EDRS (European Data Relay Satellite) Constellation
EDRS is an ESA project within the ARTES-7 (Advanced Research in Telecommunications Systems) program, a constellation of two geostationary data relay satellites, intended to provide links to satellites in LEO satellites, and possibly other spacecraft, enabling real-time communications between these spacecraft and their respective Control Center.
ESA initiated already a precursor data-relay oriented program with the development and launch of Artemis in 2001. Artemis has demonstrated the many operational and performance benefits that the availability of a data-relay satellite off ers. In the meantime, the demand for real-time high volume fi les is expected to increase dramatically with the beginning of operation of the Copernicus (formerly GMES) Sentinel system and future Earth observation and other missions. At the same time, the capacity of optical intersatellite links and their reduction in terms of mass and power requirements have jumped forward by at least one order of magnitude, as also radio systems have improved.
The EDRS program aims to create a new type of satellite services. It intends to bring the development and implementation of the system to a sufficiently mature stage, so that the resulting services can be provided by a satellite operator on a commercial basis. 1) 2)
Figure 1: Artist's rendition of the EDRS constellation architecture (image credit: ESA)
Legend to Figure 1: Various LEO spacecraft optically uplink their data to GEO stations which feed the data via conventional Ka-band links to the ground segment.
There are a number of key services that will benefit from this systems infrastructure right from the start:
• Earth Observation applications in support of a multitude of time-critical services, e.g. monitoring of land-surface motion risks, forest fires, floods and sea ice zones
• Government and security services that need images from key European space systems such as the Copernicus (formerly GMES) program
• Rescue teams that need Earth observation data in disaster areas
• Security forces that transmit data to Earth observation satellites, aircraft and unmanned aerial observation vehicles, to reconfigure such systems in real time
• Relief forces that operate among their units in the field and require telecommunication support in cut-off areas.
With the implementation of the joint European Commission/ESA Copernicus program, it is estimated that the European space telecommunications infrastructure will need to transmit a few TB (terabyte) of data every day from space to ground. The present telecommunications infrastructure is challenged to deliver such large data quantities quickly, and conventional means of communication may not be sufficient to satisfy the quality of service required by users of Earth observation data. 3)
To fulfil that demand, EDRS will become the first operational European data relay system (Figure 1) aiming at improving the quality of the data service and thereby enhancing the operational reliability and independence of European and Canadian space infrastructures such as the joint European Commission / ESA Copernicus initiative, and many national space assets. In particular, the objectives of ESA's EDRS program are:
• Provide ESA with the necessary data relay and related services via satellite. The Copernicus program will be the first customer of the EDRS service (Sentinel 1A/B and 2A/B LEO satellites). Hence, priority is currently given to the provision of services to the Copernicus/Sentinel users, given the fact that their needs are more mature and the associated timing is defined. However, the long-term objective for the EDRS program is to provide full and global data relay services to user communities related to ESA and partners of ESA.
• Foster the development of the satellite data relay services market through the exploitation of the infrastructure with commercial / institutional users beyond ESA.
• Support the standardization and adoption of optical and Ka-band data relay technology by means of the availability of technological solutions for the EDRS infrastructure as well as for the user community (Earth observation satellites, UAVs, etc.).
• Achieve a cost efficient program through a PPP (Public Private Partnership) scheme with satellite operators / service providers for the development of the infrastructure.
Unlike other ESA programs, the objective of EDRS is to develop a commercially sustainable data relay service, rather than developing only the necessary technical infrastructure (satellite and/or ground segment). Consequently, support to the adoption of EDRS by users beyond Copernicus is an important element of the program.
Events/milestones on the way toward EDRS realization:
• ESA approved the program in November 2008 following a strong push from Germany, which is the lead investor (50% share). After more than two years of negotiations, the governments of Europe have secured the full funding package for ESA to build a data-relay satellite system whose initial customer will be the European Commission’s (EC's) Earth observation program.
• In October 2010, ESA selected Astrium GmbH (Business Unit Services) as the prime contractor and operator of the EDRS (European Data Relay System) consortium that will make these services available for the GMES (Global Monitoring for Environment and Security) program.
• In January 2011, final approval for the program was given by the Joint Communication Board, based on the mission technical baseline negotiated between ESA and Astrium, and on the funding from the Participating States. In the PPP concept, Astrium Services will operate the EDRS as a profit-making business, once the system is launched. The EC (European Commission) will be Astrium's anchor tenant, but the company will be free to seek other customers as well. 4) 5)
• In particular, ESA has selected Astrium Services to manage the development and operations of EDRS that would feature one dedicated satellite (EDRS-C) and one hosted payload (EDRS-A). Both of them will be positioned in the geostationary orbit with visibility over central Europe.
