of any or very little information, the current state of the infrastructure in the capital city of Port-au-Prince was of great service to the relief workers. The quickly generated reference maps were giving an overview of the road network as well as of important buildings and facilities such as the airport as they were before and after the earthquake. 56)
• In October and November 2009, high-resolution TerraSAR-X data were acquired in Antarctica in the left-looking observation mode. The areas of scientific interest were located within glacier basins and ice streams that flow through the Transantarctic Mountains and into the Ross Ice Shelf. Detailed ice velocity patterns on the Nimrod glacier basin and Starshot glacier were studied. 57)
• In the summer of 2009, two years after launch, two dedicated calibration campaigns were performed, one for re-calibration of TerraSAR-X, and one for the experimental DRA (Dual Receive Antenna) mode. The effective and exact calibration techniques already successfully applied for the commissioning of TerraSAR-X in 2007 have been shown once again how accurately the complex TerraSAR-X system can be adjusted. Moreover, deriving the stability of the whole SAR system by real measurements two years after launch, the accuracy could be improved further on. - In total, 40 campaigns against reference targets and about 150 acquisitions across the Amazon rainforest were successfully executed and evaluated. The stability and the accuracy of the whole TerraSAR-X system and especially the radar instrument itself is still of unprecedented quality. 58)
In particular, the radiometric stability could be derived by real measurements, i.e. by a comparison of measurements performed in summer 2009 with those performed during the commissioning phase in 2007. The slight offset of only 0.15 dB over a period of two year is more than 10 times better than the requirement with 0.5 dB over six month. This improves also the specified absolute radiometric accuracy down to 0.39 dB for StripMap and down to 0.52 dB for ScanSAR basic products.
• In 2009, the measurement of ground object motions with the SAR ATI (Along-Track Interferometry) data acquisition capability was demonstrated from a spaceborne SAR instrument in different contexts, two typical applications are traffic flows and water surface currents. 59) 60)
- With regard to traffic flow, the focus lies on detecting a number of small (compared to the image resolution) separate moving objects to derive traffic information for a whole area or larger sections of a road network from it. With its large-area data acquisition and weather and daylight independence, SAR offers great potential to augment existing networks of traffic sensors or sometimes to be the only source of traffic data. For traffic measurements, the task is to detect objects of interest at first within the clutter and then to estimate their velocity and true position. Typical object velocities range from 10–50 m/s.
- For surface current measurements (on larger water bodies), the velocities are 1-2 orders of magnitude smaller. Here no detection is needed, because we deal with a distributed radar target of large extent and of more or less known position.
An automatic traffic data extraction system with near-real time (NRT) capability was developed. This TTP (TerraSAR-X Traffic Processor) includes SAR focussing, vehicle detection and measurement for public roads as well as the generation of an easily distributable traffic data product. The TTP makes use of GIS data at different stages of processing. Road data are extracted from a data base for the processed scene and enable to restrict processing to only relevant image areas, to enhance moving object signatures by adaptive filtering of the SAR data and to provide velocity measurements for detected objects based on azimuth displacement. Vehicles are extracted using a combination of ATI and DPCA detectors.
- Spacecraft and ground segment are fully operational
- Image products (Spotlight, Stripmap, ScanSAR) are calibrated and released. The product quality is within initial specification or better
- Operation of the SAR instrument proved to be very stable
- Demonstrations accomplished: Repeat pass interferometry, along-track interferometry, persistent scatterer evaluation, TOPSAR (while TOPSAR was demonstrated, implementation is pending), total zero Doppler steering, and demonstration of quadpol mode.
- Demonstration of quadpol mode
- The use of TerraSAR-X data was demonstrated for geo-scientific applications, oceanography and disaster monitoring during commissioning phase.
• LCTSX (LCT on TerraSAR-X) FSO (Free Space Optics) communication demonstrations: In a series of ISL (Intersatellite Link) tests that began in Jan./Feb. 2008 (and continued for several months), the LCT (Laser Communication Terminal) on TerraSAR-X as well as the one flown on the DoD NFIRE (Near Field Infrared Experiment) spacecraft, have exchanged data simultaneously at rates of 5.625 Gbit/s ((equivalent to ~200,000 A4-pages per second). According to Tesat-Spacecom GmbH, it has taken < 25 seconds , on average, for the terminals to lock onto each other and begin transmissions. A key feature of the system is its ability to establish and maintain a link, even when the sun is directly behind the target spacecraft. 65) 66) 67) 68) 69)
The first LEO-LEO intersatellite link was performed above the Pacific Ocean near Central America as shown in Figure 42. Continuous free-space optical transmissions were maintained for as long as the two spacecraft were within line-of-sight position of each other (both spacecraft in LEO) amounting to about 20 minutes on an average pass. On these free-space transmissions, the measured BER (Bit Error Rate) was < 10-9. Since the NFIRE spacecraft is not producing its own imagery, a closed-loop link was configured where the data from TerraSAR-X was directly re-transmitted from NFIRE to establish "duplex operations" at 5.625 Gbit/s. - Further tests are planned with transmissions to ground stations in Germany and in Spain.
The orbits of the two LEO satellites (TerraSAR-X and NFIRE) propagated in opposite directions to each other. This required the LCT to track its counter terminal across an azimuth range of about 80º. The elevation range was about 10º. The link distance varied between 3,700 km and 4,700 km, with a maximum range rate of 8,500 m/s.
