COSMO-SkyMed (Constellation of 4 SAR Satellites)
COSMO-SkyMed (Constellation of Small Satellites for Mediterranean basin Observation) is a 4-spacecraft constellation, conceived by ASI (Agenzia Spaziale Italiana), and funded by the Italian Ministry of Research (MUR) and the Italian Ministry of Defense (MoD), Rome, Italy. The program is managed in cooperation of ASI and MoD. The contract was assigned to an Italian industrial team, that is in charge of the project development. Thales Alenia Space Italia (TAS-I) is the prime contractor of the end-to-end system and leads an industrial team of small and medium sized Italian companies including many from the Finmeccanica group. Telespazio is the main Ground Segment contractor responsible for the development of the control center for the constellation, and of the user's ground segments for acquiring, processing and distributing the data gathered by the satellites for civil and defence applications. -The COSMO-SkyMed program represents the largest Italian investment in space systems for Earth Observation.
Each of the four satellites is equipped with a SAR (Synthetic Aperture Radar) instrument and is capable of operating in all visibility conditions at high resolution and in real time. The overall objective of this program is global Earth observation and the relevant data exploitation for the needs of the military community as well as for the civil (institutional, commercial) community.
Sample applications of COSMO-SkyMed data are seen the following fields:
• Defense and security applications: Surveillance, intelligence, mapping, damage assessment, vulnerability assessment, target detection/localization
• Risk management applications: Floods, droughts, landslides, volcanic/seismic, forest fire, industrial hazards, water pollution
• Other applications: Marine and coastal environments, agriculture, forestry, cartography, environment, geology and exploration, telecommunication, utilities and planning
• Provision of commercial imaging services
• The high revisit frequency offered by the four X-band SAR spacecraft is also expected to provide a unique potential to the operational meteorological user community through provision of ancillary data and/or data on meteo-correlated phenomena, in particular as regards sea ice monitoring and study of ocean wave patterns.
• A strong emphasis is given to the dual-use (civil and military) nature of the system. The IEM (Interoperability, Expandability and Multi-sensoriality) concepts are also stressed, since these qualities bring COSMO-SkyMed to be a versatile system able to expand its architecture toward a set of "Partner missions."
In 1996 the Italian Government provided initial funding for the realization of a national Earth observation program. In 1997 the general guidelines for the 1998-2002 Italian National Space Plan (PSN) were approved including the activities on Earth observation. The strategic element in this plan is the COSMO-SkyMed dual-use program of ASI. 1) 2)
In the summer of 2001, the Italian Ministry of Defense became a partner in the COSMO-SkyMed program (a welcome funding partner for the Italian Ministry of Research). However, the dual-use nature of the COSMO-SkyMed program, i. e. civil (research and commercial) and military use of its data products, resulted in a virtually classified program. A great disadvantage of the new arrangement is that rather sparse technical information of the mission can only be made available to the public. - The funding the the COSMO-SkyMed program is split between ASI and I-MOD with 75% and 25%, respectively, of the system resources distribution.
In this arrangement, Civilian (Scientific, Institutional and Commercial) and Military users share system resources under appropriate regulations. ASI manages and coordinates institutional and scientific users allowing the utilization of the service for acquisitions and products ordering by mean of the COSMO-SkyMED website, whereas the commercial users can access at the system through the commercial provider e-Geos, an ASI-Telespazio Company.
Although the first generation constellation satellite SAR instruments (SAR-2000) will observe in X-band (9.6 GHz with a wavelength of 3.1 cm), multi-mode scenarios (X-, C- L- and P-band) are planned for the future.
The overall system architecture is composed of a space segment, a constellation of SAR satellites, and a ground segment including full featured user services. The requirements call for the following general performance characteristics: 3) 4)
• Full global observation coverage with all weather, day/night acquisition capability
• Collection capability of large areas within a single pass, with along-track stereo imaging during a single pass
• High image quality, to allow a robust image interpretability at the requested scale of analysis (data sets are characterized by adequate spatial and spectral resolution suitable to perform analyses at different scales of detail)
• Ground track repeatability: the satellites of the SAR constellation shall have a ground track repeatability of better than 1 km
• Fast response times (from the data/service user request up to the data/service delivery to that requiring user).
The SABRINA project was started in 2004/5.
The COSMO-SkyMed space segment is composed of a constellation of four SAR satellites. The PRIMA [Piattaforma Riconfigurabile Italiana Multi-Applicativa (Reconfigurable Italian Platform for Multiple Applications)] bus of Alcatel Alenia Space is being employed. TAS-I (Thales Alenia Space-Italia) is also the prime contractor of the space segment [funding by ASI and MoD (Ministry of Defense)].
Figure 1: Artist's rendition of a COSMO-SkyMed spacecraft in orbit (image credit: ASI)
Each COSMO-SkyMed spacecraft is three-axis stabilized, consisting of the main body (bus), two deployable solar arrays, and a SAR antenna. The bus provides all support functions like: AOCS, electrical power (power generation, storage and distribution), data handling, thermal control, RF communications, and on-orbit propulsion for orbit injection and maintenance. The platform mechanical configuration consists of two elements or modules, namely: 5)
• SVM (Service Module) at the bottom of the bus which contains all bus subsystems including the propulsion module
• PLM (Payload Module) at the top, dedicated to the payload complement, the PDHT (Payload Data Handing and Transmission) subsystem, and the AOCS (Attitude and Orbit Control Subsystem) with star trackers gyros actuators.
The bus structure material is CFRP (Carbon Fiber Reinforced Plastic) while SVM and PLM consist of aluminum alloys. The interfaces of the SAR antenna, star trackers and gyros are mounted on the CFRP structure for pointing precision and stability (a star tracker is being used as well as a high-quality GPS receiver). The SAR antenna bore sight is pointing with an incidence angle about 38º to the right side of the spacecraft ground track.
• AOCS provides an antenna steering capability of ±2º in yaw as well as for a re-pointing capability to the left side of the ground track.
• ICS (Integrated Control Subsystem): ICS is the controlling system on board the spacecraft for the collection and distribution of information (commands, telemetry, on board data, and timing) and for the supervision of COSMO-SkyMed bus and yayload subsystems.
• TPS (Telecommands Protection System): TPS provides on-board decryption of the telecommands received from ground
• TT&C (Telemetry Tracking & Command): TT&C provides the two-ways S-band communication links between the satellite and the TT&C ground station
• PS (Propulsion Subsystem): Each S/C in the constellation features a RCT (Reaction Control Thruster) system for orbit maintenance. The PS includes six thrusters, arranged in two independently operable branches of three thrusters each, and the propellant and the pressurant, which are stored in a common tank
• TCS (Thermal Control Subsystem): TCS consists of elements that insulate the external surfaces of the satellite, heat pipes and thermal doublers to spread the heat load to be dissipated, radiator panels, and automatic electrical heaters placed under ICS control.
• PDHT (Payload Data Handling and Transmission): PDHT manages all data handling and transmission of the science data generated by the SAR payload. It includes all the necessary interfaces for acquiring telecommands and ancillary data from ICS and for storage, formatting, encryption and ground downlink of the science data from the SAR instrument.
The current TAS-I PDHT reference architecture for medium - large satellites is the one already flying on RADARSAT-2, and on the COSMO SkyMed Constellation (FM1, FM2, FM3). The architecture is depicted in Figure 2. 6)
The following acronyms of Figure 2 are defined:
- TXA (Telemetry X-band transmission Assembly). TXA is composed by two X-Band transmission chain and it includes: a) Differential-QPSK modulators: for each transmission carrier the modulators are dual redundant; b) Coaxial switches matrix to opportunely route modulator output signals towards the power amplifiers; c) HPAs (High Power Amplifiers); d) Output ports filtering and diplexing; e) Waveguide switch matrix to opportunely route signals to be transmitted towards the antennas.
- XBAA (X-Band Antenna Assembly). XBAA includes two deployable isoflux antennas in right and left looking observation configuration, to operate in both attitudes and ensure redundancy.
- DSHA (Data Storage and Handling Assembly). DSHA is composed by two identical Mass Memory Storage Unit (MMSU). The main elements included within each MMSU are: a) Supervisor with On Board Software (OBS). Supervisor is dual-redundant; b) Science Data Interface (SDI); c) Mass Memory Modules stack for storage of SAR payload science data; d) Wideband Data Handler (WBDH) for science data encryption and auxiliary and science data formatting over downlink channels 1 & 2.
Figure 2: PDHT reference architecture for medium - large satellites (image credit: TAS-I)
GPS/GNSS receiver: The SSTI (Satellite-to-Satellite Tracking Instrument) is the LAGRANGETM GPS receiver. LAGRANGE is a space-qualified, dual-frequency, GPS receiver designed for navigation purposes. The receiver is built by TAS-I (Thales Alenia Space-Italia), Milan (formerly LABEN SpA). LAGRANGE processes the received GPS and GLONASS signals in both the L1 and L2 frequency bands, allowing compensation of ionospheric delays. A special codeless tracking scheme is implemented to process the encrypted P(Y) signals transmitted in the GPS L2 frequency band.. The instrument is equipped with one hemispherical coverage antenna with (nominal) boresight pointed to the Zenith direction. The receiver uses the AGGA-2a signal processing chip developed by ESA and is currently a payload of several ASI, ESA, ISRO (Indian Satellite Agency) and CONAE (Argentina) missions. 7)
LAGRANGETM onboard COSMO-SkyMed and RADARSAT-2 is designed to process only GPS signals. The on-board orbital determination strategies performed on board for the COSMO-SkyMed and Radarsat-2 missions have successfully reached the required accuracies necessary to guarantee high level SAR products related to Earth imaging.