The EDRS system will be implemented in a so-called PPP (Public-Private Partnership) arrangement, an innovative structure in which ESA leads the creation of the initial system and infrastructure that is later taken over for full exploitation and further development by a commercial partner. EDRS will boost European-developed technology and make use of a cutting-edge intersatellite laser communication system as well as new data dissemination infrastructure on the ground. 6)
• On Oct. 3, 2011, ESA/TIA (Telecommunications & Integrated Applications) and Astrium Services signed a PPP contract in Paris for the development of the EDRS system. Under the terms of the agreement the partners will jointly finance the EDRS. With EDRS services starting in 2014, all suitably equipped future European Earth observation satellites will be able to perform quicker data transfers and transmit for longer periods. Astrium has the overall responsibility for designing and developing the complete space and ground infrastructure. Astrium will then acquire ownership of EDRS and is committed to its operation for the next 15 years. 7)
• On June 25, 2012, DLR, Astrium Satellites (prime contractor to ESA) and SES Astra TechCom S.A. (Luxemburg) signed contracts for large parts of the ground segment of the new EDRS. DLR has been appointed as a subcontractor by Astrium and is responsible for constructing large parts of the ground segment and for controlling the payloads on the first satellites, referred to as EDRS-A. DLR will also manage and control the EDRS-C relay satellites during routine flight operations that will last for at least 15 years. For this purpose, a dedicated EDRS control center will be developed within DLR's GSOC. The two geostationary relay satellites will transmit the data collected by the lower-orbiting Earth observation satellites to a total of four receiver antennas, which will be located on the sites of the existing ground stations at Weilheim (DLR) and Redu (Belgium), and at Harwell (United Kingdom). SES TechCom S.A. will supply the four antennas and will operate the antenna at Redu on behalf of DLR. 8) 9) 10) 11)
• The SRR (System Requirements Review) has been closed-out in July 2012. In parallel the EDRS-A PDR (Preliminary Design Review) has been successfully completed in April 2012.
• November 2012: The design of Europe’s data relay satellite system – EDRS - has been completed and approved. This marks the moment when it moves ahead with a green light from its first customer, the GMES (Global Monitoring for Environment and Security) initiative from the European Union. A design review board of senior members from ESA, Astrium and the DLR German Aerospace Center approved the entire system design: from the satellites to the support that will be required from the ground. 12)
• Delivery of the EDRS-A / EDRS-C payloads to corresponding satellite integrators is scheduled for the end of 2013 and for mid-2014., with a satellite launch planned for the end of 2014 and early 2016, respectively.
- Launch: A launch of the EDRS-A payload is planned for the end of 2014, hosted payload on the Eutelsat- 9B satellite.
- Launch: A launch of the EDRS-C spacecraft is planned for late 2016.
EDRS space segment:
The EDRS will be a constellation of GEO satellites intended to relay user data between LEO satellites, as well as UAVs (Unmanned Aerial Vehicles) in the future, and ground stations. EDRS will allow visibility between GEO and LEO satellites for the larger part of the orbits, offering significantly extended communication periods when otherwise LEO satellites only have a very reduced visibility from any ground station. EDRS is envisaged to significantly improve the stringent timeliness requirements of demanding Earth observation missions (i.e., time critical services).
The EDRS space segment is composed of two elements (Ref. 3):
1) The EDRS-A hosted payload, which contains a Laser Communications Terminal (LCT) and a Ka-band terminal for O-ISL (Optical- Intersatellite Link) and Ka-band ISL, respectively. The EDRS-A payload will be placed as a piggyback payload on-board Eutelsat-9B commercial telecommunication GEO satellite manufactured by Astrium Satellites SAS (France). The Eutelsat- 9B satellite, which is based on Astrium’s Eurostar E3000 bus heritage, will be launched in late 2014 and will be positioned at 9°E.
2) The EDRS-C platform (dedicated satellite manufactured by OHB of Bremen, Germany) carrying the EDRS-C payload (which includes an LCT for O-ISL). The EDRS-C satellite is based on the OHB developed SGEO (Small GEO) platform with an increased payload capability of about 360 kg and 3 kW and a lifetime of 15 years. The EDRS-C satellite is planned to be launched early 2016. The final orbital location is currently under assessment taking into account the needs from other payloads on board.
Figure 2: EDRS infrastructure currently under development depicting the EDRS-A and EDRS-C nodes of the space segment (image credit: ESA)
The development and manufacturing of both the EDRS-A payload and the dedicated EDRS-C satellite (including the EDRS-C payload) are executed in parallel. EDRS-A and EDRS-C payloads (including the LCTs) are manufactured by Tesat Spacecom (Germany).