The counter LCT was visible for 217 s. Spatial acquisition started after 42 s with uncertainty cones of 530 µrad and 1000 µrad and was closed after 13 s. It took 28 s to lock the phases for homodyne BPSK. The ISL communication with a bit error rate better than 10-9 lasted for 134 s until the counter LCT was no longer visible.
As was verified in later experiments the pointing accuracy of the LCT allows to close spatial acquisition between NFIRE and TerraSAR-X significantly faster than 10 s. Frequency acquisition has been optimized to lock the phases within 20 s. The bit error rate was always is better than 10-9.
• TOR (Tracking, Occultation and Ranging) payload: The TOR radio occultation measurements were activated permanently onboard TerraSAR-X on Feb. 16, 2009.
- The IGOR receiver of TOR was powered up shortly shortly after deployment of the TerraSAR-X mission delivering continuously tracking data for POD (Precise Orbit Determination) support. From independent precise orbit computations performed by GFZ and DLR, a 3D accuracy of < 10 cm can be estimated from comparisons of SLR data that are globally distributed. The IGOR derived orbit fits better than 4 cm with laser ranging residuals. 70) 71)
- The radio occultation measurements (TOR-RO) were enabled for a test period of 4 weeks from January 15 through February 15, 2008. This campaign yielded a daily total of about 250 neutral atmospheric profiles for temperature and humidity as well as additional ionospheric data of the vertical electron density distribution.
- SLR measurements tracking the TerraSAR-X spacecraft are being regularly performed by the global SLR community. The ranging campaign yielded a total of 735 laser passes for a period between June 16, 2007 and Oct. 1, 2007. Single shot accuracies of 3-4 cm are being reported from the especially equipped ground station at Graz, Austria.
• A bistatic X-band experiment was successfully performed in early November 2007 (Figure 43). TerraSAR-X (TSX) was used as transmitter and DLR's new airborne radar system F-SAR, which was programmed to acquire data in a quasi-continuous mode to avoid echo window synchronization issues. The F-SAR system was used as bistatic receiver in this configuration. Precise phase and time referencing between both systems, which is essential for obtaining high resolution SAR images, was derived during the bistatic processing. The experiment was considered a success after data analysis. 72) 73)
For the first time, a spaceborne-airborne X-band acquisition has been successfully conducted, including high-resolution SAR processing. The bistatic image shows an improved resolution, SNR and no range ambiguities, as well as a different perspective of the imaged scene.
Figure 43: View of the backward scattering configuration (not drawn to scale) of the bistatic TSX / F-SAR experiment (image credit: DLR)
• A successful ORR (Operational Readiness Review) of TerraSAR-X took place on Dec. 13-14, 2007 with all systems tested and validated - and with nominal operations of the spacecraft. The mission was officially declared operational as of Jan. 7, 2008. Scientists and engineers from DLR and EADS Astrium have spent the past few months calibrating and commissioning the satellite . They appear to be completely satisfied with the exceptional performance of the TerraSAR-X system. It turned out that the commissioning phase was completed successfully right on schedule. 74) 75) 76)
- The results of calibrating TerraSAR-X approve the accuracy calculated before launch and put the described strategy to calibrate efficiently a multiple mode SAR system like TerraSAR-X on a solid base. The key element of this strategy is an antenna model approach and TerraSAR-X is the first SAR satellite calibrated with this innovative method. It has been shown that all calibration systems are working very well. The stability and accuracy of the system and especially the radar instrument itself is of unprecedented quality. - All requirements and/or goals have been achieved even better than predicted. By this successful demonstration of an effective and exact calibration technique a new benchmark has been settled not only for calibrating complex SAR systems but in principle for future highly accurate spaceborne SAR sensors like TanDEM-X or Sentinel-1. 77)
- The SAR products were operationally released 5.5 months after launch. This was possible due to an intensive combined test program of the ground segment and the space segment. The CP (Commissioning Phase) planning tool allowed flexible planning and re-planning throughout the CP. Over 12000 DTs (Data Takes), i.e. scenes, were acquired and processed.
- The main objectives of the CP were the calibration and verification of the entire SAR system chain in order to achieve the specified SAR image and product quality as well as the operationalization and validation of the ground segment functions. This involved in particular the tuning and adjustment of the TMSP (TerraSAR-X Multi-Mode SAR Processor) to meet the in-orbit data characteristics and to optimize the SAR focusing results. The TMSP consistently generates phase-preserving SSC (Single-look Slant-range Complex) data sets from the imaging modes Stripmap, ScanSAR and Spotlight for all specified polarization modes. Derivation of multi-look detected products (MGD, GEC and EEC) is based on SSCs as an interim production stage, depicted in Figure 44. 78)
Figure 44: Overview of the TMSP processing concept (image credit: DLR)
• Demonstration of the novel TOPSAR (Terrain Observation with Progressive Scan) SAR operations support mode concept on TerraSAR-X (on behalf of ESA). The TOPSAR technique employs a very simple counter-rotation of the radar beam in the opposite direction to a "spot observation"; hence, the name TOPS (Terrain Observation with Progressive Scan). The first TOPSAR images and interferometric results were demonstrated on the TerraSAR-X spacecraft during the commissioning phase in the fall of 2007. TerraSAR-X was able to provide this demonstration because its TSX-SAR instrument because it was able to electronically steer the antenna azimuth pattern. This capability, together with the high flexibility of the satellite commanding, provided the opportunity to implement on the satellite the TOPSAR acquisition mode. 79) 80)
Note: TOPSAR is an ESA-proposed acquisition mode for wide swath imaging which aims at reducing the drawbacks of the ScanSAR mode. The basic principle of TOPSAR is the shrinking of the azimuth antenna pattern (along-track direction) as seen by a target on ground. This is obtained by steering the antenna in the opposite direction as for Spotlight. The TOPSAR mode is intended to replace the conventional ScanSAR mode. The technique aims at achieving the same coverage and resolution as ScanSAR, but with a nearly uniform SNR (Signal-to-Noise Ratio) and DTAR (Distributed Target Ambiguity Ratio). 81) 82) 83)
The TOPSAR technique will be used by the ESA Sentinel-1 C-SAR sensor. The Sentinel-1 spacecraft is part of the Copernicus (formerly GMES space component) in its main interferometric wideswath mode (planned launch in 2014).