Figure 3: LAGRANGE navigation antenna (left) and L1 and L2 gain pattern (right), image credit: TAS-I)
Figure 4: Photo of the LAGRANGE receiver (image credit: TAS-I)
Figure 5: Block diagram of the LAGRANGE receiver (image credit: TAS-I)
EPS (Electric Power Subsystem): The mission imaging capability requirements call for very high peak power loads of up to 14 kW. The overall power consumption associated with a specific operating mode profile is:
• Spotlight mode: about 460 to 650 A, max duration of 10 s (single spotlight)
• Stripmap mode (reference to HImage): about 330 to 450 A, max duration of 10 minutes in continuous acquisition.
The solar arrays of the S/C (total area of 18.3 m2, built by Galileo Avionica) provide a minimum power of 4 kW (BOL) and 3.6 kW (EOL, 5 years) using triple-junction solar cells. The EPS (Electrical Power Subsystem) configuration is based on an unregulated bus architecture with a bus voltage in the range from 26 to 37.8 V. The EPS uses the advanced Li-ion technology (Sony US18650 hard carbon lithium ion cells) for its ultra-high energy density. The battery requirements are summarized as follows: 8) 9) 10)
- Battery nominal capacity of 336 Ah
- Maximum depth of discharge of 35%
- Maximum discharge current of 735 A for the SAR peak imaging
- Maximum discharge voltage of 26 V
- Battery total mass of 136 kg
- Minimum battery life of 5 years
- Minimum battery reliability of 0.999 over the mission design life.
The large Li-ion battery design consists of 2016 cells (a 9S-224P configuration - or a topology of 9 cells in series and 224 strings in parallel), capable of supporting 17.8 kW of peak power demand for some SAR support modes. The battery consists of an assembly of eight identical modules paralleled through two electrical junction boxes and two power bus-bars that connect the junction boxes to the CUS (Current Unit Sensor). The versatility of the S-P topology and the COM DEV design (COM DEV International Ltd., Cambridge, ON, Canada) solution enabled this flexible design to adapt to any power requirement by simply varying tray sizes and the number of modules with minimal impact on design, schedule and cost.
Figure 6: Illustration of the first Li-ion battery for SAR support modes (image credit: COM DEV Ltd.)
To minimize the magnetic moment of 775 A peak discharges, the two trays in each module are rotated 180º with respect to each other. Since the current in both trays will be of equal value but in opposite directions, this causes the magnetic moment from the top tray to cancel the magnetic moment produced in the bottom tray. The COM DEV battery design represents a technology enabler for the COSMO-SkyMed mission.
The raw power coming from the SA (Solar Array) is conditioned by the PCU (Power Control Unit) using a MPPT (Maximum Power Point Tracking) algorithm. The PCU manages also the charge of the battery, using a control loop algorithm in collaboration with the CUS (Current Unit Sensor), providing the power to the thermal control heaters and drives the on-board pyro devices. The measured power of the SA is > 4.5 kW @ BOL. 11)
• The spotlight mode uses about 13 kW for a maximum duration of 10 s (single spotlight)
• The stripmap mode uses about 7 kW for a maximum duration of 10 minutes as continuous acquisition.
Figure 7: Block diagram of the EPS (image credit: ASI)
Figure 8: Illustration of the deployed COSMO-SkyMed spacecraft (image credit: ASI)
Figure 9: The COSMO-SkyMed spacecraft in launch configuration (image credit: ASI)
The mass of each spacecraft is about 1700 kg. The design life is 5 years. Each spacecraft in the constellation provides an onboard operational autonomy for a period of about 24 hours.
Launch: The launch of the first COSMO spacecraft in the constellation took place on June 8, 2007 (UTC). The launch was provided by the Boeing Company on a Delta-2 (7420-10 configuration) vehicle from VAFB, CA. This represented the first commercial Delta-2 launch since the formation of the United Launch Alliance (ULA) in December 2006. 12)
Table 1: Overview of COSMO-SkyMed constellation launch schedule 13)
RF communications: Onboard payload recording is provided with a 300 Gbit solid-state memory. The payload data are compressed and encrypted and downlinked in X-band at 300 Mbit/s. The TT&C function is provided in S-band.
Orbit of constellation:
Circular sun-synchronous dawn-dusk orbit, nominal altitude = 619.6 km, inclination = 97.86º, period = 97.1 min, with LTAN (Local Time of Ascending Node) at 6:00 hours (dawn/dust orbit), 14.8125 rev./day (or 14 13/16). All spacecraft of the SAR constellation will be positioned in the same orbital plane with a phasing outlined in Table 2. The nominal repeat cycle is 16 days; however, each single satellite will have a near revisit time of 5 days. 14)
• Orbit planes: during nominal operations all satellites of the SAR constellation shall operate on the same orbital plane
• Nominal orbit: dawn-dusk SSO (Sun Synchronous Orbit) with 16 days of orbit cycle in 237 orbit per orbit cycle
• Satellite phasing: during nominal operations, the satellites of the SAR constellation shall be equi-phased on the orbital plane
• Ground track repeatability: the satellites of the SAR constellation shall have a ground track repeatability of better than 1 km
The phasing of the S/C in the orbit plane has been selected to achieve optimum performance in terms of accessibility and revisit time with respect to the number of satellites. Table 3 summarizes the achievable observation performance capabilities of the SAR satellites within the latitudinal coverage of ±20º - 60º.
Table 2: SAR satellite phasing scheme
Figure 10: Illustration of COSMO-SkyMed constellation phasing scheme (image credit: TAS-I)
Potential of bistatic SAR applications: By changing one satellite inclination slightly, to generate a positive drift in the right ascension of the ascending node (RAAN), permits a number of bistatic observations under variable bistatic angle. 18)
Figure 11: COSMO-SkyMed constellation in nominal configuration (image credit: ASI)
Table 3: Accessibility parameters for the various constellation configurations
Figure 12: Artist's view of the orbiting COSMO-SkyMed constellation (image credit: Telespazio) 19)
Table 4: Overview of operational support modes
The COSMO-Skymed constellation can be operated in two basic configurations, namely in the "nominal orbital configuration" or in the "interferometric orbital configuration."
• The key requirement of Earth observation for the nominal orbit configuration is "ground track repeatability," needed to keep the subsatellite point within a given accuracy (tolerances of better than ± 1 km) with respect to the nominal ground track. This implies active spacecraft maneuvers for constellation geometry maintenance (compensation for the effects of in-plane and out-of-plane perturbations on the nominal orbit).
• The interferometric tandem configuration is designed to permit observations of 3-D SAR imagery by combining two radar measurements of the same target from slightly different incidence angles. The control requirement for this configuration calls for the maintenance of the interferometric baseline within an accuracy of tens of meters. Hence, in the interferometric configuration, the constellation control involves ground track control (analogous to the nominal configuration) plus the maintenance of the interferometric baseline. This baseline maintenance can be achieved by:
- Tandem-like interferometry (over a period of 1 day). Interferometric observations are achieved within a 24 hour delay.
- The nominal tandem interferometry offers two support modes:
1) The in-plane tandem configuration sets two satellites in the same orbital plane, in close proximity, to obtain the required baseline.
2) Tandem configuration of different orbital planes. This configuration sets the two satellites of the tandem, separated both in phase and in LTAN, with two different orbital planes with slightly different nodes such as to obtain the 'same' ground track.
Figure 13: Tandem interferometric configuration in different orbital planes (image credit: ASI)
• September 2016: One of the most challenging tasks of COSMO-SkyMed mission is the maintenance of such a complex system. Process and information flows were jointly designed and developed by Italian Space Agency (ASI), Italian Ministry of Defence (It-MoD) and Industrial staff in order to continuously guarantee the performances and the availability of the system in a Dual (Civilian/Military) context. 20)
- In order to assure the system functionality, a lifelong system coordination and engineering support has been deployed by industrial team. This support is guaranteed by maintenance and analysis activities provided by two dedicated teams: an on-site support team deployed in the ground centers (constituted by industrial personnel directly involved in day by day operation and maintenance activities) and a support engineering team located at industrial premises that works in strict conjunction with the Customer to report the end-to-end service performance and efficiency trend during the operative phase. Furthermore, this team is devoted to maintain/improve the design of the system through the analysis, evaluation and management of the occurred non-conformities. Based on that, it can propose system modifications and enhancements.
- The overall system performances are monthly monitored in order to verify the stability with reference to the committing system requirements. They are separately evaluated for defense and civilian users. In particular the system performance is assessed through a dedicated set of effectiveness (Ei) and availability (D) parameters. Furthermore the Support System itself is subjected to an assessment in terms of efficiency through the evaluation of a dedicated set of (Ki) parameters. This allows measuring, in a monthly time frame, if the support system operated in an efficient way.
- This careful organization allowed achieving that, since the start of the operative phase, in the real operational scenarios, the end-to-end system is able to maintain the required performance stability over time with the required efficiency.
- The continuous monitoring of image quality and geo-location performances of each satellite, achieved through the submission of specific acquisitions, confirms that the System is able to guarantee the required performance:
a) for all SAR payload imaging modes
b) within the time constraints imposed by the mission
c) in all system operative modes (Routine, Crisis, Very Urgent)
d) sustaining the associated operational loads
e) granting the required level of service in terms of availability and efficiency.