The Eutelsat 9B satellite, in addition to Eutelsat’s main payload and the EDRS-A payload, will also host the so-called “ASI Opportunity Payload,” funded by ASI (Italian Space Agency. Furthermore, the EDRS-C Satellite will also embark the so-called HP (Hosted Payload), an opportunity offered by ESA to fill up the spare payload capacity on the EDRS-C platform. The Hylas-3 Ka-band payload has been selected by ESA for embarkation as a HP. The contract between ESA and the commercial satellite operator Avanti Communications (UK), the owner of the Hylas-3 payload, has been signed in July 2012. The Hylas-3 payload is procured by Avanti Communications, and the integration and test activities with the EDRS-C platform will be conducted jointly with OHB. 13)
ESA will act as a major customer for Astrium paying for relay services of data from the Sentinel satellites. However the system is designed such that more customers can be integrated.
Figure 3: The EDRS system architecture including the various elements of the space and ground segments (image credit: EDRS consortium)
Figure 4: Overview of the EDRS optical services (image credit: EDRS consortium)
Figure 5: Overview of the EDRS Ka-band services (image credit: EDRS consortium)
EDRS ground segment:
In addition to the space segment, EDRS will develop the necessary ground segment infrastructure, consisting of (Ref. 3):
• EDRS SCC (Satellite Control Center) facilities: The dedicated EDRS SCC is linked to the EDRS-C spacecraft Operator (i.e., DLR in Oberpfaffenhofen), while for the EDRS-A payload this is a PCC (Payload Control Center) operated by DLR in conjunction with the Eutelsat operated SCC for the 9B satellite.
• EDRS MOC (Mission Operations Center) and the Back-up Mission Operations Center (B-MOC), which are the interface to the users for the planning of the EDRS services requests. The primary MOC will be in Ottobrunn (Germany), while the backup MOC will be installed at Redu Space Services (Belgium). The MOC function is provided by Astrium Services.
• EDRS DGS (Data Ground Stations), enabling reception of user data on ground. Two EDRS DGS shall be operational to provide service after completion of the EDRS-A LEOP (Launch Early Orbit Phase) operations and in-orbit commissioning.
• FLGS (Feeder Link Ground Station) and the B-FLGS (Backup -Feeder Link Ground Station), enabling user data reception as well as providing EDRS-C TM/TC capability. The FLGS and the B-FLGS shall be operational in line with the deployment of the EDRS-C satellite.
In June 2012, Astrium Services signed a contract with DLR (German Aerospace Center) to implement and operate major parts of the ground network. The agreement covers the design, implementation, delivery and operation of four ground stations: two DGS for the EDRS-A satellite in Weilheim (Germany) and in Harwell (United Kingdom), respectively, and the FLGS / B-FLGS for EDRS-C in Weilheim (Germany), and in Redu (Belgium). As part of the agreement, DLR will also implement and operate the PCC for EDRS-A and the SCC for EDRS-C in Oberpfaffenhofen (Germany). Major parts of the Ground Segment have been co-funded by DLR and the government of Bavaria.
The space and ground segment for the EDRS user are intrinsically part of the end-to-end system, including space-to-space, space-to-ground as well as the ground-to-ground interfaces. A joint Copernicus/EDRS System team has been established in support to the definition and implementation of all aspects of the end-to-end link involving the Sentinels. This includes the definition of key performance indicators, applicable to a SLA (Service Level Agreement) between Copernicus and Astrium Services. Up to four Sentinels satellites are currently planned to be served simultaneously, with an average of 10 minutes of communication per orbit each. The current draft SLA expects a start of the service by 2015, extendable in phases until about 2030.
The EDRS concept of operation can be summarized as follows: Communication link sessions are planned and coordinated between the EDRS MOC and the user’s MOC, making use of the visibility windows available between EDRS and the user satellites. The EDRS and the user’s space infrastructure will be configured according to each link session parameters. User data will be transmitted from LEO user satellites to either of the EDRS payloads (i.e., EDRS-A, EDRS-C) and relayed to the DGS and/or FLGS/B-FLGS on the ground, from where it will be made available through terrestrial network to the users’ sites (Figure 3). The users can also operate their own user ground stations to receive directly the user data.
Figure 6: Layout of the EDRS ground system (image credit: DLR) 14)
In addition to the downlink capabilities, the EDRS can also provide quasi real-time access from ground to the LEO user satellite during periods of direct line-of-sight, which can be used to reconfigure the EO (Earth observation) LEO payload / satellite, and hence, shortening the reaction time of EO LEO satellites in case of emergency events (i.e., request on-demand).