Legend to Figure 45: Two TOPS data takes have been acquired over a flat and high coherent region. The chosen area is the Uyuni salt lake, Bolivia, one of the largest in the world. The data takes were recorded on October 10th, 2007 and October 21st, 2007.
• Contact with the spacecraft was established shortly after launch. All systems are functioning nominally. Just four days after the launch, brilliant first satellite images have been received (June 19, 2007). All standard imaging modes (strip, spot & scan) were exercised in this early phase. Hence, LEOP (Launch and Early Orbit Phase) was successfully completed on June 21, 2007. Thereafter, TerraSAR-X was put into its commissioning phase; it was expected to remain in this status until the end of 2007.
Figure 46: The first TerraSAR-X image of the Tsimlyanskoye reservoir, Russia, taken on June 19, 2007 (image credit: DLR)
Some notes to Figure 46: In the upper half of the image, the Tsimlyanskoye reservoir can be seen. Here the River Don is dammed with the water being used for power generation. In the upper right corner of the image, a channel with a weir is visible. In the immediate neighborhood, the meandering oxbow river bends can be seen as dark surfaces. Calm water surfaces are typically very dark in radar photographs, since the radar radiation hitting them is reflected away. In the center-left of the image, a railway bridge over the River Don can be seen with the railway line disappearing towards the northwest. 84)
In the lower half, large, agricultural areas dominate. The fields form regular patterns, meandering tributaries can be seen. The different brightnesses result from the varying vegetation and the particular stages of their annual growth cycles.
During this survey, a thick cloud cover prevailed. Nevertheless, radar satellites such as TerraSAR-X offer imaging capability even in case of cloudy skies and at night. However, exceptional strong precipitation events like heavy thunderstorms may influence even radar imaging. Such an event can be seen at the upper left part of the radar image as a bright "veil".
TerraSAR-X sensor complement (TSX-SAR, TOR, LRR, LCT):
TSX-SAR (TerraSAR-X SAR instrument). TSX-SAR is an active phased array X-band antenna system providing high-resolution and multipolarization SAR imagery (H and V), permitting the operational modes of "stripmap," "spotlight," and "scanSAR." The beam-forming capability and quality of the active phased array technology introduces a range of flexibility, permitting the acquisition of high-resolution imagery as well as of wide-swath imagery. The active phased array front-end is structured in azimuth direction (along-track) into three antenna leafs, each comprised of four antenna panels. One antenna panel is made up of 32 active sub-arrays in elevation, each comprising an HP (Horizontal Polarization) and a VP (Vertical Polarization) slotted waveguide radiator. Each of the 384 sub-arrays (32 x 12) is equipped with a T/R (Transmit/Receive) module - also referred to as TRM. The dual-polarized waveguide radiator allows the polarization selection via a polarization switch in the T/R module (TRM). In toggle mode it can switch the polarization from pulse to pulse. This allows for simultaneous acquisition of two image polarizations. 85) 86) 87) 88) 89) 90) 91) 92)
The front-end is controlled by ACE (Antenna Control Electronics), providing programmable real-time control of the antenna beam shape, pointing and polarization in transmission, and reception. For each commanded antenna beam, one of 256 stored elevation beam configurations is combined with one of 256 azimuth beam configurations; the resulting excitation coefficients are transferred to the T/R modules. Beam steering in azimuth (± 0,75º) and elevation (± 20º) is performed by ACE, which provides programmable real-time control of antenna beam shape, pointing and polarization in transmission and reception. The switching of a antenna beam can take place at a maximum rate of 275 Hz. ACE is controlled by CE (Control Electronics), consisting of DCE (Data & Control Electronics), ICU (Instrument Control Unit) and RFE (Radio Frequency Electronics). RFE contains the USO (Ultra Stable Oscillator), the up- and down conversion and preamplifying stages and provides programmable signals for internal calibration. CE provides the following functions:
• Generation and transmission of the Tx signal
• Reception and A/D conversion of the Rx signal
• SAR data buffering, compression and formatting
• Instrument timing and control.
The transmit signal is produced in a digital chirp generator. An AWG (Arrayed Waveguide Grating) writes up to 8 different waveforms of commanded length and a bandwidth of up to 300 MHz in a waveform memory. One of these 8 waveforms is selected for each pulse and can be switched from pulse to pulse. In the receive path, one of three anti-aliasing filters which is matching to the ADC sampling rates of 110, 165 or 330 MHz, can be selected. The data are compressed online with a BAQ algorithm, after the time extension buffering. The BAQ processing works on blocks of 128 consecutive samples with a selectable compression rate of 8 to 4, 3, 2 bits per sample. A transparent mode also allows to bypass the data compression.
