TSX-NG (TerraSAR-X Next Generation)
The TerraSAR-X NG mission is intended to take the TerraSAR-X data and service continuity well beyond 2025 taking benefit of a 9.5 years satellite lifetime (TerraSAR-X and its twin TanDEM-X). The Space Segment, initially a single spacecraft, will be launched into the TerraSAR-X reference orbit while the first generation TerraSAR-X systems will still be operational.
The TerraSAR-X NG Mission will benefit from an advanced SAR sensor technology allowing a spatial resolution down to 0.25 meter depending on allowable chirp bandwidth in the future. Besides advanced very high resolution modes, TerraSAR-X NG will provide heritage modes enabling TerraSAR-X data continuity and improved wide swath modes to support large area applications. In addition, the TerraSAR-X NG mission will feature full polarimetry and improved near real time capabilities. The TerraSAR-X NG mission and potential extensions will be subject to a partnership model, “WorldSAR”, in which partners can participate through co-investment, subscription, and ownership of additional satellites operated in constellation.1) 2)
TSX-NG is a project of Airbus Defence and Space Geo-Intelligence/Infoterra GmbH(formerly Infoterra GmbH, Astrium Geo-Information Services), Friedrichshafen, Germany. The TSX-NG mission, implemented as commercial and civil program, constitutes the next step in the German X-band SAR roadmap and is designed to guarantee the TerraSAR-X data and service continuity for commercial and public end-users well beyond the year 2025. With new very high-resolution products and improved performance parameters, the TSX-NG mission will provide new products and services to the user community while remaining heritage products. 3)
Some background: The German TerraSAR-X and TanDEM-X, the Italian COSMO-Skymed and the Canadian RADARSAT-2 are major Earth observation programs which are operationally in orbit and which are based on state-of-the-art SAR (Synthetic Aperture Radar) instruments. The Sentinel-1 mission in ESA's Copernicus program was launched on April 3, 2014; the PAZ mission of Spain is scheduled for launch in Q4 2014. The SAR instruments of these missions utilize large phased array antennas for greatest mode flexibility and high performance. Specifically they can switch between "large swath width / medium resolution" and "medium swath width / high resolution" modes. 4)
Airbus Defence and Space (formerly 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 2015. 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.
In 2013, the product portfolio for TerraSAR-X was enhanced with two new operational modes:
- ST (Staring Spotlight) mode: with 0.8 m x 0.25 m resolution
- SCW (ScanSAR Wide) mode: with 40 m resolution and a footprint of up to 270 km depending on incidence angle. This new mode is particularly intended to satisfy the increasing demand for maritime domain awareness.
The success of these programs is already leading to the definition of follow-on programs with increased demands on the imaging performances of the radar sensor. The performance of the state-of-the-art instruments with "classical" SAR antennas can significantly be enhanced by new antennas employing DBF (Digital Beam Forming) technology. In fact, with DBF antennas a wide swath can be achieved simultaneously with high resolution and high sensitivity by utilizing the full gain of a large antenna. The new generation of SAR instruments based on DBF is referred to as HRWS (High- Resolution Wide-Swath SAR). 5) 6)
As a next step in the TerraSAR-X roadmap, a next generation X-band SAR satellite will be deployed. Data continuity will be guaranteed through dedicated heritage modes that will provide the same performance characteristics as the current TerraSAR-X spacecraft. However, the TerraSAR-X NG mission will benefit from an advanced SAR sensor technology allowing on the one hand a spatial resolution of down to 0.25 m. On the other hand, the instrument features new TOPS (Terrain Observation by Progressive Scan) modes that will support large area applications such as vessel detection and oil slick monitoring (swath width of up to 400 km). In addition to improved instrument capabilities the TerraSAR-X NG mission will provide improved overall system responsiveness and service capabilities through the utilization of a network of DAS (Direct Access Stations) also including near polar Ground Station(s) for satellite commanding and data reception. 7)
The objective of WorldSAR is to provide NRT (Near Real Time ) remote sensing information - on a global scale -through a network of three to five TerraSAR-NG type satellites operated by entities in regulated partner nations. This establishes a weather independent high quality SAR satellite constellation with outstanding NRT data access and high speed workflow / processing capability for the benefit of the users.