- Experimentation campaigns: Based on dedicated studies and continuous monitoring of the system it has been possible to identify and propose to Customers potential enhancements to increase the performances with respect to the original specifications. The designed improvements were mainly related to resolution and swath dimensions for spotlight modes. The high flexibility of CSK system allowed configuring new tables to host the configurations of the new spotlights modes without impacting the existing ones. The improvements have been possible by stressing all margins and all the available in-flight technology. The achieved performances are also the result of an improved capability of the on-ground planning tools and of image processors, in terms of technology and new models definition and implementation.
- Figure 14 shows some results concerning the IRF (Impulse Response Function) profiles and main performance parameters. The expected performances (resolution and swath) have been reached assuring also the performances related to PSLR (Peak to Sidelobe Ratio) and SSLR (Spurious Sidelobe Ratio) requested for all the standard SAR operative modes of COSMO-SkyMed. The results have been fully in line with those expected and, based on those outcomes, Customers and Industry agreed to make operative the new sensor modes.
- Even though the improvement in the performances could be simplified in an improvement in resolution, it is worth to be noted that the real improvement is also in radiometry due to the multi-look effect achievable in azimuth direction (to obtain a pixel square). In Figure 15, this effect is clearly visible in the information coming from the shadowing area that in the experimental Spotlight (named Spotlight 2A) case shows a sharper profile.
Figure 14: IRF (Impulse Response Function); Left: Range profile; Right: Azimuth profile (image credit: TAS-I, ASI, Telespazio)
• August 29, 2016: The first satellite images of the areas struck by the earthquake on 24 August have been made available by the Italian Space Agency: the "eyes" of the COSMO-SkyMed system, at work right from the first hours after the disastrous earthquake that hit the Apennine areas in central Italy, were set in emergency mode and pointed towards the zones affected by the earthquake that hit at 2.36 hours last week. The data acquired were processed and analysed with the help of the Civil Protection agency and the INGV to manage the emergency. 21)
• June 2016: Although all four satellites of the COSMO-SkyMed (CSK) mission are now beyond their design life, the performances of the constellation are fully preserved. This is due to a careful monitoring of the entire system to maintain the required performance stability over time with the required efficiency. The continuous maintenance serve of image quality and geolocation performances of each satellite, achieved through the submission of specific acquisitions, confirms that the System is able to guarantee the required performance: 22)
- for all SAR payload imaging modes
- within the time constraints imposed by the mission
- in all system operative modes (Routine, Crisis, Very Urgent)
- sustaining the associated operational loads
- granting the required level of service in terms of availability and efficiency.
Based on dedicated studies and continuous monitoring of the system it has been possible to identify and propose to customers potential enhancements to increase the performances with respect to the original specification. The designed improvements were mainly in terms of resolution and swath dimension for spotlight modes. The high flexibility of CSK system allows configuring new tables to host the configurations of the new spot-lights modes without impacting the existing ones. The improvements have been possible by stressing all margins and all the available in-flight technology. The achieved performances are also the result of an improved capability of the on-ground planning tools and of image processors, in terms of technology and new models definition and implementation.
Performance comparison: Even though the improvement in the performances for civilian modes could be simplified in an improvement in resolution, it is worth to be noted that the real improvement is also in radiometric performances due to the multilook effect achievable in azimuth direction to obtain a pixel square. In particular, this effect is clearly visible in the information coming from the shadowing area that in the Spotlight 2A case shows a sharper profile (Figure 15).
Figure 15: Enhanced Spotlight (left) versus Spotlight 2A (right) by direct comparison on images (image credit: TAS-I, Telespacio, ASI)
After a successful experimental campaign, the upgrades are now ready to be implemented in the current system.
• August 2015: COSMO-SkyMed represents the largest Italian mission and it is still operating with four mid-sized satellites placed in LEO. Each spacecraft is managed by a full-featured Ground Segment to exploit observation capabilities and providing SAR imaging services on a global scale, and thus guaranteeing a global revisit time in the order of 12 hours (worst case) for the 100% of the Earth sites (in the nominal configuration). Currently, three of the four satellites have completed their nominal operational life (5.25 years), but the constellation is still operating and providing data with the required image quality (the nominal End Of Life due to the "consumables", i.e. fuel sizing, batteries life etc, is 7 years). 23)
- In the last year, through the CEOS (Committee on Earth Observation Satellites), which ensures international coordination of civil space-based EO programs and promotes exchange of data to optimize societal benefit, ASI communicated the extensions of the operational phase, as indicated in Table 5.
- ASI is developing the CSG (COSMO Seconda Generazione) program with the aim of pursuing the twofold need of ensuring operational continuity to the first generation constellation, while achieving a generational step ahead in terms of functionality and performances so as to further improve the observing power of the existing constellation. The CSG constellation will consist of two satellite SAR constellation, similar to the one on-board of the actual system but with some enhancements: from the performance point of view the quality of the imaging service will be improved, providing the End Users with new enhanced capabilities in terms of higher number of equivalent images and of increased image quality (larger swath and finer spatial and radiometric resolution) with respect to the first generation, along with additional capabilities (e.g. full polarimetric SAR acquisition mode).
• Feb. 25, 2015: For the first time ever, the Italian Space Agency (ASI) is offering complete data sets from the CSK (COSMO-SkyMed radar satellite Constellation) free of charge. Two separate Open Calls were published today, targeted at the national and international scientific community and national private sector SMEs (Small and Medium sized manufacturing Enterprises), start-ups and university spin-offs. A similar initiative took place four years ago, although with different criteria and offered exclusively to the scientific community. The goal - according to the two calls - is to "promote the development of new algorithms" while also contributing to "improving existing products and services" and the development of "innovative technological applications": all exclusively for civilian use. 25) 26)
• January 2015: All four COSMO-SkyMed satellites are reported in good health. But the two that entered service in 2007 and 2008 are expected to need replacement in 2017 and 2018 to assure data continuity.
- A last-minute Italian government allocation of funding for the second-generation COSMO-SkyMed dual-use radar Earth observation program came in time to permit the prime contractor TAS-I (Thales Alenia Space -Italia)to assure the satellites' launches in 2017 and 2018, Thales Alenia Space officials said. 27)
• August 2014: A magnitude 6.0 earthquake struck southern Napa county northeast of San Francisco, CA, on Aug. 24, 2014, causing significant damage in the city of Napa and nearby areas. The earthquake deformed Earth's surface and this deformation was measured by precise analyses of data from nearby Global Positioning System (GPS) tracking stations of the Plate Boundary Observatory (operated by UNAVCO for the National Science Foundation) and the Bay Area Rapid Deformation network (operated by Berkeley Seismological Laboratory). 28)
- NASA/JPL scientists, in collaboration with the Italian Space Agency's (ASI) Center for the Interpretation of Earth Observation Data and the UB (Università degli studi della Basilicata), also analyzed radar images from ASI's COSMO-SkyMed satellites to calculate a map of the deformation of Earth's surface caused by the earthquake, as shown in this false-color map that has been combined with shaded relief topography in gray. The colors indicate the amount of permanent surface movement that occurred almost entirely due to the earthquake, as viewed by the satellite, during the one-month interval between two COSMO-SkyMed images, one before and one after the earthquake.
The red areas south of Napa moved about 10 cm to the east and upward during the earthquake. The dark green and blue areas north of Napa moved about 10 cm to the west and downward due to the earthquake. Scientists use these maps to build detailed models of the fault and associated land movements so that they can better understand the impact on future earthquake activity.
ASI activated crisis response for this event. The first overflightof the area by the COSMO-SkyMed X-band radar occurred on August 26, 2014. The data was delivered to the ARIA-MH (Advanced Rapid Imaging and Analysis for Monitoring Hazards) data system on Aug. 26, at 23 hrs local time. The ARIA generated map of deformationfield ("interferogram" Figure 17) was provided on Aug. 27, at 4 hr. Each fringe represents 1.5 cm of motion. 29) 30)
Figure 16: COSMO-SkyMed false-color map of the stricken region (image credit: ASI, NASA/JPL)
These maps cover only a portion of the full COSMO-SkyMed imagery, measuring approximately 55 km x 50 km, processed by the NASA JPL-Caltech ARIA (Advanced Rapid Imaging and Analysis) team using X-band interferometric synthetic aperture radar data from the Italian Space Agency's COSMO-SkyMed satellite constellation.
• June 2014: Although the first two COSMO-SkyMed satellites have already reached their nominal EOL (End-of-Life), the constellation is still able to provide the products with the required image quality and geolocation performance, fully sustaining the associated operational loads and granting the required level of service in terms of availability and efficiency. 31)
The four coplanar satellites, together with a suitable network of ground stations for science data downloading, allow to grant short revisit and response times. The constellation is capable of downloading up to 1800 images/day. During the last 6 years COSMO-SkyMed has been employed to provide "Institutional Awareness", in order to make proper decisions to prevent, respond and manage world-wide crises, to assess damages and supply fundamental information to rescue from catastrophic situations.
• The COSMO-SkyMed constellation is operating nominally in 2013. One important aspect of the mission is the quick provision of imagery for disaster and emergency management due to its daily global coverage capability. 35)
• ASI is providing a certain amount of imagery of selected "geohazard supersites" of the COSMO-SkyMed constellation for the study of major geophysical hazards such as earthquakes and volcanic eruptions, in which member organizations of GEO (Group on Earth Observation) and GEOSS (Global system of Earth Observation Systems) have been involved in for many years.