Figure 47: Functional block diagram of the TSX-SAR instrument (image credit: DLR, EADS) 93)
Figure 48: Illustration of a SAR antenna panel (image credit: EADS Astrium)
Figure 49: RF architecture of the XFE (X-band T/R Frontend) of TSX-SAR (image credit: EADS Astrium)
Figure 50: TerraSAR-X T/R Frontend (image credit: EADS Astrium)
The TSX-SAR instrument elements are fully redundant, i.e. a main and a redundant functional chain exists. This feature makes it possible to activate both functional chains at the same time, one being the master for timing purposes, thus permitting operations in an experimental DRA (Dual Receive Antenna) mode where the echoes from the azimuth antenna halves can be received and then separated during ground processing, e.g. to serve the application of ATI (Along Track Interferometry). For ATI support the SAR antenna can be grouped into two segments, each of 2.4 m. 94) 95) 96)
Figure 51: Illustration of DRA mode scheme (image credit: DLR)
Due to the bistatic configuration (one transmitting antenna, two receiving antenna elements), the effective ATI baseline is half antenna separation, i.e. 1.2 m, which corresponds to a time lag of 0.17 ms. Ideal ATI time lags for oceanic current measurements at X-band should be on the order of a few milliseconds, i.e. about 20 times longer than this; thus, the sensitivity of TSX-SAR to small current variations will be quite low. The ATI feature permits the measurement of surface currents with a spatial resolution of about 1 to 2 km. 97)
Table 7: TSX-SAR instrument parameters
TSX-SAR provides a variety of Spotlight, Stripmap and ScanSAR image products. The full operator access to the active phased array antenna together with the 300 MHz mode allows for a large number of custom-designed high-performance image products. 98)
The spotlight mode is realized on a "sliding spotlight operation" concept. 99) Compared to starring spotlight operation, sliding spotlight has the advantage of more uniform NESZ (Noise Equivalent Sigma Zero) performance achieved in along-track direction due to the averaging of the gain variation of the main beam in azimuth. The signal bandwidth is maintained constant for the whole incidence angle range in order to provide a constant slant range resolution.
Table 8: Performance overview of the TSX-SAR instrument products
The primary image performance parameters, provided in Table 8, are NESZ (Noise Equivalent Sigma Zero) and DTAR (Distributed Target Signal to Ambiguity Ratio). They show performance variations within the image as well as over the incidence angle range. NESZ is that value of sigma nought (σo) of a uniform scene that produces a processor output signal to noise of unity. The "access range" is the cross-track accessible (viewable) range on the ground provided by electronic beam steering. The instantaneous range of TSX-SAR is e.g. 30 km (max) in stripmap mode and 100 km for ScanSAR mode. In Stripmap operation, the synthetic aperture is typically given by the -3dB beamwidth of the antenna azimuth pattern defining the processed Doppler bandwidth. In order to provide sufficient performance swath width near nadir, a broadening of the elevation beamwidth is needed. This is achieved by amplitude/phase tapering of the active antenna. 100)
TSX-SAR operations and support modes:
• TSX-SAR is capable to image on either side of the subsatellite track, this is accomplished by a roll maneuver of the spacecraft. The S/C roll feature extends the FOR (Field of Regard) of the instrument for possible event coverage. However, observations to the right side of the subsatellite track are considered to be the preferred (and default) operations mode, due to power constraints in the left-side configuration (the solar array isn't pointing into the sun anymore; also, the communications link to the ground is obstructed).
• There are also experimental dual-receive modes for wide bandwidth (300 MHz), providing even higher resolution, as well as for full polarization and along-track interferometry (ATI), the latter two being achieved by splitting the receive antenna into two azimuth halves (split-antenna stripmap mode). The dual-receive mode provides the potential for ATI velocity measurements of ocean currents, and a full polarimetric mode, by receiving simultaneously the H and V components with the two apertures (the dual-receive mode is also being used to demonstrate traffic velocity measurements on highways). The redundancy concept in the TerraSAR-X receiving chain and the front-end design offer the possibility to use the second spare receiving channel in parallel to the main receiving channel.
• Capability of repeat-pass as well as Along Track Interferometry (ATI)
- SM (Stripmap Mode). The ground swath is illuminated with a continuous sequence of pulses while the antenna beam is fixed in elevation and azimuth. This results in an image strip with continuous image quality in azimuth.
- SC (ScanSAR Mode). SC mode provides a large area coverage. The wider swath is achieved by scanning several adjacent ground sub-swaths with simultaneous beams, each with a different incidence angle. Due to the reduced azimuth bandwidth the azimuth resolution of a ScanSAR product is lower than in StripMap mode.
Figure 52: Overview of the TerraSAR-X scanning modes (image credit: DLR)
- HS (High-resolution Spotlight Mode). HS provides the highest geometrical resolution. Therefore the size of the observed area on ground is smaller than the one in all other modes. During the observation of a particular ground scene the radar beam is steered like a spotlight so that the area of interest is illuminated longer and hence the synthetic aperture becomes larger. The Maximum azimuth steering angle range is ± 0.75º.
- SL (Spotlight Mode). The HS and SL modes are very similar. In SL mode the geometric azimuth resolution is reduced in order to increase the azimuth scene coverage.
• The instrument features selectable or dual polarization (support of single, dual and full polarization modes). Further flexibility is provided by the large number of possible antenna beam configurations. The number of beams in elevation for SM or SC support is about 12. The number of beams in elevation for HS or SL support is about 95.
- Single polarization: The radar transmits either H or V polarized pulses and receives in H or V polarization. The resulting product will consist of one polarimetric channel in one of the combinations HH, HV, VH or VV. It can be operated in all different modes HS, SL, SM and SC.