In contrast to a constellation, owned and operated by a single mission operator, who assumes all risks, the collaborative approach of a CCC (Coordinated Constellation Concept) entails the sharing of risks and benefits between partners, each of whom owns and operates a part (WorldSAR Component) of the constellation. First experiences with this concept will be gathered through the TerraSAR-X / PAZ constellation services from 2014 onwards.
The PPP (Public-Private Partnership) between DLR and Airbus Defence and Space (previously Astrium GmbH) foresees, that a new-generation SAR (TerraSAR-NG) will be financed and developed by industry. 8)
Figure 1: Overview of the SAR development line — German radar missions (image credit: DLR, Ref. 8)
TSX-NG System Overview :
The TerraSAR NG system will be composed of a space segment with the TerraSAR-X NG satellite, TT&C (Telemetry, Tracking & Command) and reception stations, a core ground segment and a service segment including DAS (Direct Access Stations).
The Space Segment is initially based on a single spacecraft (TSX-NG) to be launched into LEO (Low Earth Orbit) while the TerraSAR-X/TanDEM-X satellites are supposed to be still operational and providing the opportunity for a constellation with satellite(s) operated by partners (“WorldSAR” partnership). As secondary payload, an AIS receiver will complement the SAR Mission in order to extend ship detection capabilities based on SAR imagery (Ref. 3).
Figure 2: Artists view of the TSX-NG spacecraft (image credit: Airbus Defence and Space)
The TerraSAR-NG System foresees a core ground segment infrastructure and a network of main and external TT&C and reception stations. To improve overall responsiveness polar station(s), reception services provided by Direct Access Partners and a corresponding high bandwidth network are envisaged. Interoperability with the on-going TerraSAR-X mission is a driving mission requirement. Therefore, the TerraSAR-NG mission will provide seamless access to data of both missions and in addition, TerraSAR-NG will provide TerraSAR-X heritage products allowing for continuation of existing data stacks and interferometric applications.
The TerraSAR-NG Service Segment will provide the information service, ordering service and delivery service to the customers. In addition, the Service Segment is responsible for a seamless access to the data and products from TerraSAR-X and TerraSAR-NG Missions.
Figure 3: TerraSAR-X NG system overview (image credit: EADS Astrium GmbH)
• The pursued extension of the bandwidth allocation to 1,200 MHz bandwidth, necessary for the VHR (Very High Resolution) mode (0.25 m), has been adopted as topic for approval on the agenda of the next World Radio Conference in 2015. Analysis of the conditions for the pursued spectrum extension is on-going in coordination with stakeholders.
• An initial CCC (Coordinated Constellation Concept) for commercial X-band SAR will start operations in 2014, comprising the German TerraSAR-X/TanDEM-X and the Spanish PAZ satellite.
• The TerraSAR-X NG system project has passed the SRR (System Requirements Review) in 2012 at Astrium Geo-Information Services and is now in the Preliminary Design Phase, accompanied by incorporation of further user feedback and key requirements from stakeholders and partners.
Time critical development activities are under way to ensure the planned start of commercial services in order to guarantee X-band data and service continuity. A launch opportunity has been secured, with options being under investigation.
TSX-NG System Capabilities:
The TSX-NG system capabilities have been derived from lessons learned from the current TerraSAR-X mission and a detailed analysis of key user requirements (Ref. 3).
A key element to achieve the demanding mission requirements is the TSX-NG SAR instrument, an advanced high-resolution X-band synthetic aperture radar based on active phased-array technology. The new SAR instrument represents a significant technology update and provides improved performance based on TerraSAR-X heritage.