For this purpose, four candidate sites have been defined to become earthquake "Supersites" (Tokyo, Vancouver-Seattle, Los Angeles and Istanbul) and 4 for volcanoes (Mt Fuji, Mount Vesuvius and Campi Flegrei, Mount Etna, and Hawaii ). 36)
Figure 18: Reliability and continuity of the COSMO-SkyMed constellation in 2012 (image credit: e-GEOS) 37)
• In June 2012, e-GEOS signed a contract for the setting up of a COSMO-SkyMed Receiving Station in Sodankylä, Finland, to be operated by FMI (Finnish Meteorological Institute) as a part of the Finnish Ministry of Transportation and to become operational before the end of 2012. This agreement will provide Finland with COSMO-SkyMed direct reception capabilities over the Baltic and a large part of the Arctic areas to monitor ice formation and movements and Arctic changes. 38)
Figure 19: Illustration of the Sodankylä station reception cone (image credit: e-GEOS)
• The COSMO-SkyMed constellation is operating nominally in 2012. The high revisit frequency of the constellation is showing new and enhanced capabilities in particular for agri-environmental targets.
Legend to Figure 20: The radar image was snapped by chance during a routine reconnaissance survey by an Italian COSMO-SkyMed satellite orbiting above the Earth, according to ASI (Italian Space Agency) and clearly shows the Costa Concordia wreckage and the entire island of Giglio. - The composite mosaic above combines the COSMO-SkyMed space radar image with the DigitalGlobe WorldView satellite photo, to provide a side-by-side comparison of the wreckage from the two different satellite systems which have different resolutions.
The ocean liner hit a reef on January 13, 2012 and tipped over. The Costa Concordia was carrying about 3,200 passengers and a crew of 1,000 when it ran aground, according to news reports. The accident killed 11 people, with more than two dozen others still missing (as of January 20, 2012, Ref. 39).
• On January 13, 2012, the companies KSAT (Kongsberg Satellite Services), ConocoPhillips, and e-GEOS (joint venture of ASI and Telespazio for commercial product services) have signed a contract for monitoring ice formation and movement throughout the 2011-12 winter season. The effort will make exclusive use of the unique capabilities of the Italian COSMO-SkyMed constellation of four VHR SAR satellites, owned by the Italian Space Agency (ASI) and exploited commercially by e-GEOS, to provide coverage over the area of interest for the entirety of the season. The frequency and duration of the coverage will allow unprecedented detail on the evolution of sea ice conditions using high-resolution, X-band (9.6 GHz) SAR technology.40)
By acquiring coverage for a full season with hundreds of SAR images, it will be possible for the first time to thoroughly analyze the patterns of ice formation, the characteristics of the ice under winter conditions, and the progression of the spring melt: how rapidly it occurs and the size and prevailing direction of the ice fragments Ref. 40).
Legend to Figure 21: SAOCOM is a L-band mission of CONAE (Argentina) co-financed by CONAE and ASI. Note: CSK is the acronym for the various COSMO SkyMed satellites.
Figure 22: COSMO-SkyMed orbital configuration in 2011/12 (image credit: e-GEOS, Ref. 41)
• Currently (summer 2011), the full constellation has been deployed having only a single couple of satellites in a one-day interferometric configuration through the second and the third satellite (COSMO-2 and COSMO-3), whereas the first and the fourth satellite of the constellation (COSMO-1 and COSMO-4) are placed with a displacement of 90º in true anomaly in order to optimize the time performances of the overall constellation (Figure 23). - Other configurations are possible in order to exploit the interferometric capabilities of the constellation. The orbital cycle is 16 days and the constellation revisit time is < 12 hours. The System Operational Lifetime is 15 years, with a satellite lifetime of 5 years. 42)
Figure 23: Actual orbital configuration of the COSMO-SkyMed constellation in 2011 (image credit: ASI)
Three different system operative modes are defined:
1) Routine support
2) Crisis support
3) Very urgent.
In the first mode (routine) the requests of the users pertaining image acquisitions are planned and sent to the constellation once a day. In the second mode (crisis) this operation is done twice a day. The third mode (very urgent) is asynchronous, allowing the servicing of an image acquisition request with the minimum possible latency.
Figure 24: User privileges of COSMO-SkyMed data products (image credit: ASI, Ref. 86)
The system is capable to satisfy a User Request (ability to deliver the image product required by an End User in a timely manner) which in the case of the first level of SAR standard products (not derived from spotlight mode acquisitions) is within 72 hours for the system working in routine mode (acquisition plan uploaded once a day), 36 hours for the crisis mode (acquisition plan uploaded twice a day) and 18 hours for very urgent mode (acquisition plan uploaded asynchronously).
The constellation average daily acquisition capability is 1800 images acquired in a 24 hours window (75 spotlight plus 375 stripmap or 150 ScanSAR for each satellite). Constellation peak daily acquisition capability is 10 minutes of continuous operation in Stripmap or ScanSAR modes or alternatively 20 spotlight images.
Currently (2011), the data volume is 475 products at the C-UGS (Civilian-User Ground Segment) and 75 products at the D-UGS (Defense - User Ground Segment), with the limitation of 560 products downloadable at the Civilian Ground Segment, whereas the System daily products number at C-UGS is 200.
Regarding the capability to change attitude in order to acquire images at both right and left side, four pointing modes are allowed: the right-looking nominal mode is the standard operation mode of the satellite, in which the SAR instrument has access capability in the incidence angle range of 25-50º, yielding optimum radar imaging performance. The left-looking nominal mode is obtained by commanding a roll maneuver to point the SAR antenna mechanical boresight on-ground to the left side. The extended mode is obtained by platform roll agility, gaining access to the incidence angle ranges 20-25º and 50-59.5º, with degraded performance. This extended access region is achieved by commanding a roll maneuver up to -7º and + 7º with respect to the nominal pointing (Ref. 42).
• Summer 2011 - image product qualification: While product characterization and calibration measurements started early, the final SAR verification had to wait for the complete verification of all input products (specifically the orbit and attitude data) and the finalization of the overall SAR system performance and verification. Thus more than a thousand "operational" products of all modes and variants have been used for final verification of basic characteristics like format, saturation degree, scene location and extent. A subset of specific acquisitions over calibration sites were processed to various product variants and analyzed with respect to point target responses and radiometry. These measurements confirmed the very excellent COSMO/SkyMed product performance in terms of: 43)
- Focusing quality and resolution: since the overall SAR system performance and stability are outstanding, some margins in the spotlight azimuth beam steering sequence and processing have been invested in an improved IRF performance. This yields ratios to spatial resolution in azimuth and ground range, over an access region (20º .. 60º), better than 1 m.
- Sensitivity: the NEσο (or NESZ) is always better than the specified value (-22 dB) for all the operation modes as well as the ambiguity to signal ratios.
- Radiometry: the calibration effort, the pointing accuracy and the operational pattern correction and normalization result in products with a radiometric accuracy better than 1 dB.
• In April 2011, the COSMO-SkyMed constellation is now fully operational and complete with the arrival of the system's fourth satellite in its final orbit position. The satellite was launched on Nov. 6, 2010 (UTC) from the Vandenberg base in California, and came fully on-stream with the transmission of the first images to the ground stations (Ref. 89).
The early orbit operations for the fourth COSMO-SkyMed satellite were once again successfully managed by the LEOP (Launch and Early Orbit Phase) team at Telespazio's Fucino Space Center, while the first radar images from COSMO-SkyMed-4 were acquired by the Matera Space Center of ASI.
Since the beginning, the constellation has been successfully used in various applications in the field of risk and emergency management such as: China's earthquake, Myanmar and Haiti flood, Abruzzo earthquake, ice monitoring (reduction of the glaciers, Wilkins Ice Shelf disintegration), multi-temporal acquisition for agriculture monitoring, interferometry, landslides monitoring, maritime surveillance and security, rapid mapping. The results revealed the strong contribution of the X-band SAR and the importance of satellite constellation to get fast response and short revisit time. 44)
• On Dec. 16, 2010, the project is reporting that COSMO-SkyMed-4 has reached its final orbit and is transmitting its first images. 45)
• On Nov. 6, 2010 (UTC), the COSMO-SkyMed constellation was completed with the launch of the COSMO-SkyMed-4 spacecraft from VAFB, CA. This makes COSMO-SkyMed the first global Earth observation constellation that has both civil and military applications (Ref. NO TAG#. The satellite telemetry was acquired on the same day by Telespazio's Fucino Space Center, which is in charge of its operations for the entire duration of the mission.
• The 3-spacecraft constellation is operating nominally in 2010. The system is able to provide the products with the required image quality and geolocation performance for all SAR payload imaging modes within the time constraints imposed by the mission in all system operative modes (Routine, Crisis and Very-Urgent), fully sustaining the associated operational loads and granting the required "Quality of Service". 46)
Figure 25: COSMO-SkyMed image of the summit of Mount Etna at 1 m spatial resolution (image credit: ASI)
Figure 26: COSMO-SkyMed multitemporal SAR image of the Icelandic volcano Eyjafjallajoekull (image credit: Telespazio)
Legend to Figure 26: The image is obtained using two shots, one taken before and one taken after the eruption (on 03/04/2010 and on 19/04/2010). In the image, it is possible to distinguish changes in the signal recorded by the radar: these can be identified by the presence of red and green areas.