- Dual polarization: In this mode the radar toggles the transmit and/or receive polarization on a pulse to pulse basis. The effective PRF in each polarimetric channel is half of the total PRF, which means that the azimuth resolution is slightly reduced. The polarimetric phase between both channels can be exploited, e.g. for interferometry or classification purposes. The product consists of two layers that can be selected out of the possible combinations. Dual polarization is possible for all image modes as well.
- Quad polarization: Quad polarization is possible in the experimental dual receive antenna mode as the signal can be received simultaneously in H and V polarization. By sending alternating H and V pulses, the full polarimetric matrix can be obtained. Currently quad-polarization is not operationally foreseen (only research support).
• Left or right side observation capability (Figure 52). The slew capability of TerraSAR-X spacecraft allows observations to be conducted on either side of the sub-satellite track. This feature is of great value for event monitoring - doubling in effect the FOR (Field of Regard).
TSX-SAR instrument calibration:
The calibration process transforms the image magnitude or power into the required physical units, which are assumed to be those of radar cross section (RCS) or back-scattering coefficient σo (sigma-zero, radar cross section per unit illuminated area). Three major tasks are being performed by the calibration as illustrated in Figure 54: 104) 105) 106) 107) 108) 109)
- Internal calibration: The compensation of instrument fluctuations. This is performed by in-orbit verification of the instrument against pre-flight results. This internal calibration yields a stabilized radar instrument and defines the radiometric stability.
- Antenna pattern calibration: The compensation of the antenna pattern within the SAR scene in order to obtain a constant gain across the whole SAR image. This is performed by a determination/estimation of the actual antenna pattern. We thus obtain relatively calibrated SAR data products and the relative radiometric accuracy.
- External calibration: The correction of the radiometric bias. This is performed by measuring the radar system against standard ground targets with known radar cross section (RCS). This external calibration yields to an absolute calibrated radar system and defines the absolute radiometric accuracy.
Figure 53: Illustration of the challenge involved in calibrating a multiple-mode, high-resolution SAR such as TerraSAR-X (image credit: DLR)
Figure 54: Overview of the calibration concept (image credit: DLR)
The internal calibration facility of TSX-SAR monitors the critical elements of the XFE (X-band Front-end), consisting of the 384 T/R modules (TRMs) of the active phased array. Each module is feeding a radiating sub-array for horizontal and vertical polarization - controlling the beam steering in azimuth and elevation direction. Three different types of calibration pulses are applied, whereby sets of these pulses are needed at the start and end of each data take. All calibration pulses have the same length and bandwidth (chirp) as is commanded for the mode.
Precise modelling of the antenna is only possible if the actual characteristics of each individual transmit/receive module (TRM) are known. A calibration network (CAL N/W) records the internal instrument behavior characterizing the instrument stability over time. The antenna performance can be monitored with an innovative characterization mode based on the so-called PN-gating method. 110)
Figure 55: The XFE of TSX-SAR with 4 of 384 TRMs - the calibration signal is routed via couplers at the TRMs and the CAL N/W, (image credit: DLR)
The essential TSX-SAR calibration facilities/references are:
• Standard ground target monitoring for the bias correction
• Ground receivers for in-flight measurements
• Different analysis and evaluation tools
- The antenna pattern model providing an estimation of the actual antenna pattern
- SARCON (SAR Product Control Software), for the point target and distributed target analysis of different calibration targets.
Introduction of new SAR calibration technology: 111)
Due to the multitude of operational modes based on the active phased antenna array with hundreds of T/R modules, a large number of different antenna beams are obtained (some 10,000 for TSX-SAR). For this situation, a conventional calibration approach is not feasible. Hence, DLR/HR developed innovative and efficient calibration methods for the TSX-SAR instrument. The two most important innovations are:
- PN-gating (Pseudo Noise-gating), a new method for internal calibration. PN-gating is a technique of monitoring the gain and phase variations in the transmit and receive paths of individual T/R modules while all 384 modules are operating - representing a characterization under the most realistic conditions with the advantage that all modules can be characterized simultaneously. For this purpose, the instrument is operated in a special module characterization mode. The simulated results of the PN-gating method are very promising (confirmed by ground tests).
- A precise antenna model, based on a new antenna pattern optimization. The modular nature of an active phased array antenna provides the capability of mathematical modeling. Such an antenna model is in fact a software tool that accurately determines the antenna beam patterns based on detailed characterization of the antenna hardware and the knowledge of the antenna control parameters. To achieve the required radiometric quality, highly accurate pre-launch characterization data was needed. After pre-launch validation against the near field pattern measurements and in-flight verification, the antenna model is being used to generate the in-orbit calibrated beam patterns required for radiometric corrections in the SAR processing throughout the satellite lifetime. The antenna model is capable of accurately determining not only the individual beam patterns, but also the relative gain variations from beam to beam. Determination of the absolute gain from measurements over external calibration targets can then be reduced to a few beams. In order to maintain the SAR antenna performance, the following steps are implemented into the TSX-SAR calibration system:
• Compensation of radar instrument drift with the help of internal calibration
• Individual TRM (T/R Module) characterization by using the novel PN-gating method.
The internal calibration facility features a coupling into an additional port of each TRM as shown in Figure 55. Calibration pulses are routed through the XFE to characterize critical elements of the transmit (Tx) and receive (Rx) path. The acquired signals can only be measured at the composite ports of the distribution networks. Three different types of calibration pulses are applied, whereby sets of these pulses are needed at the start and end of each data acquisition. All calibration pulses have the same length and bandwidth as the transmit pulse commanded for the mode. In orbit, absolute power level degradation is calibrated via external targets like transponders or corner reflectors. Thus, only relative characterization results are of interest.