Main system characteristics and improved capabilities of the TSX-NG mission compared to the TerraSAR-X mission can be summarized as follows:
• X-band service continuity for operational users
• New imaging modes and improved performance
• Improved maritime service capability
• Improved system responsiveness, NRT (Near-Real-Time) capability and mission lifetime.
The X-band service and data continuity is a driving requirement for the TSX-NG mission and will be ensured through TerraSAR-X heritage modes that will enable data and service continuity across the TerraSAR-X generations. Particularly, interferometric applications and existing TerraSAR-X data stack will be supported by the TSX-NG mission.
The following imaging modes and improved performance will be implemented in the TSX-NG SAR instrument, including advanced VHR (Very High Resolution) modes up to 0.25 m resolution at considerably improved NESZ (Noise Equivalent Sigma Zero). The TSX-NG SAR instrument supports: SpotLight (sliding and staring) modes, StripMap and TOPS (Terrain Observation by Progressive Scan) modes, a ScanSAR improvement avoiding scalloping effects, with single, dual/cross and quad polarization, as well as a MAPS (Multi Azimuth Phase Center) processing capability. Table 1 summarizes some key parameters for selected VHR SpotLight modes:
Table 1: Selected TSX-NG VHR SpotLight modes
Besides the advanced VHR modes, the TSX-NG mission will provide improved StripMap modes to support various mapping applications. Among the new modes, also a SM (StripMap) mode with 1 m ground resolution will be offered. Table 2 provides some key parameters for selected StripMap modes.
Table 2: TSX-NG StripMap modes
To support various maritime services, a series of modes will be implemented that particularly support large area and maritime awareness applications. For TSX-NG the TOPS imaging mode will be implemented taking benefit from improved DTAR (Distributed Target Ambiguity Ratio) and NESZ performance compared to traditional ScanSAR modes. As for the StripMap modes, the TSX-NG TOPS modes full performance incidence angle range will be 20º to 50º and the acquisition length will only be limited by the maximal instrument on-time, but at least 1500 km.
In addition, the TSX-NG system will feature a dedicated ship detection mode with an extended swath, specifically designed to enable the detection of vessels of 25 m in length and larger at a very low false alarm rate. The improved maritime services will be complemented by an AIS (Automatic Identification System) receiver on-board of the TSX-NG spacecraft to allow maritime services, based on the fusion of radar images and AIS information. For this purpose, the AIS receiver will be operated in conjunction with SAR imaging modes but with a different acquisition duration (several minutes) driven by the frequency at which ship messages are transmitted (between 2 and 3 minutes dependent on ship velocity). Key parameters of the large area modes are summarized in Table 3.
Table 3: TOPS (Terrain Observation by Progressive Scan) modes of TSX-NG
Based on the analysis of an alternative launcher scenario, an extended antenna design for TSX-NG is under investigation that already includes first steps towards an HRWS (High Resolution Wide Swath) system the next major technology milestone of the German SAR roadmap. Key features of this extended instrument concept are:
• Improved SAR products with regard to footprint and/or NESZ
• Increased data access range
• Improved radiometric sensitivity
• GMTI (Ground Moving Target Indication) capability.
To provide improved system responsiveness and NRT capability in particular, the TSX-NG system will use a network of main and external ground stations including polar station(s), reception services provided by Direct Access Partners and correspondingly a high bandwidth network to reduce overall product delivery time significantly. Compared to the TerraSAR-X mission, the upcoming TSX-NG mission will also support significantly more order deadlines per day. The order deadlines follow the uplink opportunities, which in turn are driven by the implemented station scenario. Also, the time from order deadline to scheduled uplink opportunities will be shortened significantly.
Furthermore, the overall mission concept foresees the possibility of distributed reception and processing of SAR data. The main advantages of this concept are significantly improved timeliness from acquisition to delivery (earlier downlink) and faster release of the satellite memory.