The new openings in the crater are clearly visible on the right of the image with the bright green edge. The large areas of dark red and green surrounding the crater and extending eastwards can be interpreted as changes caused by the melting ice and deposits of ash expelled by the volcano during the eruption. - The blue zones represent areas where there has not been any change. [Red: 03/04/2010; Green: 19/04/2010; Blue: Coherence]
Figure 27: COSMO-SkyMed multitemporal SAR image of the Icelandic volcano Eyjafjallajoekull (image credit: Telespazio)
Legend to Figure 27: The false color image, obtained from a different elaboration of the same data (as presented in Figure 26), shows changes in the red and light blue area. On the left of the image are clearly visible traces left by researcher on the surfaces of the glacier.
• October 2009: TAS (Thales Alenia Space) handed over the management of the COSMO-SkyMed ground network to the Italian Defense Ministry, an event that marks the beginning of full operational use of the system by its principal customer, the Italian Defense Ministry and the ground network provider Telespazio of Rome. 47)
In the fall of 2009, the three launched satellites are in a "Tandem like" configuration in which the mutual angular distance of satellites COSMO-2 and COSMO-3 is 67.5º (Figure 28). The constellation is currently providing products with a maximum revisit time of 37 hours (in routine mode) and making available acquisitions with a de-correlation time of 8 and 16 days (two consecutive acquisitions with the same geometric conditions).
• The COSMO-SkyMed-3 spacecraft completed the commissioning phase in July 2009 and is operational. During commissioning the end-to-end system performance has maintained the required stability and the ILS&OPS (Integrated Logistics & Operations) support system has fully demonstrated its efficiency. COSMO-SkyMed-3 is positioned in the so-called "one-day interferometry configuration", allowing the constellation to detect interferometric acquisitions with a de-correlation time equal to one day and to improve the number of data take acquisitions (minimizing the acquisition conflicts). 48) 49) 50) 51)
Figure 28: COSMO-SkyMed constellation in the 2009 configuration (image credit: ASI)
The system is extensively used by Italian and international civil users, in particular Italian Civilian Protection and ASI pilot projects users who are exploiting the COSMO-SkyMed data for several applications.
Example: On April 6, 2009, a magnitude 6.3 earthquake hit a large area near L'Aquila city, in Central Italy. The COSMO-SkyMed system was the first to catch images over the area, thanks to its very low revisit time (about 37 hours). - After the quake, the system began to gather up to 6 images/day, in all possible modes and viewing geometries. During the following month, 3 images/day were acquired, for a total of 100 images. To reach this target, all orbital passes, in ascending ascending and descending nodes, the right- and left- side viewing capabilities, as well as different incidence angles were used (Ref. 48).
Figure 29: COSMO-SkyMed interferogram of the L'Aquila-Abruzzo Earthquake (image credit: ASI)
Legend to Figure 29: Co-seismic ascending Cosmo-SkyMed interferogram covering the April 6, 2009 L'Aquila earthquake: pre-event image April 4, post-event image April 12. Each fringe indicates a ground subsidence (in the satellite Line of Sight) of 1.5 cm, for a total of about -20 cm between L'Aquila and the Fossa village. This subsidence took place during the earthquake (co-seismic deformation) and is the surface response due to the dislocation at depth along the seismic fault plane (Ref. 35).
• Nov. 25, 2008: One month after its launch and while still in its commissioning phase, the 3rd satellite of the Italian radar constellation COSMO-SkyMed-3 has produced its first images. 52)
• In 2008, a new company, e-GEOS, was set up for the provision of commercial products and services. e-GEOS is a joint venture between ASI and Telespazio (a Finmeccanica/Thales company). The objective is to bring data from the COSMO-SkyMed satellite constellation to the global market. 53)
Figure 30: Company profile of e-GEOS (image credit: e-GEOS) 54)
• The COSMO-SkyMed-1 and -2 spacecraft are fully operational since August 1, 2008 when the commissioning phase ended and the two spacecraft were declared "operational". The observation data are being used by both civilian (institutional and commercial) and defense users. 55) 56) 57)
• The commissioning phase has been performed in parallel for COSMO-SkyMed-1 and COSMO-SkyMed-2. During the commissioning phase the functionality and performance of all subsystems and payloads were tested and validated including all calibration activities. - During the commissioning phase the first two satellites have begun to acquire thousands of images all over the globe, showing the full potential of the system.
• The images acquired by the COSMO-SkyMed satellites during February and March 2008 over the Northern Caspian Sea and the Antarctic region show the great potentiality of the X-band SAR data in ocean and ice applications.
Sensor complement: (SAR-2000)
SAR-2000 (Synthetic Aperture Radar-2000):
The SAR-2000 instrument was designed and developed by TAS-I (Thales Alenia Space Italia), formerly Alenia Spazio, Rome. SAR-2000 is a multi-mode instrument, a programmable system providing different performance characteristics in terms of swath size, spatial resolution, and polarization configurations. The SAR transmitter/receiver system operates through an electrically steerable multi-beam antenna which concentrates the transmitted energy into narrow beams in the cross-track direction while the characteristics of the transmitted pulses and the echo signal determine the spatial resolution and coverage. 58) 59) 60)
The objectives call for the following design features:
• Very large instantaneous bandwidth
• Electronic beam steering in range and azimuth
• Multi-polarization and full polarimetry support (data takes with selectable polarization and multiple polarizations on the same scene using the ping-pong technique)
• Programmable PRF, pulse width and bandwidth
• Multiple imaging mode support
• Onboard hardware calibration techniques
• Onboard data compression processing techniques (both analog and digital)
The instrument is composed of two major elements: a phased array antenna subsystem (external equipment), also known as SAA (SAR Active Antenna), and of the central electronics module (internal equipment), also known as SIE (SAR Internal Electronics). The SIE module in turn consists of three parts:
• RFA (Radio Frequency Assembly). RFA has four units: XDU (X-band Driver Unit), XSU (X-band path Switch Unit), DCU (X-band Receiver), and FGU (Frequency Generation Unit).
• DESS (Digital Electronics Subsystem)
• CPSU (Central Power Supply Subsystem)
The SAA is in charge to provide distributed power amplification, beam steering capabilities and low noise reception of echoes through the beam forming functions. The SIE is in charge to provide functions in electronics power supply, signal generation, frequency up-conversion, TX/RX signal routing, frequency down-conversion and data production.
The key design elements of the instruments are: the pulse generation, the band stretching and the up conversion, the active antenna, the deramp on receive for the spotlight mode which replace the first down-conversion scheme of the two stage superheterodyne receiver, the multi-rate A/D conversion with selectable compression algorithm based on BAQ.
Figure 31: Block diagram of the SAR-2000 instrument (image credit: TAS-I)
The pulse generator is based on baseband (BB) generation followed by SSB modulation: the BB generation is done with a memory read out scheme which allow the generation of the selected waveform by the simple calculation of the samples. Presently chirp waveforms are implemented (with both length and bandwidth programmable in accordance to the specific mode configuration) with the possibility to compensate the effects of both the H/W distortions versus the frequency and the temperature through some predistortion coefficients computed during the on-ground instrument characterization and selectable from a LUT (Look Up Table) according to the specific programmed configuration.
The up-converter is based on a first stage conversion (from chirp carrier to 2400 MHz) followed by a x4 frequency multiplication; the results of such a scheme if twofold: 1) it implements the up conversion to X-band (9600 MHz) carrier frequency; 2) it performs a bandwidth stretching to achieve the resolution requirements in spotlight mode. On the opposite site, that architecture imposed a very tight control on the image rejection and spurious level suppression to avoid any reduction in signal dynamic after deramping.
The Rx chain is based on a two stage superheterodyne scheme where the first stage is replaced by a deramping function for the spotlight mode only. The IF stage implements four bandpass filters to be selected according to the specific operation mode and configuration. The last stage is the I-Q demodulator that operates directly at 2400 MHz IF. The digital section is based on a programmable multi-rate A/D converter followed by the pre-processor (decimator) and the Block Adaptive Quantizer (8:4, 8:3, 8:2, and 8:1 processing schemes and the possibility to by-pass the compression). The data are formatted in CCSDS like standard packets for the transmission on a high speed link (HotLink) with the nominal data data rate of 600 Mbit/s.
Antenna: The SA-2000 X-band antenna subsystem, also referred to as SAA (SAR Antenna Assembly), is a large deployable planar phased array operating in dual linear polarization, able to generate and to steer T/R beams, capable of assuming many different shapes along the antenna elevation plane. Moreover the antenna has in-orbit calibration features and the possibility to upload newly designed beams. 61) 62) 63)
The mission requirements call for the following features:
• Beam steering on the two main antenna planes
• Elevation beam shaping according to different masks (up to 68) required by the instrument to implement the different operating modes
• High speed selection of the two linear polarizations
• High peak power generated inside the antenna
Figure 32: Schematic of SAA configuration (image credit: ASI)
The SAA antenna has dimensions of 5.7 m x 1.4 m (elevation steering range of ±15º), consisting of 40 identical tiles arranged in five identical electrical configurations (5 horizontal electrical panels of 8 vertical tiles each). Each tile consists of:
• 32 T/R (Transmit/Receive) modules, grouped in 4 electronic front ends (EFE), to amplify and control the RF signal
• 2 DC/DC tile power supply units (TPSU) to supply the required energy to the modules
• The intermediate digital controller (SBC) to control and to compensate the modules settings
• A true time delay line, to stabilize the beam pointing in the frequency band and to amplify both the Tx and Rx RF signals.