The calibration pulses are applied to the XFE to characterize the instrument influence on the radar signal. The three types of calibration pulses account for the transmit path, the receive path, and for differences in the routing of the first two pulse types. The acquired signals can be measured at the receiving ports of the distribution networks. Evaluating the amplitude and phase of the calibration signals provides information how to model the instrument drift during data acquisition. This drift is corrected during SAR image processing to obtain the high-quality SAR products.
The spaceborne results from repeated measurements of the same instrument and antenna conditions prove the high measurement accuracy of the PN-gating technique. The repeated measurements were in a time frame of weeks. Still, the accordance of the estimation results to each other is almost perfect.
Figure 56: Block diagram of the antenna pattern subsystem (image credit: DLR)
Calibration results: During the commissioning phase, the baseline calibration procedure approach of TSX was subdivided into six major tasks. The successive baseline calibration procedures were: 112) 113) 114)
• Geometric calibration, to assign the SAR data to the geographic location on the Earth's surface.
• Antenna pointing determination, to obtain a correct beam pointing of the antenna.
• Antenna model verification, to ensure the provision of the antenna patterns of all operation modes and the gain offset between different beams.
• Relative radiometric calibration, for radiometric correction of SAR data within an illuminated scene.
• Absolute radiometric calibration, for measuring the SAR system against standard targets with well known radar cross section (RCS).
The results of calibrating TerraSAR-X approve the accuracy calculated before launch and put the new strategy to calibrate efficiently a multiple mode SAR system like TerraSAR-X on a solid base. The key element of this strategy was an antenna model approach. TerraSAR-X is the first SAR satellite calibrated with this innovative method.
All requirements and/or goals have been achieved even better than predicted. By this successful demonstration of an effective and exact calibration technique, a new benchmark has been reached not only for calibrating complex SAR systems but in principle for future highly accurate spaceborne SAR sensors like TanDEM-X or Sentinel-1 (Ref. 112).
TerraSAR-X offers a number of new perspectives to SAR interferometry when compared to ERS and also to Envisat: 115)
• High resolution of 3 m or better in stripmap and spotlight mode
• The option for a burst synchronized scanSAR mode
• The high range bandwidth will allow large baselines and the option for highly precise DEM generation
• X-band will show new scattering properties
• High observation frequency due to the short repeat cycle and variable incidence
• An ATI (Along Track Interferometric) mode.
The rather short orbit repeat cycle of 11 days and the electronically steerable antenna allow fast and frequent imaging of a certain site. At average latitudes an interferometric pair can be acquired within only 12 days. And within 22 days 12 interferometric pairs with different observation geometries can theoretically be acquired. Note that the nominal antenna look direction is to the right. The left-looking mode is possible but has some operational deficiencies so that it will only be used in high priority situations.
Secondary payloads of TerraSAR-X (TOR, LRR, LCT)
TOR (Tracking, Occultation and Ranging)
The TOR experiment is furnished by GFZ Potsdam, Germany. and CSR (Center of Space Research) at UTA (University of Texas at Austin), USA. The TOR payload consists of the dual-frequency GPS receiver IGOR (Integrated GPS Occultation Receiver), developed and built by Broad Reach Engineering Company of Tempe, AZ, and LRR (Laser Retro Reflector) for evaluation of GPS-based orbit data as an independent tracking technique. The IGOR design of is of BlackJack heritage of NASA/JPL, flown on such missions as CHAMP, SAC-C, Jason-1, and GRACE. - The overall objectives are: 116) 117)
1) To collect atmospheric and ionospheric radio occultation (RO) data using IGOR and to augment the global RO data sets obtained from other LEO satellites (namely CHAMP and GRACE) to be used for improvements of numerical weather forecasts, climate change studies and space weather monitoring. Derived fundamental atmospheric and ionospheric quantities (e.g. temperature, water vapor) are being used as complementary information for SAR data error correction.
2) Use of the resulting high-quality orbit and atmospheric correction products in conjunction with DLR-provided TSX-SAR data for improved analysis. Specific objectives for the TSX-SAR science studies are:
- Landslide, rock fall and urban land subsidence
- Ice sheet applications
- TSX-SAR digital terrain model for surface water availability modeling
- Seismic loading cycle studies
3) The IGOR GPS tracking capabilities are being used for ground-based POD (Precise Orbit Determination) algorithm processing with accuracies of < 5 cm in position (IGOR is a 48 channel space qualified GPS receiver). The orbits can be made available within less than 3 hours after data reception and are being used for SAR science analysis and eventually for NWP analysis. 118)
The IGOR payload suite consists of a dual-redundant dual frequency (L1/L2) GPS receiver unit, a built-in SSR(Solid-State Recorder), a built-in PC, and an antenna set comprised of:
• Two POD (Precision Orbit Determination) L1/L2 patch antennas
• Two RO (Radio Occultation) antennas 1x4 L1/L2 patch arrays
The IGOR instrument [GPS, SSR, antennas, PC] consists of a box of size 200 mm x 240 mm x 100 mm with a mass of about 4.2 kg.
