HYLAS (Highly Adaptable Satellite)
HYLAS-1 is a small geostationary communications satellite within a PPP (Public Private Partnership) project of ESA with the UK operator Avanti Communications Plc of London. This is a multi-spot beam Ka-band satellite with very sophisticated flexible transponder technology developed in ESA projects. The overall objective is to develop and demonstrate a series of innovative payload technologies and to provide broadband data services for end users in Europe. The goal of the project is to:
- Construct a flexible Ka- and Ku-band payload, based on state-of-the-art payload components that are fully configurable in orbit
- Launch and operate the payload on the HYLAS satellite
- Demonstrate the efficient delivery of Ka-band broadband services with this payload and prove the case for low-cost, satellite-delivered services to underserved markets across Europe.
A key feature of HYLAS is its ability to support a dynamic business environment and provide a system that can be adapted to respond to changes in market requirements for broadband and broadcast satellite services. Part of this capability is built into the inherent flexibility of the satellite payload, the rest into the design and implementation of the application-related aspects of the system, specifically the HYLAS ground facilities. 1) 2) 3) 4) 5) 6)
The HYLAS project was initiated in May 2006 when ESA and Avanti Communications announced the signing of a contract providing support to the development of the most innovative elements of this new system. BNSC is a funding partner in this project through ESA's ARTES-3 (Advanced Research in Telecommunications Systems-3) project. Avanti Communications owns and operates the spacecraft.
EADS Astrium Ltd., UK is the prime contractor leading the design and manufacture of HYLAS and is responsible for developing the advanced Ku- and Ka-band payload. ISRO (Indian Space Research Organization) is providing the satellite platform. Other European and Canadian companies including Tesat-Spacecom, ComDev Europe Ltd., and EADS Casa Espacio (ECE) are providing essential equipment for the communications payload.
The HYLAS mission is intended to address the key cost drivers behind the delivery of satellite broadband services – the per MHz cost of satellite bandwidth and the high cost of end user equipment. These will be minimized through the utilization of Ka-band spot beams, which allows efficient point-to-point communications with low cost user terminals, and through the in-orbit configuration of the payload to match satellite resources to market requirements.
Figure 1: Artist's rendition of the HYLAS spacecraft in orbit (image credit: ESA) 7)
The HYLAS-1 spacecraft structure is based on the I-2K bus of ISRO (Indian Space Research Organization) and its commercial arm Antrix Corporation. The satellite was assembled and tested at ISRO's facility in Bangalore, before being flown more than halfway round the world to the launch site in French Guiana.
The I-2K platform was procured for HYLAS-1 by Astrium, prime contractor for the mission's space segment. It is being employed as a result of a 2005 strategic partnership struck by Astrium with Antrix. The partners are marketing the design to the commercial market for telecommunication satellites requiring less than 4 kW of power in the 2-3 ton class. Small modifications have been introduced to the basic design by Astrium/ISRO to align it with the HYLAS-1 mission requirements. 8)
The platform is a 3-axis stabilized design capable of delivering about 3 kW of power over a mission lifetime of ~ 15 years. Its structure is based around a load-bearing central cylinder made of CFRP (Carbon Fiber Reinforced Polymer), hosting a pair of propellant tanks in its interior and incorporating a standardized launcher interface.
Aluminum sandwich panels attached to this central cylinder host the majority of the equipment. In addition, these panels incorporate optical radiators and embedded heat pipes for thermal control.
Figure 2: Illustration of the HYLAS-1 spacecraft (image credit: BBC)
The propulsion subsystem consists of a bipropellant liquid apogee motor – to place the satellite into its final geostationary orbit – and 16 reaction control thrusters – to maintain its desired attitude once orbit has been achieved.
The power subsystem consists of two sun-tracking solar arrays with triple-junction solar cells and two lithium-ion batteries, enabling continued operations while HYLAS-1 is out of contact with the sun.
The I-2K family of satellites is built almost entirely from Indian technology, with the exceptions of US-sourced solar cells; the momentum wheels were produced by Germany-based Rockwell Collins; the batteries are from the French firm Saft, and the transmitters and receivers were procured for TAS (Thales Alenia Space) in France and Spain.
