SAR-Lupe is a SAR (Synthetic Aperture Radar) reconnaissance satellite imaging project of the German government, in particular the German Ministry of Defense (BMVg) and the Federal Office of Defense Technology and Procurement, referred to as BWB (Bundesamt für Wehrtechnik und Beschaffung - Federal Office of Defense Technology and Procurement), Koblenz, Germany (BWB manages the procurement of the ground and space segments). The overall objective is to provide high-resolution radar imagery to German defense forces over a period of ten years starting in 2006. SAR-Lupe is in fact the first dedicated reconnaissance satellite imaging project of Germany. More specific objectives are: 1) 2) 3) 4)
• To provide an event monitoring observation capability independent of weather and illumination conditions, in particular for regions of crisis or emergency situations (such as natural disaster coverage) to support the government in the assessment of intelligence information
• To assist the military to plan and to prepare operations
• To support deployed forces with frequent event-driven intelligence information.
The construction and launch contract of the five identical satellites was awarded to a consortium in Aug. 2001, with OHB-System AG of Bremen as prime contractor (signed in Dec. 2001). Some consortium partners in the SAR-Lupe project are: TAS (Thales Alenia Space) formerly Alcatel Alenia Space, Toulouse; Saab Ericsson of Göteborg, Sweden (SAR antennas); Carlo Gavazzi Space, Milan, Italy; TESAT-Spacecom GmbH, Backnang, Germany (development the high-performance amplifier); RST Radar Systemtechnik GmbH, Salem, THALES of Ulm, Germany, EADS (ground segment), DLR [LEOP (Launch and Early Orbit Phase) and operational backup function].
Background: Germany’s development of this program was directly related to its experiences during the NATO action in Kosovo (1998-1999), particularly to difficulties in getting the US to share satellite intelligence of direct relevance to the protection and security of non-US allied forces. These experiences convinced Germany of the need for its own spaceborne intelligence-gathering assets to deal with crisis management. 5)
In October 2003, the BWB tasked OHB-System to conduct a second-phase study focusuing on the specifics of SAR-Lupe and Helios II operating in tandem. The study involved arriving at a detailed description of the needed technical adjustments to the SAR-Lupe ground station and the precise interfaces required, as well as a clear understanding of program costs and schedules.
OHB-System undertook its initial hardware tests involving a radar transmitter tube in November 2002. The company and the BWB completed the CDR (Critical Design Review) of the system during the first half of 2003. The engineering test model for the SAR-Lupe satellite was completed by 2004 in OHB-System’s Bremen integration facility, which includes a radome.
In addition, a bilateral agreement with the French government was signed for data from SAR-Lupe to be provided in exchange for data from Helios.
Figure 1: Artist's rendition of the SAR-Lupe spacecraft in orbit (image credit: OHB-System AG)
The S/C structure has a size of about: 4 m x 3 m x 2 m with a fixed parabolic SAR/communication dual-use reflector antenna (antenna size: 3.3 m x 2.7 m). Three-axis stabilization is used.The ACS (Attitude Control Subsystem) features high performance attitude knowledge and control capabilities. Attitude is being sensed by sun sensors, star sensors and gyroscopes; attitude control is provided with reaction wheels and magnetic torquers. With the special agile bus the satellite and thus the antenna boresight can be aligned with great precision to a specific location on the ground via its three-axis stabilization.
The EPS (Electric Power Subsystem) features a solar panel of 2.4 m2 of GaAs solar cells providing 550 W of power at EOL. Use of 2 x 66 Ah Li-ion batteries for power storage (ABSL, UK).
The robust monopropellant hydrazine system allows for compensation of the altitude decay due to the residual air drag and for EOL de-orbiting.
Figure 2: Photo of the Li-ion two module battery sytem on SAR-Lupe (image credit: ABSL)
The SMU (Satellite Management Unit) is providing the service functions relevant for OBDH (Onboard Data Handling), power and control distribution, AOCS, etc. The SMU is based on a radiation tolerant DSP processor, TSC21020. To achieve maximum integration and miniaturization, the SMU design makes extensively use of FPGA components. Various interfaces are implemented in the SMU such as standard serial lines (RS422 and IEEE1355) as well as standard ESA OBDH interfaces. Each S/C is furnished with two SMU units for redundancy.
Figure 3: Illustration of the SMU (image credit: Carlo Gavazzi Space)
RF communication is provided in X-band for encrypted downlink transmissions (the constellation satellites use the same antenna for image acquisition and X-band data downlink; however, each function requires exclusive antenna use). The uplink communication is in S-band (also encrypted). In addition, an intersatellite link (or cross-link) in S-band is provided for data product transfer. The satellite control data routing is selectable either via ground station and/or via an inter-satellite link.