The EDRS provides different types of optical and Ka-band services, using the O-ISLs (Optical Intersatellite Links) and Ka-ISLs (Ka-band Intersatellite Links), respectively (Figures 4 and 5). Both ISLs between the LEO user and the GEO satellites are bi-directional, and are referred as the RTN (Return link), from LEO user to GEO) and the FWD (Forward link), from GEO to the LEO user. The EDRS-A and the EDRS-C payloads offer Optical services, whereas the Ka-band services are implemented on the EDRS-A payload only.
The Optical services are split into RTN and FWD services. The Optical RTN service is a high data rate channel transferring the user data from the LEO user satellite to ground via the GEO relay satellite. The user data rate for the Optical RTN service is 600 Mbit/s for the so-called Sentinel mode (e.g., for Sentinel-1A and Sentinel-2A) and 1.8 Gbit/s for the so-called “advanced mode” to cover future user needs. The EDRS RF feeder link is accordingly dimensioned to support these user data rate requirements. The Optical FWD service is a low data rate channel (limited by the uplink TM/TC channel) to transmit telecommands to the LEO user satellite in quasi-real time (e.g., for reconfiguration of the Earth observation LEO payload / satellite).
Alternatively to the Optical RTN service, the EDRS also offers the Ka-ISL RTN service, which relays the user data from the LEO user satellite to ground at a user data rate up to 300 Mbit/s.
Table 1: Maximum handling capacity of the EDRS system currently under development considering one or two GEO nodes and operating in Sentinel / Advanced mode
LCT (Laser Communication Terminal):
Compared to typical RF communication systems laser communication terminals (LCT) offer the advantage of higher data rate and larger link distance at lower size, weight and power. The major factor is the four orders of magnitude shorter carrier wavelength translating into higher antenna gain. As additional benefits, laser communication links are free of interference problems, they provide secure transmission and the user is not limited by ITU regulations. Of the available optical technologies homodyne BPSK (Binary Phase Shift Keying) is superior due to the merits of: 15) 16)
• Spatial filtering by the homodyne detection cone (the narrowest possible for a given aperture)
• Frequency filtering by phase locking loop (far more selective than available optical coatings)
• Leveraging the signal amplitude (superposition with orders of magnitude larger than the local oscillator amplitude).
The objective of the LCT is to operate a duplex communication link for binary digital data between two satellites or a ground station via a single optical carrier at 1.064 µm wavelength. The demonstration LCTs, accommodated on the TerraSAR-X (launch June 15, 2007) and NFIRE (Near Field Infrared Experiment) spacecraft of DoD (launch April 24, 2007), are considered to be verified on orbit throughout multi-year routine operations. More than 100 ISLs (Intersatellite Links) with bidirectional communication have been established so far. The first 2nd generation LCT , with the TDP1 (Technology Demonstration Package No 1), will be flown on AlphaSat-1 / InmarSat I-XL, a GEO spacecraft scheduled for launch in Q2 2013 (positioned at longitude 25º E).
The EDRS O-ISLs are based on 2nd generation LCTs (Laser Communication Terminals) which are developed and qualified by Tesat Spacecom (Germany) under DLR German national funding. These LCTs feature a significantly increased data transmission rate compared to SILEX technology, and at the same time reduced mass and size (Ref. 3). 17) 18)
Table 2: Performance comparison between SILEX O-ISL technology and Tesat LCT technology (Ref. 3)
The EDRS LCTs will benefit from the space heritage attained in the following in-orbit demonstrations led by DLR (German Aerospace Center):
• The in-orbit verification of the 1st generation Tesat LCT as part of the LEO-LEO O-ISL between TerraSAR-X (German LEO satellite) and NFIRE (US LEO satellite), which took place in 2008, at a data rate of 5.6 Gbit/s over link distances of about 5000 km. It demonstrated the feasibility of beaconless spatial acquisition and the communications performance of the homodyne BPSK detection scheme (data stream of 5.625 Gbit/s with a BEP <10-9).
• The in-orbit validation, in cooperation with ESA and the Swiss Space Office, of the 2nd generation Tesat LCT as part of the LEO-GEO O-ISL between the GEO LCT embarked on ESA’s Alphasat satellite, (Figure 7) and the LEO LCTs on board Sentinel-1A/-2A satellites. It will demonstrate a LEO-GEO bidirectional link. Furthermore, it will perform end-to-end preoperational experiments between Sentinels-1A/-2A satellites and Alphasat (i.e., EDRS precursor) at 600 Mbit/s of user data rate. The launch of AlphaSat is scheduled for Q2 2013, whereas the launch of Sentinel-1A and Sentinel-2A satellites is expected in 2013 and 2014, respectively.