The satellite has a launch mass of 2540 kg, a payload mass of 200 kg and a payload power of 2 kW. It is designed to have a lifetime of 15 years. 9)
Figure 3: The HYLAS-1 spacecraft in launch configuration at Kourou (image credit: ESA) 10)
Figure 4: Overview of the HYLAS-1 system (image credit: ESA, Ref. 3)
Figure 5: Photo of the Hylas-1 satellite during mating to its Ariane-5 ECA adapter at Europe's Spaceport in French Guiana,Nov. 3, 2010 (image credit: ESA, S. Corvaja) 11)
Launch: The HYLAS-1 spacecraft (along with Intelsat-17) was launched on November 26, 2010 on Ariane-5 ECA (V198). The launch site was Kourou and the provider was Arianespace. The second telecommunication satellite on this mission was Intelsat-17. The total launch mass of both spacecraft was 8,867 kg (with adapters). 12) 13)
Orbit: Geostationary orbit of HYLAS-1 (altitude of ~35,786 km) at longitude 33.5º W.
Following launch, there will be a 1 year demonstration phase during which the flexibility of the payload and a series of advanced broadband applications will be demonstrated.
Table 1: Overview of HYLAS-1 parameters
Launch: HYLAS-2 of AVANTI Communications (3,325 kg, 5 kW), developed by Orbital Sciences Corporation on a STAR-2 platform, was launched on August 2, 2012 on an Ariane-5ECA vehicle from the Guyana Space Center, Kourou, French Guyana. 14) 15)
Also lofted with HYLAS-2 on this mission was Intelsat's Intelsat-20 satellite (6,090 kg, developed by SSL on its 1300 platform), which is to replace the Intelsat-7 and Intelsat-10 spacecraft in orbit, and provide a broad range of communication services for Europe, the Middle East, Russia and Asia. Intelsat-20 is riding to orbit as the upper position of the payload stack.
Orbit: Geostationary orbit of HYLAS-2 (altitude of ~35,786 km) at longitude 31º East.
HYLAS-2 features 24 active and 6 gateway Ka-band transponders covering Northern and Southern Africa, Eastern Europe and the Middle East. HYLAS -2 is also equipped with steerable spot-beams to direct extra capacity in required areas when needed. The satellite provides 8,280 MHz of bandwidth, whereas its predecessor HYLAS-1 offers only 3,000 MHz.
• May 9, 2017: A consortium consisting of the Fraunhofer IIS (Institute for Integrated Circuits), Erlangen, Germany, WORK Microwave GmbH of Holzkirchen, Germany, and AVANTI Communications recently completed highly successful over-the-air trials on Avanti HYLAS satellite capacity employing the latest DVB-S2X modulator and demodulator equipment. The demonstrations used wideband carriers exploiting full transponder bandwidth and outperforming the target throughput of more than one gigabit per second (1 Gbit/s) using a single communication carrier. The technology demonstration was part of a development funded by ESA (European Space Agency) under the "ARTES Advanced Technology" program. 16) 17)
- The experiments demonstrated a variety of carrier bandwidths and modulation schemes. This included closing the forward link with a 480 MHz carrier and successfully receiving and demodulating this signal on the ground. The sustained throughput to a single end-user terminal was measured at 1.27 Gbit/s, leveraging from the DVB-S2X time slicing capability that allows the receiver to selectively skip and ignore parts of the incoming signal and thus save on processing power. DVB-S2X as the latest satellite communication standard allows for exceptionally efficient use of spectrum.
- Fraunhofer IIS teamed up with WORK Microwave and Avanti Communications to implement a DVB-S2X wideband transmission system over satellite, including modulator and demodulator. Fraunhofer IIS and WORK Microwave were actively involved in the development, specification and validation of DVB-S2X, from standardization work to the implementation. Fraunhofer IIS developed a DVB-S2X receiver IP, which was used for the technology demonstration. WORK Microwave develops and manufactures satellite communication equipment and developed a high-performance wideband DVB-S2X modulator, which was used for the communication on the uplink.
- The FPGA-based, modular design of modulator and demodulator enables different products and throughput ranges, up to the maximum spectral efficiency supported by DVB-S2X on wideband carriers. UK-based AVANTI Communications provided engineering support and HYLAS Ka-band transponder capacity for the live demonstration. The activity was sponsored by ESA under the ARTES Advanced Technology contract "Ultrahigh throughput transmission through wideband Ka-band transponder".