Orbit: The five satellites are launched into three orbital planes. Average altitude of about 500 km, inclination for near-polar orbits (98.2º). Orbital plane 1 contains 2 S/C; orbital plane 2 contains 1 S/C, orbital plane 3 contains 2 S/C. The angle between orbital plane 1/2 is 64º; the angle between orbital planes 2/3 is 65.6º. The phase angles of the S/C are: Orbital plane 1 = 0º+69º; orbital plane 2 = 34.5º; orbital plane 3 = 0º+69º. The five spacecraft constellation offers very short response times and a high system redundancy.
SAR-Lupe-1 (international code: 2006-060A) has a perigee of 468 km, an apogee of 505 km, inclination = 98.2º, period= 94.3 minutes.
Figure 4: Alternate view of the SAR-Lupe spacecraft (image credit: OHB-System AG)
Figure 5: Illustration of the three orbital planes of the SAR-Lupe constellation (image credit: OHB-System AG)
Launch: The 5-satellite constellation was launched in the time frame 2006-2008 (single launches in half year increments), initiating operations from 2006 onwards and full utilization of services starting in 2008. The launch provider was Cosmos International GmbH using the Cosmos 3M launch vehicle.
• A successful launch of the SAR-Lupe-1 spacecraft of the constellation took place on Dec. 19, 2006 from the Plesetsk Cosmodrome, Russia (service provider: Eurockot). Initial S/C control and monitoring services in LEOP (Launch and Early Orbit Phase), involving S/C injection/deployment, system check-out, positioning, etc., are being provided by DLR/GSOC (German Space Operations Center).
Germany and France have already agreed on cooperation regarding mutual SAR-Lupe and Helios-II data exchange services. Eventually, a European reconnaissance capability may evolve consisting of the following systems: SAR-Lupe, Helios-II, COSMO/SkyMed, Pléiades, and TerraSAR-X [also within the context of the European initiative GMES (Global Monitoring for Environment and Security)].
Table 1: Overview of SAR-Lupe mission schedule 8)
Figure 6: Several views of the SAR-Lupe spacecraft configuration (image credit: OHB-System AG)
Status of the SAR-Lupe constellation:
• The complete SAR-Lupe constellation of 5 spacecraft is operating nominally in 2013. 9)
• The complete SAR-Lupe constellation of 5 spacecraft is operating nominally in 2012. The on orbit spacecraft service time has accumulated to more than 20 years of flawless uninterrupted operations for the constellation.
• In Sept. 2011, OHB completed a System definition study of a SAR-Lupe Follow-On (BWB, BMVg) constellation.
• The complete SAR-Lupe constellation of 5 spacecraft is operating nominally in 2011. 10)
Figure 7: SAR Imaging from Space: Mission Timeline (image credit: Astrium) 11)
• The complete SAR-Lupe constellation of 5 spacecraft is operating nominally in 2010.
• In 2009, the SAR-Lupe constellation experienced more than 800 close encounters with orbital junk or other operating satellites, including 32 passes at less than 1 km from another SAR-Lupe spacecraft, and one that required a collision-avoidance maneuver. Due to these collision concers, Germany inaugurated in 2009 the GSSAC (German Space Situational Awareness Center), located in Uedem, which is near Kleve in North Rhine-Westphalia, Germany. - In addition, Germany’s defense forces, in a rare move, have invested cash in an ESA-led program to design a European space surveillance system starting with ground-based radars already existing in Germany and France.
• On December 4, 2008, the SAR-Lupe radar reconnaissance system was officially handed over to the German Bundeswehr. 12)
• The commissioning phase of SAR-Lupe-5 was completed and declared as “operational” in Oct. 2008 (launch of SAR-Lupe-5 on July 22, 2008). Hence, the project commenced the phase of full operational service provision in Oct. 2008 which is expected to last until the end of 2017. 13)
• In early July 2008, the SAR-Lupe-4 commissioning phase was completed and the spacecraft was integrated into the operational constellation. Operations of the full constellation (5 spacecraft) is expected to start in October 2008.
• On April 3, 2008, the FQR (Flight Qualification Review) took place for SAR-Lupe-3, thus ending the commissioning phase - the spacecraft was declared “operational” as of April 3, 2008. There are now 3 operational spacecraft in the constellation.
• In early December 2007, the German Bundeswehr accepted officially the current SAR-Lupe-System as an operational user. With two fully functional spacecraft of the projected five S/C constellation in service, a certain operational base level can be maintained. After a period of intensive tests, the BWB (Koblenz) confirmed the required performance and high quality of the SAR imagery of the two spacecraft. Hence, with a current response time of 24 hours, the system was declared as “operational” - starting in turn the projected 10-year service period (design life) of the constellation as specified by the prime contractor. 14)
• SAR-Lupe-2 was declared operational in mid-August 2007.