Figure 7: Tesat 2nd generation LCT instrument showing the space side with hemispherical coarse pointing unit (image credit: Tesat Spacecom)
The LCT block diagram is shown in Figure 8. Major changes are: For the 2nd generation LCT an off-axis telescope is chosen, the optical power amplifier is changed to a 5 W device, the receiver is optimized for a user data rate of 1.8Gbit/s. For GEO applications, the electronics were redesigned to operate the adapted devices and to match with the GEO radiation environment for 15 years of continuous service. The thermal system is improved, the mechanics scaled for the bigger units.
LCT generic design/qualification approach: The LCTs are built such, that the LCTs for GEO and LEO application are same for its units design and their qualification. The GEO and LEO LCTs have identical optical space interfaces with same performance.
Figure 8: Block diagram of the LCT instrument (image credit: Tesat
The LCT one-unit design consists of a central rectangular base structure, a coarse pointer (gimbal) mounted on space side and the optics unit reaching through this structure on the S/C side. The frame unit structure houses the entire laser communication terminals electronics and active optics.
The optics unit comprises the receive/transmit optics, fine steering mechanisms and the receiver. A single telescope as optical antenna serves as common transmit and receive path. The coarse pointer is designed for hemispherical tracking of the counter terminal. In park position, the optics are protected during non-operational modes against contamination; a launch lock secures the coarse pointer during launch.
Together with Renishaw (UK), Tesat has successfully developed and tested a space qualified optical encoder. While it was tailored to the operation in a LCT under harsh environmental conditions outside the spacecraft in GEO, it can be used as well for many other applications in high precision, long life space instruments. 19)
Table 3: Performance parameters of the optical encoder
IOV (In-Orbit Verification) of LCT for EDRS (Ref. 3):
EDRS operations will take advantage of the LEO-GEO O-ISL experiments between the GEO AlphaSat satellite and the LEO Sentinel-1A/-2A satellites planned after the launch of the first Sentinel. The payload data rate requirement for the Sentinel-1A/-2A satellites is 600 Mbit/s, which is well within the capabilities of 2nd generation Tesat LCT. The objectives of these experiments with AlphaSat and Sentinel-1A/-2A satellites are:
• to optimize the technical performances of the LEO-GEO O-ISL (including GEO-ground tests)
• to perform an early validation of the end-to-end data relay system, which includes the LEO-GEO O-ISL, the RF feeder link between AlphaSat and its ground segment, and the interfaces and operations with the Copernicus ground segment.
The experience gained from these experiments will be taken into account during the IOV activities of the LCTs on board the Eutelsat-9B and EDRS-C satellites and for the validation of the end-to-end EDRS performances.
The approach for the IOV of the LCTs is as follows:
- LCT self-test: this test is performed during commissioning to check out the LCT performances after launch. A functional end-to-end test of the entire data transmit and receive chain (including the telescope and the coarse pointer) can be performed while the LCT is in parking position thanks to a mirror mounted in the parking position unit.
- IOV with an OGS Optical Ground Station): these trials are required in order to calibrate the LCT setting parameters (e.g., acquisition scanning parameters, misalignment matrix between satellite coordinate reference system and LCT coordinate reference system, clock offset and other propagator parameters, etc.) and to verify the full PAT (Pointing Acquisition and Tracking) performances (e.g., uncertainty cone, link acquisition time, etc.). The baseline is to use the ESA’s OGS on Tenerife Island (Spain) with some adaptations (Figure 9). As a mean of cross support, DLR’s mobile OGS can, on request, be used as backup. The DLR mobile OGS is intended for experimental space-to-ground optical links with GEO satellites (e.g. AlphaSat) or lower altitude vehicles (e.g. Sentinel-1A, Sentinel-2A).
- IOV with LEO user satellite: these tests are to further fine tune the LCT setting parameters, and to validate the LEO-GEO O-OISL acquisition, tracking and communications performances. The LCTs on board the LEO Sentinel-1A/-2A satellites will be used as counter terminals.
After successful completion of the remaining commissioning activities (e.g., EDRS space segment with EDRS ground segment commissioning, joint EDRS Copernicus service commissioning) to verify the full operational capability of the overall EDRS system with the user (i.e., Copernicus), the EDRS will enter into operational phase by mid of 2015.
Figure 9: The telescope of ESA’s OGS on Tenerife used for the experiments is a Zeiss 1 m O Ritchey-Chrétien/Coudé telescope supported by an English mount (image credit: ESA)
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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.