- Thomas Fröhlich, CEO of WORK Microwave said that by using DVB-S2X waveforms and their wideband ground equipment the capabilities of high throughput satellites can be fully exploited. This gives satellite operators the future perspective to meet the growing bandwidth demands of broadband applications.
- "It has been a tremendous team effort developing this DVB-S2X enabled broadband technology and putting all pieces together to demonstrate such a high throughput system", said Michael Schlicht, Head of Communication Systems Division, Fraunhofer IIS. "We are very pleased seeing it now working live and flawlessly on an actual satellite."
- "This trial confirms the excellent performance of our fleet of satellites when using the DVB-S2X waveform and state-of-the-art ground equipment. We are keeping a close eye on such wideband technologies to ensure we can meet the future bandwidth demands of broadband users", said an AVANTI Communications spokesman.
• January 31, 2017: AVANTI has launched their service across Spain and Portugal after signing a 10 year contract that was signed in Q4 FY2016 with capacity from AVANTI's HYLAS-1 Ka-band satellite to provide connectivity across Spain and Portugal. 18)
- The contract will enable Eurona to grow its business and support Spain's broadband coverage goals set by the European Union in the Digital Agenda for Europe (DAE), as part of the Horizon 2020 program. The service delivers packages with speeds of 30 Mbit/s using the new Jupiter 2 platform, which can support throughputs of up to 100 Mbit/s. With extensive capacity of Spain and Portugal acquired by Eurona, and exclusivity of consumers in these regions, the leading Service Provider will be able to provide a more preferential service to new and existing customers. This includes providing more than 15,000 consumers across Spain and Portugal with a higher quality and more flexible service from 2017 onwards.
• January 16, 2017: Improved and newly devised techniques for single carrier operation mode are employed based on the DVB-S2X standard. The objective is to demonstrate clear performance and efficiency advantages compared to operation with multiple carriers per transponder. - Existing tools for calculating exact link budgets shall be enhanced to allow performance and throughput prediction based on measurements under controlled conditions. 19)
- The performance target is to achieve a throughput of more than 1 Gbit/s at IP level. Therefore all components deployed within this activity will be enhanced to practically handle such throughput. In order to realize ultra-high throughput transmission over a satellite wideband transponder newly devised ACM (Adaptive Coding and Modulation) techniques have to be applied. This implies the implementation of feedback loops between demodulator and modulator / encapsulator.
- Verification of performance via on-field-trials through a Ka-band transponder of Avanti's HYLAS-2 satellite in the test phase January to March 2017).
Figure 6: Implementation of laboratory testbed components suitable to support more than 1 Gbit/s (image credit: Fraunhofer IIS)
• In 2012, after more than one year in space, HYLAS-1 is performing very satisfactorily with its eight Ka-band spot beams powered up and with users in all of the European regions in the satellite's coverage. 20)
• In the summer of 2011, HYLAS-1 has just completed seven months in orbit and has continued with solid and steady progress through the commercial operations phase since 4 April, 2011. 21)
- All onboard satellite resources are fully operational and the satellite has activated all its eight Ka-band spot beams specifically designed for broadband services over selected European market areas.
- The Ground Segment is also fully functional to support the high availability required by broadband users and includes novel features that will allow monitoring and control of the accurate pointing of the satellite antenna spot beams by a network of sensing stations, for improved communication performances.
- In ESA, the HYLAS project will be completed in June 2012 with an assessment of the satellite and ground segment performances over a period of 15 months of broadband service routine operations (Ref. 21).
• Since 4 April, the existing 4000 broadband customers of Avanti Communications are being migrated to HYLAS-1 from the Ku-band capacity currently leased on another telecommunication satellite (Ref. 23).
• In late March 2011, the HYLAS-1 spacecraft was commissioned and is now ready for commercial service. An extensive series of tests has checked the performance of its communications payloads. Conducted from ESA's new testing facilities in Redu, Belgium, the tests have confirmed that the payload, including the antennas, is in good health, operating correctly and shows no ill effects from space. 22)
• On Feb. 24, 2011, HYLAS-1 reached its designated geostationary position (33.5º W). The payload was switched on and configured on Feb. 27 for IOT (In-Orbit Testing). The ESA ground station of Redu, with its newly updated Ka-band IOT antenna, has hosted facilities and the engineering teams from ESA and industry for the full campaign. Test activities lasted two weeks, during which key payload and antenna performances were measured and validated by comparison with ground test data. All active payload equipment and redundancies were tested. The review of test results has confirmed the excellent performances of the payload through all of its 66 active items. Testing of the platform had already been performed shortly after launch.