Consequently, the TSX-NG system layout for the Ground and Service Segment provides a constellation capability with the TerraSAR-X/TanDEM-X and PAZ satellite(s) and future TSX-NG (TerraSAR-X NG) satellites of the “WorldSAR” partnership, allowing seamless ordering of data from all partner missions of the constellation.
The TSX-NG spacecraft will be designed to support a lifetime of at least 9.5 years while the Ground and Service Segment will provide an operational lifetime of 10 years or more in order to provide a long lasting X-band data continuity.
In contrast to a constellation, owned and operated by a single mission operator, who bears all risks, the collaborative approach of a CCC (Coordinated Constellation Concept) entails the sharing of risks and benefits between partners, each of whom is responsible for a part (WorldSAR component) of the constellation. First experience with this concept will be gathered through the TerraSAR-X / PAZ constellation services, starting from 2014 onwards. There are different collaboration schemes possible for WorldSAR:
• System Partnership: partners own and operate a WorldSAR component
• Operations Partnership: partners own and a central entity operates a WorldSAR component
• Service Partnership: partners own the capacity or parts of the capacity of a WorldSAR component, which is owned and operated by a central entity.
The respective collaboration scheme is dependent on partner nation’s compliance with German export and satellite data security regulations.
Figure 4: High level description of WorldSAR system where partner satellites have the flexibility to operate in coordination and/or as standalone systems (image credit: EADS Astrium GmbH)
Key WorldSAR objectives can be summarized as follows (Ref. 3):
1) Provide fresh information with minimum latency globally: This will be achieved through a coordinated spacecraft constellation supporting near real time data flows. To optimize regional observation capability, the WorldSAR concept already includes the option of SSOs (Sun-Synchronous Orbits) and inclined orbiting satellites flying together in a coordinated constellation as shown in Figure 4.
2) The goal is to build an optimized constellation of 3-5 satellites to achieve a revisit time of better than 10 hours globally, with much better revisit capabilities in higher latitudes (Figure 5).
3) Joint investment into a full-fledged constellation will limit risk and maximize the access to observation capacity. Subscribers in the alliance will own variable observation capacity of a single system and will benefit from a constellation. Free capacity for partner’s satellites can be shared by constellation partners.
Figure 5: Example of a WorldSAR constellation (image credit: EADS Astrium GmbH)
TSX-NG supported applications:
The current TerraSAR-X mission already provides a number of widely renowned advantages, namely its high orbit precision and stability and correspondingly its unique geolocation accuracy as well as its service reliability. While these features will be maintained or even improved for the TSX-NG mission, a considerable benefit for main applications is expected particularly taking the improved imaging modes of TSX-NG and the constellation capability with TerraSAR-X/TanDEM-X, PAZ and WorldSAR into account. TSX-NG specifically will stimulate applications that require more detailed target and feature detection and increased revisit capabilities. In the following, some of the relevant applications and the related benefit TSX-NG will provide, are briefly introduced:
1) Situational awareness applications, for defense and intelligence purposes will benefit from the increased geometric resolution up to 0.25 m and improved radiometric sensitivity. In addition, also coherence change detection capabilities will be significantly enhanced.
2) Monitoring critical infrastructure, like traffic infrastructure (road and railway networks, airports, harbors, inland waterways), energy infrastructure (electricity, pipelines, and power plants) will similarly take advantage of the improved geometric resolution. Furthermore, it can be expected that along with the very high resolution the number of persistent scatterers will increase dramatically leveraging the surface motion applications into a new dimension.
3) DEMs (Digital Elevation Models) based on higher resolution SpotLight and StripMap data provided by TSX-NG will allow more precise elevation models on the local and regional scale. Also, the accuracy of ground control point extraction will be improved by use of very high resolution data from TSX-NG. - Besides applications such as Disaster Monitoring and Environmental Monitoring, which will mainly benefit from improved StripMap Modes, also large area applications will be supported.