Within a tile there are all functions needed for beam forming and reception. Each tile is a self-consistent active antenna element including all functional support (thermal, RF, digital and power). The antenna has a total of 1280 T/R modules.
A photo of the antenna panels with the 40 tiles is shown in Figure 33. The tiles are arranged into a double staggered grid in order to spread the grating lobes energy and reduce their peak level. The tiles are mounted on an aluminum panel frame, which supports also the mechanisms for hold down and deployment and the antenna harness.
Figure 33: Photo of the active SAR antenna in stowed (left) and deployed configuration (right), image credit: TAS-I
Two beam forming networks (one for the RF Tx / Rx distribution and one for calibration purposes) are mounted on the tiles supporting frames of the three panels. The digital control of the antenna is achieved by five 1553 digital busses, one for each column of 8 tiles.
The following operational acquisition modes are supported: Stripmap, ScanSAR, and Spotlight. The SAR data localization accuracy is 25 m without GCP (Ground Control Point).
The single orbital plane constellation offers a number of interferometric applications/features such as:
• A full interferometric accessibility (i.e. a double acquisition of all sites with the same incidence angle within the 16 day orbit repeat cycle
• No degradation of the mean revisit time with respect to the performances achievable with the same number of satellites
• A fixed time period of one day between two interferometric acquisitions.
SAR product examples: Multidate basic products, stereo pairs for radargrammetry, interferograms (for DEM and DTM) or differential interferogram, coherence products, etc.
Figure 34: Illustration of the SAR operational modes (image credit: ASI)
Although the swath width of the SAR instrument is between 10-200 km (depending on support mode), a total FOR (Field of Regard) of 1300 km in the cross-track direction is available for event monitoring applications (for revisit times of < 12 hours for the full constellation).
Figure 35: Illustration of the potential coverage region of COSMO-SkyMed provided through electronic steering (image credit: ASI) 64)
Figure 36: Illustration of multi-mode acquisition capability (image credit: ASI)
Table 6: Acquisition modes of the SAR-2000 payload
SAR instrument calibration:
The required radiometric stability is achieved through the compensation of instrument fluctuations. This is done by in-flight verification of the instrument against the pre-flight instrument characterization. 65) 66)
The critical part of the instrument is SAA. The instrument stability is based on two mechanisms: a) the compensation of the TRM (T/R Module) variations versus temperature, and b) the internal calibration. The compensation of the TRM variation vs temperature is implemented by the tile control units (SBC) in the background of the instrument operation. The TRM compensation is based on the complete characterization of each module during the ground test. The characterization data are being stored in the SBC look-up table to select the TRM settings according to those commanded and the actual temperature value of the specific TRM. The refresh rate of the TRM setting is equal to PRF (Pulse Repetition Frequency).
The internal calibration subsystem of the SAR instrument monitors all critical parts of the radar (i.e. the passive linear arrays are outside the loops) in a special calibration mode (TRCAL). The internal calibration is performed by sending dedicated pulses along the signal path, according to the specific system timeline. Three different calibration pulses are used:
• CalTx is used to perform the calibration of the transmit path (V and H polarizations can be selected)
• CalRx is used to perform the calibration of the receiving path.
• Short Cal is used to perform short calibrations at the electronics level.
The relative radiometric accuracy depends mainly on the compensation of the antenna pattern. The in-flight determination of the actual patterns would lead to an increase of the commissioning period without the availability of a reliable antenna model.
The absolute radiometric accuracy depends mainly on the estimation and compensation of the radiometric bias. This activity is done with the acquisition of standard target imagery or known RCS (Radar Cross Section).
Split SAR antenna design for multi-beam operational modes:
In the time frame 2004/5, the SAR antenna design was modified into a multiple sub-aperture configuration. This improved "split-antenna design" provides a number of extra support mode and performance capabilities such as: 67) 68) 69)
• Clutter cancellation and detection of slowly moving ground targets for surveillance mode support. This feature is referred to as MTI (Moving Target Indication). MTI systems employ the STAP (Space-Time Adaptive Processing) technique to cancel the background clutter. STAP offers a means of detecting targets that compete with clutter located within the skirts of the mainlobe.
• Sea current/wave monitoring by along-track SAR interferometry (ATI) using two independent receiving channels with the largest possible horizontal along-track baseline
• Exact positioning and imaging of moving targets in the high-resolution SAR imagery; this feature is also referred to as "Reloc" (Relocation of moving targets in high resolution SAR images)
• Increased monitoring capabilities by collecting for instance fully polarimetric imagery; this feature is also referred to as "MultiPol" (Multi-Polarization imagery).
The phased array antenna design, composed of 5 horizontal electrical panels of 8 vertical tiles each - is being split up into five channels by acting at the level of electrical panels, and into up to 40 sub-apertures, acting at the level of tiles. This operational scenario does not require any modifications of the tiles and their T/R modules (this means limited impact on original antenna).
The new multi-beam antenna configuration considers all symmetric configurations obtained by splitting the whole aperture into 2, 3, 4, 5 horizontal subapertures and into 2 vertical subapertures. These configuration instances are described in Table 7, where the value `0/1' specifies whether a given panel is "not used/used" in a specific sub-array, the symbol `+' splits up sub-arrays whose outputs are used simultaneously, and panels represented in bold are shared by adjacent sub-arrays.
• The STRx (Standard Receive) configuration corresponds to the actual single channel antenna design
• The SPAN No (Split Antenna) configurations refer to the horizontal partitions of the array into multiple (No) sub-apertures
• The SPAN 2V (Vertical Split Antenna) configuration corresponds to a vertical partition of the array into 2 subapertures
• The ALRx No (Alternating Receiver) configurations are based on an alternate reception with a leading and a trailing subaperture that uses a certain number (No) of external antenna panels.
Table 7: Overview of multi-beam antenna configurations
New operational modes have been conceived for the multi-beam applications, of three types:
1) Wide Area Surveillance (WAS) modes (based on a ScanSAR acquisition mode) optimized to obtain large swath width with a reduced ground resolution (conceived for MTI and experimental for ATI)
2) Continuous Imaging (IMA) modes with intermediate values of swath width and resolution corresponding to a StripMap acquisition mode
3) High Resolution Imaging (HRI) modes (based on a SpotLight acquisition mode) optimized to obtain high resolution on reduced dimension swath (conceived for MultiPol and Reloc, and experimental for MTI due to the very high value of the signal bandwidth).
The design of the multi-beam operational modes included the specification of the product geometric characteristics (swath width, resolution, number of looks) and the design of appropriate sets of beams (PRF + off-nadir angle + antenna beam-width + pulse width) that assures the coverage of the ground access area (incidence angles from 25º to 57º) without range ambiguities and avoiding ALE (Altitude Line Echo). One of the main limitations in the design of the new multi-beam modes is the required data rate, which is mainly affected by:
• The number of receiving channels
• The range resolution (namely the chirp bandwidth)
• The oversampling factor (OVS) of the range bandwidth (specifically, for MTI an higher value has to be used in order to perform the digital equalization of the receiving channel required to obtain an effective clutter cancellation
• The number of bits used to encode the samples (I+Q) with reference to the BAQ (Block Adaptive Quantizer) compression level acceptable for each Multi-Beam application.
A trade-off between increase of data rate and performance in the multi-beam techniques suggests to optimize the operational modes for two different antenna configurations:
1) A "Reduced Solution" (2 Rx channels) usable either for SPAN 2b or for STRX + 1 auxiliary signal for ECCM (Electronic Counter Counter Measure) techniques
2) A "Complete Solution" (5 receiving channels), usable for: (a) SPAN 5 (or SPAN 4 through digital beamforming), (b) SPAN 2b/SPAN 3b+3/2 auxiliary signals for ECCM, or STRX + 4 auxiliary signals for ECCM.
Table 8: Reduced solution modes (2 Rx channels)
Table 9: Complete solution modes (5 Rx channels)
SABRINA (System for Advanced Bistatic and Radar Interferometric Applications)
In the framework of COSMO-SkyMed follow-on activities, ASI plans to design, develop and operate a bistatic and interferometric mission, SABRINA. The SABRINA mission concept uses a passive satellite, namely BISSAT (Bistatic and Interferometric SAR Satellite), which is to co-orbit in close formation with one of the COSMO-SkyMed constellation spacecraft. The overall objectives of the SABRINA mission are: 70) 71) 72) 73)
• To conduct experiments involving a bistatic and interferometric configuration - using two antennas operating simultaneously
• To investigate new applications by complementing monostatic SAR data with multi-angle, multi-baseline bistatic-interferometric data, to add value to the data products and to attain further objectives of ASI's National Space Plan (PSN)
• To operate a formation in multiple configurations, involving such functions as: orbit control and navigation, stable attitude pointing, signal synchronization, baseline measurement and control, and switching from interferometric to bistatic configurations.
The following scientific applications and products are expected from the SABRINA mission:
- Characterization of bistatic scattering and the study of its effects on imagery. Bistatic measurements help to discriminate the physical scattering mechanisms inherent to surface clutter; in particular, they are useful when the monostatic radar cross section of a terrain is rather weak. The studies involve a spectrum of items: effects of baseline decorrelation, multi-angle observation on data classification, polarimetric analysis of bistatic data, differences between monostatic and bistatic scattering of sea surfaces driven by wind, etc.
- Development of techniques for obtaining digital elevation, slope and velocity maps (radargrammetry, multi-angle Doppler analysis, bistatic scattering from rough surfaces for slope determination, etc.)