Control of the satellite was then handed over from the ISRO Master Control Facility in Hassan, India, to Inmarsat in London for the routine satellite operations. IOT was followed by the System Acceptance Test to prepare for the commercial rollout of broadband customers. A series of end-to-end tests was performed to validate all the elements of the ground segment (gateways, hubs, network infrastructure) required for the provision of satellite connectivity services. 23)
• Since liftoff, HYLAS-1 has remained in good health, with all systems operating within their anticipated current and temperature limits, including the Generic Flexible Payload. 24)
• On Dec. 1, 2010, the HYLAS-1 spacecraft reached its Geostationary orbit. Initially, it was injected into GTO (Geostationary Transfer Orbit) with a perigee of 250 km, an apogee of 35,906 km and inclination of 1.99º. 25)
• After launch of the spacecraft, the IOT (In-Orbit Test) campaign was run from ISRO's MCF (Mster Control Facility) in Hassan for the platform part - and from the SCC at Inmarsat for the payload part. The payload IOT campaign measurements were performed by RSS (Redu Space Services) at the ESA Redu ground station in Belgium, supported by a collocated team of engineers from Astrium, ESA and Avanti (Ref. 20).
GFP (Generic Flexible Payload):
The agile payload was designed and developed by EADS Astrium Ltd. and partners with support from ESA via the ARTES-3 program. The coverage is implemented by means eight spot beams providing high gain and thereby high EIRP and G/T for compatibility with small and inexpensive transmit/receive end-user terminals. New repeater technology has been exploited to provide extensive service flexibility and re-configurability in-orbit. Specifically: 26) 27)
• The GFP equipment provides the means to flexibly allocate spectrum (bandwidth) to beams in orbit.
• "Flexible" TWTAs (offering an in-orbit adjustable saturated output power at near-constant efficiency) provide the means to flexibly allocate power to beams in orbit.
The Generic Flexible Payload is aimed at FSS (Fixed Service Satellite), DBS (Direct Broadcast Satellite) and broadband missions in the C-band, Ku-band and Ka-band. The payload design will enable in-orbit adjustment on channels with flexible routing from input to output.
Figure 7: The architecture concept of the GFP (image credit: ESA, Astrium)
The GFP solution works by converting the frequency of all signals received by the satellite to a common IF (Intermediate Frequency), of approximately 6 GHz. This function is performed by an electronic unit called the AIDA (Agile Integrated Down-converter Assembly). Variants of the AIDA have been produced that cover Ku-band for fixed satellite services (Ku-FSS), Ku-band for broadcasting satellite services (Ku-BSS) and Ka-bands. For payloads that use C-band uplink frequencies no AIDA is required because the received signal is already at the correct IF.
Figure 8: Photo of the AIDA device (image credit: ESA, Astrium) 28)
Extensive connectivity is inherent to the GFP architecture: the IF output signals from the AIDA are all input to a sold-state switch matrix, called RASE (Routing And Switching Equipment). The RASE allows connectivity between any pair of satellite uplink and downlink beams. In addition, the RASE allows links to be operated in ‘broadcast' mode whereby a single uplink signal can be transmitted in multiple downlink beams simultaneously.
Having established the desired end-to-end connectivity path through the payload, the uplink signal is then ‘conditioned' by a piece of equipment which constitutes the heart of the GFP concept called the SCACE (Single Channel Agile Converter Equipment). The SCACE comprises three principle elements/functions (Figure 9):
• A reconfigurable bandpass filter that allows the transponder center frequency and bandwidth to be independently adjusted under control from the ground.
• A gain control element that allows the link to be operated in either Fixed Gain Mode where the gain of the overall transponder can be adjusted via ground control, or ALC (Automatic Level Control) Mode. In ALC mode, variations of the receive signal strength (for example due to rain fading) can be automatically compensated for by adjustment of the satellite transponder gain.
• A frequency converter to translate the signal to the required transmit frequency.