Figure 57: The IGOR instrument (image credit: GFZ, Broad Reach Engineering)
The main components of the infrastructure for TerraSAR-X GPS occultations are: the GPS receiver onboard the satellite and the ground segment. It consists of the Polar receiving station at Ny-Ålesund (Spitzbergen), the fiducial GPS ground network, the ultra rapid precise orbit determination facility, the operational occultation processing system and, for archiving and distribution: the TerraSAR-X data Center.
The generation of the TerraSAR-X GPS occultation data products will be performed by an automatic data processing system, which will be based on the CHAMP/GRACE processing system. This system is designed to be extendable for the processing of additional GPS occultation missions, as of TerraSAR-X.
Table 9: Data products of the TOR experiment
Figure 58: Photo of the POD antennas on choke rings during spacecraft integration (image credit: EADS Astrium)
Figure 59: Accommodation of MosaicGNSS and IGOR antennas on the TerraSAR-X spacecraft (image credit: DLR)
Two independent GPS receiver units are used on TerraSAR-X for increased redundancy. For each receiver an independent passive GPS antenna (Seavey SPA-16C/S) and low noise amplifier (Delta Microwave L5690) are employed, which enable representative signal-to-noise ratios (C/N0) of 45 dB-Hz near the boresight direction. To provide an adequate sky coverage during right- and left looking SAR operations, the MosaicGNSS GPS antennas are oriented opposite to the SAR antenna. In the nominal flight configuration (right-looking SAR), the boresight points to the left of the flight direction with a 33.8º offset from the vertical. The IGOR choke ring antennas, in contrast, are exactly zenith pointing (Ref. 17).
LRR (Laser Retro Reflector)
LRR is of CHAMP and GRACE heritage developed at GFZ. A passive optical device for accurate satellite tracking from ground laser ranging stations of the SLR network. SLR tracking to spaceborne laser retro-reflectors is performed by a global network of about 40 ground stations of the ILRS (International Laser Ranging Service). The SLR data provide an independent observation type at a comparable accuracy level to the onboard GPS tracking data for the quality assurance of the TerraSAR-X POD (Precise Orbit Determination).
Figure 60: Illustration of the LRR device (image credit: GFZ Potsdam)
LCT (Laser Communication Terminal)
TSX-LCT is an experimental secondary payload, developed and built by Tesat-Spacecom GmbH of Backnang as prime contractor (formerly Bosch SatCom GmbH; Tesat Spacecom is owned by EADS Astrium), Germany. LCT development contributions come also from EADS Astrium and Zeiss Optronik. LCT funding comes from DLR and from BMBF. The LCT (optical communication) technology and implementation was chosen due to its potential of providing higher data rates, lower mass and lower power than required by conventional RF communications. 119) 120) 121) 122) 123) 124)
The LCT objective is to provide either a bidirectional communications link for binary digital data transfer between two satellites, (like LEO-MEO) or between one satellite and an optical ground station.
LCT on TerraSAR-X, also referred to as LCTSX, is designed as a COTS (Commercial-Off-The-Shelf) product and is already being made available to other missions. Tesat-Spacecom provides the full ground test equipment especially the so-called STB (System Test Bed) to verify the complete sequence of LCT operational states from start up over acquisition and tracking to communication under space environment, i.e. link distance, satellite's in-orbit vibrations, sun light. - The intent is to use LCT on future high data rate missions, in Earth observation as well as in commercial communications. 125) 126) 127)
Figure 61: View of the LCTSX instrument (image credit: Tesat-Spacecom)
Background: The on-orbit verification of the first coherent optical communication system on a satellite builds on more than 15 years of European efforts under programs and studies like SOLACOS (Solid State Laser Communications in Space), DLR-LCT, MEDIS (Multimedia Experiment and Demonstration System), and SILEX (Semiconductor Intersatellite Link Experiment). SILEX, an ESA design, was the only project that was realized, with a LEO terminal on SPOT-4 (launch March 24, 1998) and a GEO terminal on ARTEMIS (launch July 12, 2001).
Note: LCT is considered the first laser communication system in space that operates on a coherent basis (homodyne digital receiver and BPSK modulation). In a coherent system the receiver operates by optically adding a locally generated field to the receiver field prior to photodetection. The prime objective is to use the added local field to improve the detection of the weaker received field in the presence of the receiver thermal noise. - In contrast, the SILEX concept employs a direct-detection system in which the desired information is intensity modulated onto an optical source and transmitted to the receiver terminal. In this concept, the photodetector at the receiver side provides basically the function of a power detection device. - While coherent detection is the most advanced technology, which enables the maximum data rate at minimum power, but it requires relatively complicated hardware and it cannot be used through atmospheric turbulence.
Table 10: Homodyne BPSK optical modulation scheme (Ref. 67)
The same LCT system of Tesat-Spacecom is also flown on the NFIRE (Near Field Infrared Experiment) spacecraft of the US MDA (Missile Defense Agency) of DoD with a launch April 24, 2007 from the Mid-Atlantic Regional Spaceport on Wallops Island, VA (Minotaur vehicle of OSC). The NFIRE spacecraft has been built by General Dynamics C4 Systems of Gilbert, Arizona (formerly SpectrumAstro). LCT is installed on NFIRE as a secondary payload to evaluate laser communication technology. The primary mission of the NFIRE satellite is to collect images of a boosting rocket to improve understanding of exhaust plume phenomenology and plume-to-rocket body discrimination. In addition, forest fires, volcanoes and ground-based rocket engine tests are on NFIRE's observation list for the two-year mission as well.
In a further verification step, an ISL (InterSatellite Link) between LCT on TerraSAR-X and LCT on the NFIRE satellite is planned to be established.