• The commissioning phase of SAR-Lupe-1 ended on January 19, 2007. While the cross-link capability couldn't been tested (only one spacecraft has been launched so far), the first images produced are as good as expected.
Sensor complement (XSAR):
In the absence of an instrument description (the SAR-Lupe mission is classified), some general observational objectives/features are provided:
XSAR (X-band SAR instrument):
XSAR of SAR-Lupe is observing in X-band (center frequency of 9.65 GHz corresponding to a wavelength of 3.1 cm).
• Global observation coverage capability
• Use of a parabolic SAR reflector antenna of size: 3.3 m x 2.7 m. The choice of using a single beam offset reflector antenna instead of an active beam-steering antenna represented a major cost saving in the development of the instrument. SAR-Lupe uses a single beam offset reflector antenna and a TWT (Travelling Wave Tube) based transmitter, illuminated by a feed horn on a deployable boom. The highly efficient TWT and a low-loss high-gain antenna are providing good power potential for radar with efficient DC power utilization. 15) 16)
Prior to an image acquisition, the satellite rolls in an appropriate position and stabilizes its attitude. Then, the SAR image is acquired. After that, the satellite rolls back into its standby attitude and continues to load its batteries preparing itself for the next SAR image acquisition.
• Number of scenes of area of interest: ≥ 30/day
• System response time: < 36 hours
• System availability: 95%
• Automated monitoring and control of the constellation via a ground control station
• Automated data reception and image processing
• LEOP (Launch and Early Orbit Phase) support is provided by DLR/GSOC
• The mean response time of the system is in the range of 10 hours. System availability is provided by the distribution of the satellites in their orbital planes.
• The modular interface design of the ground segment permits also future integration into an international reconnaissance network (mutual utilization of the system, etc.).
• SAR imaging modes provided: stripmap and spotlight. Stripmap imaging involves antenna pointing into a fixed direction (normally in cross-track). Internally, these modes are referred to as “Strip-SAR” and “Slip-SAR.” Strip-SAR observations are conducted in the nadir direction. In Slip-SAR mode, the entire spacecraft is rotated into the direction of the target to increase the integration time and therefore the in-track resolution.
• Spatial resolution of SAR data: 0.5 m in spotlight mode for a scene of about 5.5 km x 5.5 km in size; a stripmap scene has a size of 60 km x 8 km.
• Satellite operations permit “spotlight imaging” of a scene. This involves rotation of the entire S/C about a target area to increase the integration time of the scene (the SAR beacon is pointable). In SAR-Lupe terminology, spotlight imaging is referred to as “Slip-SAR.”
• An onboard image storage capability of 128 Gbit (EOL) is provided.
• The main image products are: 1) stripmap scenes of size 60 km x 8 km, and 2) square scenes of 5.5 km x 5.5 km in size.
• The following additional products can also be generated: a) elevation models from multipass interferometric products, b) multipass stereo products, c) change detection products, d) products with enhanced radiometric resolution.
The calibration concept of the SAR instrument consists of two parts: 1) internal calibration and 2) external or end-to-end calibration. The internal calibration feature is used for continuous calibration of the SAR antenna and the high power amplifier. External calibration is employed for two purposes: 17)
- Ensure a high image quality after the commissioning phase
- To ensure nearly continuous image quality in the routine phase of the mission by re-calibration in certain time intervals (e.g. every year).
The calibration is using new developments on Active Radar Calibration (ARC) targets in X-band and L-band specially developed for that purpose.
Figure 8: Illustration of the SAR-Lupe ARC in X-band (image credit: RST)
Figure 9: Block diagram of the calibration of the XSAR antenna
The SAR-Lupe constellation is being monitored and controlled (TT&C function) by OHB-System AG, Bremen. The user ground segment services, including payload data reception, processing and archiving, and user interface, are provided at a service center located in Gelsdorf, near Bonn. 18)
As part of the bi-national collaboration between France and Germany in the area of satelliteborne reconnaissance, OHB-System AG has implemented a further SAR-Lupe ground station in France, allowing the French army to directly access the reconnaissance information collected by the German SAR-Lupe radar satellite system. On April 1, 2010, the German Federal Office of Defense Technology and Procurement [BWB (Bundesamt für Wehrtechnik und Beschaffung), Koblenz] awarded OHB a contract to operate this ground station for a period of 5 years. 19) 20)
The ground stations permit the French and German military forces access to each other’s optical and radar satellite systems. The contracts are part of a bilateral French-German program called E-SGA (Europäisierung der satellitengestützten Aufklärung - or Europeanization of Satellite-aided Reconnaissance) and are an example of progress made among European nations to harness space-based assets that were financed and built separately into a system usable by several nations. 21)
The partnership will provide Germany with access to the two French Helios-2 optical and infrared imaging satellites, while French forces will be able to use Germany’s constellation of five SAR-Lupe radar spacecraft. OHB is the contractor and for implementing the E-SGA program. The main tasks are: (Ref. 19)
- to modify the national SAR-Lupe ground segment so that it can function as a multinational ground segment which can be addressed by partner nations requesting or providing image data
- to develop, supply and operate a partner ground segment for France, referred to as FSLGS (French SAR-Lupe Ground Segment)
- to connect the German HELIOS-2 ground segment, called CPHD (Centre Principale HELIOS Germany), to the multinational SAR-Lupe ground segment. In addition, OHB will be operating the CPHD.