For HYLAS-1, the required transmit signal is amplified prior to retransmission by an IOA-MPM (In Orbit Adjustable Microwave Power Module). This allows the transmit power signal to be adjusted while maintaining a near-constant efficiency. The IOA-MPM on HYLAS-1 represents its first use on a spaceborne mission.
This innovation, developed by Tesat-Spacecom, Backnang, Germany, enables the available satellite power to be reallocated between transponders as demanded by evolving operational conditions.
Figure 9: Photo of the SCACE instrument (image credit: EADS Astrium)
Figure 10: Schematic view of the broadband TWTA device (image credit: ESA)
Figure 11: Photo of the IOA-MPM qualification unit (image credit: ESA, Tesat-Spacecom)
Figure 12: Photo of the GFP assembly (image credit: ESA) 29)
Ka-band dual reflector antenna (DRA):
There are two main antennas on HYLAS-1, one on either side of the satellite. The larger, double antenna operates in the Ka-band, and has been engineered to serve hundreds of thousands of broadband users throughout Europe in parallel. This antenna splits its Ka-band beam into eight highly concentrated spot beams targeting different European regions.
The HYLAS-1 satellite payload features 2 Ku-band transponders with coverage of western and central Europe and 6 Ka-band forward transponders with 8 spot beam coverage of various European target market countries. 30) 31)
Figure 13: HYLAS Ka-band dual reflector assembly (image credit: EADS CASA Espacio)
The Ka-band mission development is based on the following key requirements:
• Reflector type: parabolic with elliptical aperture (1620 mm x 1350 mm); two reflectors.
• Frequency: Ka band from 19 GHz up to 30 GHz
• Polarization: circular
• Reflection loss: Tx < 0.1 dB ; Rx < 0.2 dB
• Passive Inter Modulation (PIM) level: less than -90 dBm 3rd order and -145 dBm, 11th order (2 carriers of 75 W)
• Total mass: < 16 kg
• Stowed minimum natural frequency: > 34 Hz
• Deployed minimum natural frequency: > 2 Hz
• Manufacturing (Mfg.) surface accuracy: Root Mean Square (RMS) < 50 µm
• Mfg. absolute angular tilt of boresight < 0.015º
• Mfg. relative angular tilt of boresight < 0.0025º
• The adjustment accuracy shall be better than 0.0025º reflector pointing
• On-orbit thermal distortion surface accuracy: RMS < 50 µm
• On orbit absolute angular tilt of boresight < 0.012º
• On orbit relative angular tilt of boresight < 0.005º
• Radiant surface: zero specular reflections
• ADTM (Antenna Deployment and Trimming Mechanism)
In order to achieve the desired Earth coverage, the HYLAS Ka-band reflector antenna is comprised of 2 reflecting dishes: South and North dish. Each dish is joined to a common support structure or platform by means of 3 isostatic supports or blades. This platform interfaces with the satellite through 2 hard points which are the HRMs and the deployment mechanism, which is attached to the end of the torsion box.
Figure 14: HYLAS Ka-band assembly description (image credit: EADS CASA Espacio)
DRA (Deployable Reflector Assembly) common support structure: To be able to meet the very stringent relative depointing requirement it is essential to have a common support structure almost which is ideally stable, mainly in the out of the mounting plane direction and also to consider a thermal design that could be able to limit the gradient cases to the minimum possible.
The design for the common support structure considers very stable materials and design configurations but also a very stiff design solution that could minimize the distortions induced by any type of asymmetry and at the same time meet the stowed stiffness requirement considering only two HRM points and a working ADTM fixation. The design based on an "I" shaped sandwich panel structure is adequate to minimize the out of plane distortions induced by any kind of possible instability contributors (thermal loads, moisture release, thermal cycling, gravity, etc).
Design and manufacturing: Once the back structure was manufactured and its main performances (stability) verified, the manufacturing of the metalized reflector dishes was undertaken. It was not possible to meet the very stringent requirements of the dish shape manufacturing distortion but it was possible to use them without any degradation of the mission performances: on one side because the use of a metalized radiant face minimizes losses allowing that some surface degradation could be accepted and on the other side because the ad hoc alignment of the dishes could also minimize the effect.
The assembly and alignment of both dishes was done in an iterative way using adjustable shims (titanium machinable shims) at each individual fixation point redefining and reworking the shims until the relative depointing requirements of the assembly were met.