4) Maritime applications such as maritime awareness (ship detection), iceberg or oil spill detection and monitoring on the open sea and coastal waters will take advantage from the dedicated ship detection mode, the improved large area (TOPS) modes and the concurrent availability of AIS information.
The TSX-NG Ground and Service Segment will be upgraded from the existing TerraSAR-X service elements to derive specific products which contribute to the above mentioned application areas. Specifically, the following products and services are envisaged:
• Amplitude and coherence change detection
• Surface deformation monitoring and satellite geodesy
• Ship, oil spill and ice detection and monitoring
• Ground control points
• Digital Elevation Models: DSM (Digital Surface Model) and DTM (Digital Terrain Model)
• Topographic and environmental mapping.
Within the German national X-band program, the definition of TSX-NG started in 2009 and is now (2014) in the preliminary design phase.
Launch: The launch of the TSX-NG spacecraft is planned for the timeframe 2018 (?).
Orbit: Sun-synchronous circular dawn-dusk orbit with a LTAN (Local Time of Ascending Node) at 18:00 hours (± 0.25 h) equatorial crossing, average altitude = 514.8 km , inclination = 97.44º, period = 94.85 minutes.
This file will be updated as new information of the TSX-NG project (spacecraft and payload) becomes available.
1) Jürgen Janoth, Steffen Gantert, Thomas Schrage, Alexander Kaptein, “TerraSAR Next Generation - Mission Capabilities,” Proceedings of IGARSS (IEEE International Geoscience and Remote Sensing Symposium), Melbourne, Australia, July 21-26, 2013
2) Jürgen Janoth, Steffen Gantert, Thomas Schrage, Alexander Kaptein, “From TerraSAR-X towards TerraSAR Next Generation,” Proceedings of EUSAR 2014 (10th European Conference on Synthetic Aperture Radar), Berlin, Germany, June 3-5, 2014
3) Thomas Schrage, Juergen Janoth, Alexander Kaptein, Noemie Bernede, Steffen Gantert, Ralf Duering, “TerraSAR-X Next Generation – Mission Overview,” Proceedings of the 64th International Astronautical Congress (IAC 2013), Beijing, China, Sept. 23-27, 2013, paper: IAC-13-B1.2.8
4) Noemie Bernede, “New X-Band capabilities: TerraSAR Improvement Program and TerraSAR Next Generation Outlook,” 2012 Pacific GIS/RS Conference, Suva, Fiji, November 2012, URL: http://www.sopac.org/.../1_04_03_TerraSAR_Programme.pdf
5) Christian Fischer, Christoph Heer, Rolf Werninghaus, “Technology Demonstration for TerraSAR-X Next Generation,” Proceedings of the 3rd Workshop on Advanced RF Sensors and Remote Sensing Instruments (ARSI), Noordwijk, The Netherlands, Sept. 13-15, 2011, URL: http://www.congrex.nl/11c11/ARSI%20papers/FISCHER_ARSI_Paper.pdf
6) Stefan Knabe, “TerraSAR-X Status and Future Plans,” SeaSAR 2010 Workshop, Frascati, Italy, Jan. 25-29, 2010, URL: http://earth.eo.esa.int/workshops/seasar2010/9_Stefan_KnabeTSX.pdf
7) Juergen Janoth, Steffen Gantert, Wolfgang Koppe, Alexander Kaptein, Christian Fischer, “TerraSAR-X2 - Mission Overview,” Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Munich, Germany, July 22-27, 2012
8) Michael Eineder,Irena Hajnsek, Gerhard Krieger, Alberto Moreira, Kostas Papathanassiou, “Tandem-L: Satellite Mission Proposal for Monitoring Dynamic Processes on the Earth’s Surface,” DLR Brochure, April 2014, URL: http://www.dlr.de/hr/en/Portaldata/32/Resources/dokumente/broschueren/Tandem-L_web_Broschuere2014_en.pdf
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org) .