Military operational use of the integrated system is expected to commence in mid-2010.
1) In German, the word Lupe means “magnifying glas,” a reference and connotation for high-resolution imagery.
3) Information was provided by Fritz Merkle, Danela Sell, Ingo Engeln, and Steffen Leuthold of OHB-System, Bremen
4) “The SAR-Lupe Program - An Industrial View,” Security and Defence Aspects of Space: The Challenges for EU, Athens, Greece, May 8-9, 2003
5) Rebecca E. Johnson, “Europe's Space Policies and their relevance to ESDP (European Security and Defence Policy),” June 2006, URL: http://www.google.de/url?sa=t&rct=j&q=europe%27s%20sapce%20policies%20and%20their%20relevance%20to%20esdp%20
6) David Koebel, Carsten Tobehn, Boris Penné, “OHB Platforms for Constellation Satellites,” 5th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 4-8, 2005, URL: http://www.dlr.de/iaa.symp/Portaldata/49/Resources/dokumente/archiv5/0304_Koebel.pdf
7) R Spurrett , N Simmons, C Pearson, “Advanced Batteries Based on Commercial Cell Technology: From Imagination to Reality,” Proceedings of the 59th IAC (International Astronautical Congress), Glasgow, Scotland, UK, Sept. 29 to Oct. 3, 2008, IAC-08.C3.3.5
8) “SAR-Lupe, Germany’s first satellite-based reconnaissance system, now completed,” DLR, July 22, 2008, URL: http://www.dlr.de/en/desktopdefault.aspx/tabid-6214/10201_read-13077/
9) Information provided by Steffen Leuthold of OHB, Bremen, Germany
10) Information provided bei Reiner Knauer of OHB, Bremen, Germany
11) Frank Döngi, “Earth Observation from Space - The European Landscape in the Second Decade,” Proceedings of SPIE, Remote Sensing 2011, Vol. 8181, Prague, Czech Republic, Sept. 19-22, 2011, URL: http://spie.org/Documents/AboutSPIE/PDF/ERS11-plenary-Doengi.pdf
12) “OHB-System AG: SAR-Lupe now officially handed over to Strategic Reconnaissance Command,” Dec. 4. 2008, URL: http://www.ohb.de/press-releases-details/items/ohb-pr_sl_%C3%BCbergabe.html
13) Information provided by Ingo Engeln of OHB-System, Bremen, Germany
14) “10-year operation period of the SAR-Lupe system for the German Federal Armed Forces now to commence,” Dec. 3, 2007, URL: http://www.ohb-technology.de/press-releases-details/items/id-10-jaehriger-betrieb-des-sar-lupe-systems-fuer-die-bundeswehr-beginnt.html
15) H. M. Braun, P. E. Knobloch, “SAR on Small Satellites- Shown on the SAR-Lupe Example,” Proceedings of the International Radar Symposium 2007 (IRS 2007), Cologne, Germany, Sept. 5-7, 2007
16) J. Zemann, et al., “The Deployable Boom Assembly for SAR-Lupe,” 28th ESA Antenna Workshop, June 2005, Estec Noordwijk, The Netherlands.
17) H. M. Braun, S. Kiecherer, “External Calibration for CRS-1 and SAR-Lupe,” Proceedings of EUSAR 2006, Dresden, Germany, May 15-18, 2006
18) Roland Mader, “SAR-Lupe, Challenge of Satellite Based Reconnaissance,” Dec. 19, 2006, URL: http://www.google.de/url?sa=t&rct=j&q=sar-lupe%2C%20challenge%20of%20satellite%20based%20reconnaissance&source=web&cd=1&sqi=2&ved
20) “OHB-System awarded contracts for the operation of the French SAR-Lupe ground station and the German HELIOS II ground station,” May 4, 2010, URL: https://www.ohb-system.de/press-releases-details/items/ohb-system-awarded-contracts-for-the-operation-of-the-french
21) Peter B. de Selding, “OHB To Support Franco-German Imagery Partnership,” Space News, May 4, 2010, URL: http://www.spacenews.com/military/100504-ohb-imagery-partnership.html
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.