- Development of new procedures for bistatic SAR data processing (focusing, motion compensation, effects of bistatic geometry, use of active and passive calibrators, etc.).
As of 2006, SABRINA and BISSAT are in their study phase at ASI.
Background on ASI-CNES agreements (international cooperations)
Since 1997 CNES (France) is studying the use of smaller satellites (a medium spacecraft size of about 900 kg instead of 3 tons as for the SPOT-5 S/C), resulting in the "3S" platform concept (Small Satellite System) standing for "Suite de Systeme SPOT" or for "SPOT Successor System." The focus is on cost reduction, technological innovation, user services, and performance upgrades for a new generation of optical imaging satellites, referred to as Pleiades. 74) 75) 76) 77) 78)
• On Jan. 29, 2001, an intergovernmental agreement (memorandum of understanding) was signed during the Turin meeting between Italy (Guiliano Amato) and France (Lionel Jospin). The objective of this agreement, referred to as ORFEO (Optical and Radar Federated Earth Observation), is the cooperation of France and Italy on a "dual high-resolution Earth observation system," comprising a two-satellite constellation in the optical region under the leadership of France, and a four-satellite constellation under Italian leadership in the microwave region of the spectrum (initially X-band SAR). The intent of this agreement is to provide a long-term perspective on a number of high-quality data products and services on the commercial market for a wide range of applications in the fields of cartography, agriculture, forestry, hydrology, and geological prospecting. The dual service concept is seen in the data requirements of the defense and civilian communities with an option of a daily revisit capability. The agreement calls for funding and development of the space segment by each country and a common sharing of the ground segment. 79) 80) 81)
• In 2000, ASI signed a cooperative agreement with CONAE (Comisión Nacional de Actividades Espaciales), Argentina's Space Agency, Buenos Aires. The agreement, referred to as SIASGE (Sistema Italo Argentino de Satélites para la Gestión de Emergencias - Italian/Argentinian satellite system for emergency management), has been signed under which the satellites in the Argentine SAOCom (SAR Observation & Communications Satellite) system will operate jointly with the Italian COSMO-SkyMed constellation in X-band to provide frequent information relevant for emergency management. This approach of a two SAOCom and a four COSMO-SkyMed spacecraft configuration offers an effective means of a twice-daily coverage capability. By joining forces, both agencies will be able to generate SAR products in X-band and in L-band for their customers. - SAOCom is the CONAE constellation of 2 L-band SAR satellites. A launch of SAOCom-1A is expected in 2015. Once entered into its operational phase, the system will be available for use by both agencies, ASI and CONAE.
Figure 37: Overview of organizations in the ORFEO agreement
In this framework the Italian and French Governments started a cooperation with the goal of Earth observation for dual-use applications (military and civil) with SAR and optical instruments based on the on-going COSMO-SkyMed and Pleiades small satellite programs, respectively. This dual-use scenario calls for missions that offer advanced observation capabilities in several modes of operation, permitting to meet the objectives of the military and civil communities at the same time. The development of innovative and complementary instrumentation in the radar field (e.g., multi-mode and flexible-support SAR's offering high-resolution data) and in the optical field (e. g., hyperspectral sensor with capabilities of variable spatial resolutions as well as high detection sensitivities in the visible and infrared spectral regions) are major objectives of the programs.
The earlier memorandum of understanding was followed by a Memorandum of Agreement (MoA), for the ORFEO system definition step, between the French space agency (CNES) and the Italian space agency (ASI). The MoA was signed on June 22, 2002 at the Paris Air Show.
The COSMO-SkyMed / Pleiades space segment is based on a constellation of small satellites combined with a fast data reception capability. The provision of data on an operational basis (of continuity and quality) is essential for the system.
The constellation will be deployed in two orbital planes, one orbit plane for the SAR satellites and one orbit plane for the optical satellites. This permits a) the SAR satellites to collect the maximum solar radiation for S/C power demands in sun-synchronous dawn-dust orbits, and b) the optical satellites to operate under optimal illumination conditions in a sun-synchronous near noon orbit.
• In 2009 an intergovernmental agreement MOU (Memorandum of Understanding) was signed between ASI and JAXA concerning the feasibility study and joint research activities for the mutual cooperation in the satellite disaster monitoring. In this framework a cooperative project was implemented for:
- Feasibility study and demonstration on improvement of observation frequency and coordination during emergencies, using COSMO-SkyMed (X-band), ALOS (L-band), COSMO-SkyMed 2nd generation (X- band) and ALOS-2 (L-band) satellites.
- Cooperation in joint SAR research activities related to disaster monitoring.
The COSMO-SkyMed / Pleiades ground segment provides all the infrastructures needed to support the mission in a dual-use scenario (functions/operations in terms of constellation control and global data management). Telespazio was the prime contractor for the development and construction of the civil and military ground segment, and now controls the constellation's in-orbit operations. e-GEOS S.p.A., a joint venture between the Italian Space Agency (20%) and Telespazio (80%) is responsible for the acquisition and processing of COSMO-SkyMed data, and sells it on the international market. e-GEOS is also the European distributor of GeoEye-1, IKONOS, QuickBird, WorldView-1/2, EROS-A/B, and Radarsat-1/2 satellite data (Ref. 54).
The various elements of the COSMO-SkyMed ground segment are:
• CPCM (Centro Planificazione e Controllo Missione - Mission Planning and Control Center). Coordination of on-board and ground activities, mission planning, and resource allocation.
• CCS (Centro Controllo Satelliti - Satellite Control Center). Provides the monitor and control function of the constellation including flight dynamics. The CPCM and CCS functions are located at Fucino, Italy.
• TT&C stations. Primary service link between the space and ground segments (dedicated communications network).
• CREDO (Centro Ricezione ed Elaborazione Dati Operativi). CREDO operates X-band stations for high-rate data acquisition and provides data archiving and processing. To the user side there is a two-fold function:
- As a civil support facility which handles civil user requests: The UGS (User Ground Segment) function is located in Matera, Italy.
- As a military support facility to handle the military service needs: The UGS for military applications is located in Pratica di Mare, Italy.
- In addition there are mobile stations supporting UGS.
Figure 38: Schematic of COSMO-SkyMed ground segment elements
Data Policy and Data Access: Due to the dual use feature of the system, COSMOSkyMed data access is regulated by means of an appropriate and well-defined data policy. Users are classified mainly in two separate domain: Civilian domain and Defence domain. In the Civilian domain users can be: 84)
• institutional users
• commercial users.
Figure 39: Resources distribution of the COSMO-SkyMed system (image credit: ASI)
Institutional users are international partners, national and international Administrations, Agencies, Ministries, Universities, Research Centres, etc. ASI manages and coordinates institutional and scientific users allowing the utilization of the service for acquisitions and products ordering by mean of the COSMO-SkyMED website, whereas the commercial users can access the system through the commercial provider e-GEOS, an ASI-Telespazio Company. In the first case, specific agreements are signed and shared among the partners; similarly, commercial users regulate their relationship with the commercial provider by means of commercial contracts.
In order to define the access rules to the COSMO-SkyMed system, it is necessary to determine the characteristics of the User, which determine the access rights and the characteristics of the service that the user may require. The User classification levels are the following:
• USER CLASS (classification according to the Data Policy): institutional or commercial.
• USER CATEGORY (classification according to the configuration defined in the agreement). There are 3 categories: A (users with agreement); B (ASI users, including specific projects approved by ASI); C (commercial users).
• USER PROFILE (classification defined at System / User Ground Segment).
Figure 40: Classification level scheme for the COSMO-SkyMed Users in the Civilian Domain (image credit: ASI)
Figure 41: Data access scheme for the Civilian User Domain (image credit: ASI)
IEM (Interoperability, Expandability and Multi-sensoriality) capability:
One of the major problems the EO user community has to face with is the high number of existing/planned spread EO facilities, each providing different specific, regional or thematic functions (e.g. near real time data production and distribution, off-line archive, added value services, etc.) 85) and Ref. 5).
Some international initiatives, such as GMES (Global Monitoring for Environment and Security), GEOSS (Global Earth Observation System of Systems), and, for Defense, MUSIS (Multinational Space-based Imaging System), aim at finding a viable way to provide cost-effective solutions for future EO systems, granting users with simple access to Multi-Mission/ Multi-Sensor (MM/MS) capabilities and streamlined operations.
The COSMO-SkyMed System architecture provides a key mechanisms for implementing the future EO systems MM/MS capabilities by having been envisaged, from the elder design phases, as a versatile system able to easily and cost effectively expand its architecture toward a set of "partner-mission" (up to 5 civilian and up to 5 defence partners) so to cover a larger variety of utilization needs (e.g. optical, hyperspectral or other radar bands).
Figure 42: Dual-use architecture of the COSMO-SkyMed constellation (image credit: ASI) 86)
Figure 44: The distributed ground centers of COSMO-SkyMed (image credit: ASI, Ref. 5)
The IEM concepts and their implementation into the COSMO-SkyMed ground segment.
• Interoperability: This is the capability of exchanging data and information with external heterogeneous systems according to predefined agreed modalities and standards, and irrespective of internal design of the cooperating parts.
The COSMO-SkyMed architecture implements standard Catalogue Interoperability Protocol based on CEOS (Committee on Earth Observation Satellites) guidelines, through which it provides access to a variety of EO systems worldwide, to cover the observation needs of the largest number and typologies of Users, mainly for civilian institutional, commercial, and scientific purposes.
• Expandability: This is the ability of an architecture to embody mission-specific components "imported" from partner's EO system, according to a well defined set of standardized interfaces and protocols.