Table 11: Performance overview of LCT (Laser Communication Terminal)
Instrument: LCT is accommodated on the anti-sun side of TerraSAR-X to guarantee a free hemispherical FOV which includes the Earth surface. The terminal consists of one physical unit with a mass of about 33 kg and an average power consumption of 136 W. For thermal control reasons, LCT is equipped with a heat pipe radiator having a radiator area of 0.57 m2 and a mass of about 10 kg. Due to an generous energy margin of the spacecraft, LCT can be operated in parallel the TSX-SAR instrument during most of the year.
The instrument is functionally divided into Optics Unit (OU) which includes the coarse pointing mechanism, telescope, fine pointer and receiver and a Frame Unit. The OU (Figure 63) consists of the beam handling elements of the terminal. The CPU (Coarse Pointing Unit) is designed as a stiff lightweight construction to guide incoming and outgoing laser beams from/to its free aperture to the telescope (124 mm aperture diameter). The aperture of the CPU is steered by two independent motors with full pointing, tracking and communications performance over the entire hemisphere. Within the CPU, the beam is being guided by two high precision mirrors keeping track the optical path onto the line-of-sight to the counter terminal. The leaving and received beams have a common path through the telescope (athermal, lightweight) for magnification/demagnification and through the Fine Pointing Unit (FPU).
Figure 62: LCTSX acquisition and communication receive path (image credit: Tesat)
Figure 63: Block diagram of the LCTSX (image credit: DLR)
The FPU employs a high-bandwidth pointing mechanism which provides the necessary tracking gain at frequencies up to 1 kHz in order to cancel satellite vibrations. The receiving beam is then separated from the leaving beam path by a polarization beam splitter and sent to the receiver. The receiver contains the 90º hybrid replacing the optical bench. There the received beam is superposed with a local oscillator laser in order to extract maximum sensitivity for a given received power level. The 90º hybrid has an added functionality of generating four separate beams with 0º, 90º, 180º and 270º relative phase shift. The acquisition, tracking and communications information contained within the four outputs of the 90º hybrid is detected in the photodiode arrays of the electrical front end, where they are immediately preprocessed. Further processing of this information (phase locking, data extraction, tracking servo) is performed by the electronics located in the Frame Unit.
The Frame Unit contains the EPC (Electrical Power Conditioner) for the terminal, the Terminal and PAT (Pointing, Acquisition and Tracking) Controller (TAPCO), data electronics and the Laser Subsystem. The Laser Subsystem contains two highly reliable diode pump modules for pumping two solid-state lasers in NPRO [(Non Planar Ring Oscillator) after Kane, Byer] configuration. One laser serves as transmit laser. The second laser serves as local oscillator and is fed directly to the receiver optical bench. The laser pump modules were developed to a large extent by researchers at the Fraunhofer Institute for Laser Technology (ILT) in Aachen, Germany, on behalf of Tesat.
Figure 64: Illustration of the LCT concept (image credit: Tesat-Spacecom)
The LCT equipment employs a high performance PowerPC processor (505 MIPS) which allows to run both tasks, real-time control loops and TM/TC data processing, at the same time on a single computer. It exhibits small dimensions (180 mm x 160 mm x 60 mm) and low mass (1 kg). The power consumption is in the range of 15 W, depending on processor load. The onboard computer provides a variety of hardware interfaces, especially high resolution A/D and D/A interfaces as well as a MIL-STD-1553B interface. Due to its modular software architecture the functionality can be easily adapted to application specific requirements. 128)
Figure 65: The LCT device mounted on the TerraSAR-X spacecraft (image credit: EADS- Astrium)
Figure 66: The LCTSX terminal (image credit: Tesat-Spacecom)
Figure 67: Artist's view of FSO (Free Space Optics) communication demonstrations between TerraSAR-X and NFIRE (image credit: Tesat)
TerraSAR-X ground segment:
• DLR/GSOC is providing the functions of spacecraft operations (commanding, monitoring, planning & scheduling, orbit and attitude determination, etc.) and data acquisition with ground stations in Weilheim and Neustrelitz. Weilheim is used as TT&C station, while Neustrelitz serves as the central receiving station of payload data at 300 Mbit/s.
• DLR/HR (DLR/Institut für Hochfrequenztechnik und Radar) is providing the IOCS (Instrument Operation and Calibration Segment) function
• PGS (Payload Ground Segment) function. The DLR research functions of data processing, analysis and simulation are provided by DFD (Deutsches Fernerkundungsdatenzentrum) and IMF (Institut für Methodik der Fernerkundung). The DLR Ground Segment provides the main functions Space and Ground Segment planning, orbit control and analysis, spacecraft telemetry reception and command, data reception and archiving, calibration and performance analysis, product generation, delivery and provision of user services.
• To comply with increased requirements on data latency, especially from the Maritime and Emergency Response Services, the TerraSAR's ground station network access has constantly been upgraded especially through additional polar station access.
Airbus Defence and Space (formerly EADS Astrium/Infoterra) is providing the service infrastructure for commercial data exploitation and customer interfaces. Additional direct payload access stations - commercial partners of Infoterra - are foreseen to extend the baseline receiving station concept.
Figure 68: Direct data reception in the US, Japan, and Germany in 2011 (image credit: Astrium Services, Ref. 11)
Figure 69: Overview of the TerraSAR-X system (image credit: DLR)
Figure 70: Overview of the TerraSAR ground segment (image credit: DLR)
Figure 71: TerraSAR-X project structure and task allocations (image credit: DLR)