The assembly activities finished with the manufacturing and integration of the thermal hardware elements that in this case are essential to control the thermal gradients which would allow meeting the in orbit relative depointing requirements.
Figure 15: DRA final assembly including thermal hardware (image credit: EADS CASA Espacio)
Figure 16: Illustration of the HYLAS-1 Ka-band antenna system (on top of the bus), image credit: ESA
Figure 17: Schematic of HYLAS-1 service provision (image credit: ESA)
The primary mission of HYLAS is to provide 2-way Ka-band satellite broadband services with the following characteristics:
• Typical use of 67 cm SITs (Satellite Interactive Terminals) of 2-4 W
• Forward-link user data rates are between 768 kbit/s – 10 Mbit/s
• Return-link user data rates are 512 kbit/s - 4 Mbit/s.
HYLAS can support up to 300,000 two-way broadband customers across Europe – depending on the service package mix.
Figure 18: HYLAS-1 Ku-band antenna deployed during testing at ISRO Bangalore, India (image credit: ISRO)
Enabling technologies: The equipment making up the HYLAS-1 payload incorporates a number of technical innovations that allow a very high degree of functionality to be contained within highly integrated, compact, analog subassemblies.
This equipment is based on the MMHT (Modular Microwave Hybrid Technology) concept developed by Astrium under a series of ESA funded ARTES projects.
- With this approach, the high-frequency elements of the design are realized by embedding active and passive circuit elements within multi-layer ceramic substrates housed within hermetically sealed cavities.
- Low-frequency and DC bias circuits are mounted on the outside of the structure to form complete functional blocks (or ‘hybrids') such as frequency-converters, amplifiers, and agile frequency synthesizers.
Used in conjunction with state-of-the-art microwave semiconductor circuit dies, the resulting GFP hybrids represent a significant advance in terms of analog RF functionality and performance for space applications.
Under the terms of the HYLAS-1 PPP (Public-Private Partnership), Avanti Communications is responsible for designing, installing, testing and operating the mission's ground segment. To this end, the company has made a series of procurement contracts with various partners. 32)
The HYLAS-1 ground segment is composed of three major building blocks:
• the SCC (Satellite Control Center) for control of the satellite and its orbit. The SCC is hosted and operated on Avanti's behalf by InmarSat. The InmarSat ground stations will be used for HYLAS-1 tracking, telemetry and control, with their operations coordinated at the Fucino ground station in Italy.
• the Common Elements, including the gateway stations and antennas used to connect the satellite to the wider Internet and the Network Operations Center, satellite resources management and back-office tools, among all other infrastructure and software elements shared between the different ground segment building blocks
• the Service Segment, which provides satellite broadband services, including gateway hubs and user terminals. The Service Segment includes the hubs that relay data between the Internet and the customer as well as the user terminals needed to provide broadband services to consumer and business customer.
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21) "HYLAS," ESA Bulletin No 147, Aug. 2011, p. 74
23) "HYLAS," ESA Bulletin No 146, May 2011, pp.92-93
24) "HYLAS status," ESA Bulletin, No 145, February 2011, p. 86
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26) Graham Weaver, Glyn Thomas, Gary Cobb, Ian Morris, "Agile Equipments for an Advanced Ku/Ka Satellite," 24th AIAA International Communications Satellite Systems Conference (ICSSC), 11 - 14 June 2006, San Diego, CA, USA, AIAA 2006-5396
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30) J. Yebrin, E. Ozores, J. L. Pardo, "High Stability Ka-band Dual Reflector Antenna," Proceedings of the 32nd ESA Antenna Workshop on Antennas for Space Applications, Noordwijk, The Netherlands, Oct. 5-8, 2010, URL: http://utopia.duth.gr/~iaitidis/ESA%20conference%202010/Papers/session-202/FCXNL-10C09-1985643-1-1985643yebrin.pdf
31) E. Ozores, J. L. Pardo, O. Castro, B. Lopez, C. Montesano, "Improvements on Antenna Reflectors," Proceecings of 33rd ESA Antenna Workshop on Challenges for Space Antenna Systems, ESA/ESTEC, Noordwijk, The Netherlands, Oct. 18-21, 2011
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 (email@example.com).