The COSMO-SkyMed architecture is designed to be able to integrate PFIs (Partner's Furnished Items) from different partner systems, such as: (1) Acquisition Chain, (2) Processing Chain, and (3) Programming Chain, to locally achieve multi-mission and multi-sensor capabilities. - Reciprocally, COSMO-SkyMed mission-specific components can be configured as PFI to be exported towards a partner's EO system.
• Multi-sensoriality: This is the system ability to request, process, and manage data related to different observation sensors. Multi-sensoriality concerns a number of end-to-end functional chains, in turn:
- Mission Programming Chain, including depositing, analysis, harmonization of Programming Requests for acquisition from multiple sensors, up to the scheduling of the related image data takes
- Image Chain, to process data generated from different sensors, then to extract and to correlate the imaging features
- User Service Chain providing End-Users with the capability of searching and ordering products from different sensors, as well as multi-sensor (e.g. co-registration) products.
The multi-sensoriality feature does not necessarily imply the co-location of all architectural elements related to different sensor data on the same UGS site. For example, a multisensor 'User Service Chain' can be implemented through standard 'Catalog Interoperability' established among EO systems "federated" to provide integrated system services to the user community.
Figure 45: IEM implementation in the COSMO-SkyMed ground segment architecture (image credit: ASI, MoD)
The growth capabilities are important flexibility elements for COSMO-SkyMed product and data distribution to external (institutional, defense or commercial) "clients". In fact, COSMO-SkyMed "clients" can not only get products acquired from main COSMO-SkyMed centers, but also build-up their own user center, in order to perform autonomously and locally programming, acquisition and processing activities. In this context, two significant examples of real COSMO-SkyMed expansion are the following:
1) French Defense User Ground Segment (F-DUGS) – expansion "by-partner"
2) Portable/Commercial User Terminals (TUP) – expansion "by-terminal"
In addition, COSMO-SkyMed is also involved in further on-going cooperation programs, namely:
• SIASGE: Italian-Argentine cooperation program, in which COSMO-SkyMed will: a) provide a COSMO-SkyMed partner user center to CONAE; and b) embody the SAOCOM L-band processors PFIs within the COSMO-SkyMed civilian UGS (User Ground Segment).
• MUSIS (Multinational Space-based Imaging System): interoperability between COSMO-SkyMed and other European Defense Earth Observation systems.
Figure 46: Schematic view of COSMO-SkyMed expansion by partner and terminals (image credit: ASI, MoD)
F-DUGS is an example of expansion "by-partner" of COSMO-SkyMed System, which means an additional complete partner UGS, developed for providing French defense users with access capability towards COSMO-SkyMed, in the frame of the ORFEO cooperation program.
The F-DUGS center is located in France and is scheduled to become operational in 2010. From an architectural point of view, F-DUGS is an additional instance within COSMO-SkyMed Defense user centers, equipped with all necessary technical means and solutions for guaranteeing security, integrity and confidentiality constraints, and provides end users with capabilities for:
• Request programming and pre-feasibility analysis, including request harmonization with other defense partners
• Satellite data acquisition, by local antenna or by means of COSMO-SkyMed external stations
• Data processing and distribution
• Archiving and cataloguing of all generated products and related auxiliary data.
The F-DUGS center is autonomously operated by French personnel, and it is operatively integrated with COSMO-SkyMed system by means of a well defined set of interfaces and processes, which allows the center to be informed on system chronologies changes, on relevant on-board activities.
FMCS (System Monitoring and Coordination Function):
A dedicated FMCS system has been designed and implemented as part of the end-to-end system to monitor performances, operations and logistics capabilities, and therefore to optimize the associated operational/supporting cost. The COSMO-SkyMed FMCS system specifically allows: (Ref. 46)
• System performance monitoring
• Support system efficiency monitoring
FMCS has been specifically designed and developed as a set of machines, tools, human operators and procedures customized onto the COSMO-SkyMed system to be coordinated and managed. The multi-mission FMCS can be seen on the one hand as an information retrieval and provision service by which its users can retrieve all necessary data related to the EO system behavior. On the other hand, it can be seen as an integrated "overall procedure" involving machines, information systems and human decision boards, at the various levels of the system hierarchy, all cooperating through schedule-driven timelines and rules to systematically monitor the global system behavior and adaptively decide about the best way to operate it.
Basic objectives to be pursued by FMCS function are:
- System management
- Operative life coordination
- Support system efficiency evaluation
- System technical balances evaluation
- Performance degradations and system/subsystem failures temporal trends retrieval
- Other functions related to the dual-use of COSMO-SkyMed.
COSMO-SkyMed Background Mission:
Since the launch of the first satellite in 2007 the exploitation of the system kept growing steadily. Initially, the COSMO-SkyMed exploitation was mostly "on demand", on the basis of specific user requests and signed agreements. When the constellation was fully deployed and became fully-operational, it was decided to exploit the wide imaging and processing capacity offered by of the system implementing a well-defined Background Mission allowing to maximize the system exploitation during the operational lifetime of the constellation and to build up consistent data sets taking into account the overall mission objectives. The idea is to generate a "historical image archive" or "strategic data set" to be used when necessary in the future, for both, institutional and commercial use. 89)
Guidelines and general requirements:
A low priority background acquisition strategy needs to be systematic and to guarantee a global coverage and/or a significant coverage, allowing to minimize conflicts with existing user requests. The acquisition plan has to be kept as simple as possible in order to be exploited with low priority modality. A systematic background mission allows to obtain regular, repetitive and comparable acquisitions: an essential aspect to build a historical data series and to provide for continuity of the observations. The global coverage allows to satisfy a wide user community for both scientific and commercial needs, to guarantee a more effective monitoring of the Earth's resources.
Nevertheless, the global coverage may not be the best strategy of coverage, when considering the high degree of complexity of a dual use system, which might limit the system flexibility.
The areas of interest for the COSMO-SkyMed Background Mission (part of which is shown in Table 10) were selected by collecting the expression of interest related to specific sites and topics coming from the scientific and institutional community and taking into account possible customers' future needs. The main area selection criteria are:
- population density (populated areas worldwide, large cities, cities, capital cities)
- economic and strategic relevance (oil and gas sites, UNESCO sites, dams, main railroads, etc)
- sensible areas (active volcanic sites worldwide, seismic areas worldwide, areas subject to subsidence phenomena, glaciers).
Table 10: Some of the main targets of the COSMO-SkyMed Background Mission
Table 11: General requirements of the COSMO-SkyMed Background Mission
The acquisition geometry has to be the same over the same area, to guarantee the continuity of the temporal series. The incidence angles selected for the acquisitions depend on the application. The polarization for the acquisitions of the Background Mission is HH selected on the basis of ERS, Envisat, and RADARSAT experience. The acquisition mode is the right looking mode, which is the nominal mode; while the used imaging mode may change according to the target even though the Stripmap mode is the most widely used. Both ascending and descending acquisition directions can be performed, considering cross acquisitions for areas with a strong orography.
Status and main results:
The Background Mission is a default planning applied in absence or in addition to the civilian user requests; the institutional part of this COSMO-SkyMed default planning occupies about 40% of the daily system imaging capability (Figure 47). All data acquired for the Background Mission are not immediately processed at the C-UGS (Civilian User Ground Segment), avoiding useless load of the processing chains that are, in this way, fully dedicated to serve the acquisition request of the user community.
Figure 47: Usage of the COSMO-SkyMed Background Mission in the period 2011-2012 (image credit: ASI)
Due to the implementation of this acquisition plan, the system exploitation increased considerably, reaching a percentage of exploitation in the civilian domain in the range 80%-100% in the last year. The trend of the COSMO-SkyMed Background Mission starting from May 2011 is visible in Figure 48.
Figure 48: Trend of the COSMO-SkyMed Background Mission (2011-2012), image credit: ASI
In the second part of 2011, the first interferometric coverage of Kenya was completed within the framework of the first interferometric mapping program of the African continent (Figure 49). The Kenya mapping project was undertaken, in addition to the Background Mission functions, using the Stripmap/HIMAGE acquisition mode with a 40 km x 40 km nominal swath, 3 m (single look) spatial resolution, and with the following main data take elements: polarization VV; right looking acquisitions; ascending orbit passes; incidence angle in the range of 20°-25°.
Figure 49: Google map of the first coverage of Kenya performed with COSMO-SkyMed constellation (July 2011- Dec. 2011), image credit: ASI
The total number of COSMO-SkyMed Stripmap data takes for a single mapping is close to 1600. The main goal of performing an (iterative) Kenya Interferometric Background Mission was to provide a specific data archive for demonstration purposes:
- Obtain a regional coverage data set archived using the best swath/resolution
- Obtain the necessary data to support land use, land cover change, oil spill, inland waters monitoring and emergency management (volcanoes, floods, etc.)
- Obtain a time-series interferometric archive for change detection applications
- Support specific technical training in Kenya and new research projects/ new services based on Earth Observation data.
Figure 50: COSMO-SkyMed stripmap/HIMAGE VV image of the Malindi region observed on Aug. 24, 2011 (image credit: ASI)
Figure 51: COSMO-SkyMed stripmap/HIMAGE VV image of the Nairobi region observed on Aug. 9, 2011 (image credit: ASI)
In January 2012, the Background Mission program started with the planning of a set of interferometric acquisitions over Sweden and over countries with high seismic risk: Japan, Iran, New Zealand (Ref. 89).
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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).