Optical Satellite Communications in Europe

Zoran Sodnika, Hanspeter Lutza, Bernhard Furcha, Rolf Meyerc

aEuropean Space Agency – ESTEC, Keplerlaan 1, 2200 AZ Noordwijk, The Netherlands; bGerman Space Agency DLR, Königswintererstrasse 522-524, 53227 Bonn, Germany

ABSTRACT

This paper describes optical satellite communication activities based on technology developments, which started in Europe more than 30 years ago and led in 2001 to the world-first optical inter-satellite communication link experiment (SILEX). SILEX proved that optical communication technologies can be reliably mastered in space and in 2006 the Japanese Space Agency (JAXA) joined the optical inter-satellite experiment from their own satellite. Since 2008 the German Space Agency (DLR) is operating an inter-satellite link between the NFIRE and TerraSAR-X satellites based on a second generation of laser communication technology, which will be used for the new European Data Relay Satellite (EDRS) system to be deployed in 2013.

Keywords: Laser satellite communication, SILEX, coherent modulation, European Data Relay Satellite System

1. INTRODUCTION Thirty years ago, in summer 1977, ESA placed a technological research contract for the assessment of modulators for high data rate laser links in space. This marked the beginning of a long and sustained ESA involvement in space optical communications. A large number of study contracts and preparatory hardware developments followed, conducted under various ESA R&D activities. In the mid 1980's, ESA took an ambitious step by embarking on the SILEX (Semiconductor laser Inter-satellite Link Experiment) program, to demonstrate a pre-operational optical communication link in space. SILEX, which started routine operations in March 2003, has put ESA in a world leading position in optical inter-satellite links.

In 1993 the Japanese space agency NASDA and ESA agreed on a cooperation to perform optical inter-satellite communication experiments and in 2006 communication links were established.

Having demonstrated the feasibility of optical communication technology ESA decided to leave the field to European industry pick-up on the lessons learned and to develop laser communication terminal (LCT) for the commercial market. This however turned out to be difficult because the wireless GSM telephone network emerged competing with global telephone networks based on communication satellite constellations (IridiumTM) and planned follow-on satellite constellations (e.g. CelestriTM and TeledesicTM) with up to 288 satellites were cancelled, which would have required large numbers of laser communication terminals to establish high-speed data exchange.

The German Space Agency (DLR) however continued the development of laser communication technology, realizing the strategic importance for its industry. The next generation of optical terminals was developed, which are now operational in orbit since 2008. They will form the backbone of the new European Data Relay Satellite (EDRS) system to be deployed in 2013.

2. SEMICONDUCTOR LASER INTER-SATELLITE LINK EXPERIMENT (SILEX) When ESA started to consider optics for inter-satellite communications, virtually no component technology was available to support space system development. The available laser sources were rather bulky and primarily laboratory devices. Initially carbon dioxide (CO2) gas laser were selected, because these were the most efficient and reliable laser at that time and Europe had a considerable background in CO2 laser technology for industrial applications. A detailed design study of a CO2 laser communication terminal was undertaken and all critical sub-systems were bread-boarded and tested.

Invited Paper

Free-Space Laser Communication Technologies XXII, edited by Hamid Hemmati, Proc. of SPIE Vol. 7587,758705 · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.847075

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This enabled ESA to get acquainted with the intricacies of coherent, free-space optical communication, but very early on, it became evident that the 10.6µm CO2 laser was not the winning technology for use in space because of weight, lifetime and operational problems.

Towards the end of the 1970’s semiconductor diode lasers operating at room temperature became available, providing a very promising transmitter source for optical inter-satellite links. In 1980, therefore, ESA placed the first studies to explore the potential of using this new device for inter-satellite links. At the same time, the French CNES started to look into a laser-diode based optical data-relay system. This line of development was consequently followed thereafter and resulted in the decision, in 1985, to embark on the Semiconductor laser Inter-satellite Link Experiment (SILEX) [1].

SILEX is a free-space optical communication system which consists of two optical communication payloads embarked on the ESA ARTEMIS (Advanced Relay and TEchnology MIssion Satellite) spacecraft and on the French Earth-observation spacecraft SPOT 4. It allows the transmission of the maximum data rate the Earth-observation camera on SPOT-4 can provide, namely 50 Mbps, from low-Earth orbit (LEO) to geostationary orbit (GEO) using GaAlAs laser diodes and direct detection.

In 1997 both terminals underwent a stringent environmental test program and the first host spacecraft (SPOT-4) was launched in March 1998 [2]. The pre-shipment review of ARTEMIS took place in ESA at the end of 1999, but the launch of ARTEMIS, which was initially scheduled on the Japanese launcher H2A for February 2000, had to be cancelled. Launcher problems made it necessary to look for an alternative launch option in order to avoid further delays and a dual launch option on an Ariane5 was negotiated.

2.1 ARTEMIS and SPOT-4

ARTEMIS was eventually launched on 12 July 2001, but due to underperformance the third stage of its Ariane5 launcher the satellite was injected into a too low elliptical geostationary transfer orbit with apogee x perigee altitudes of 17500km x 590km instead of 36000km x 860km. Within 10 days, and by using most of its on-board propellant for a total of 8 apogee motor firings, ARTEMIS was brought out of the radiation belts and into a circular, however below geostationary, orbit (with 31000km altitude, a 0.8° inclination and an orbital period of 20 hours).

Fig. 1: Schematic depiction of the SILEX inter-satellite link between SPOT-4 and ARTEMIS (left) and the first image transmitted by the SILEX optical data relay system on November 30th, 2001 (right). It shows Southern part of the island of Lanzarote, Canary Islands, Spain.

The first inter-satellite link between ARTEMIS and SPOT-4 was established on November 21 of which a schematic is shown in Fig.1 on the left and the first image, which was obtained on November 30, 2001 by SPOT-4, optically transmitted to ARTEMIS and relayed via Ka-band to a ground station in Toulouse is shown on the right [3].

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With help of its ion thrusters – initially foreseen for North/South station keeping – ARTEMIS was spiraled out towards its nominal orbital position of 21.5° East in GEO. During the maneuver, which lasted from February 2002 until February 2003, no data-relay operations were possible, because the ion-engines thrust direction required a spacecraft attitude different from its nominal Nadir pointing.

2.2 SILEX Technology

SILEX is based on-off keying modulation and direct detection of laser beams in a wavelength range between 800nm and 850nm. The two SILEX terminals on ARTEMIS and on SPOT-4 use wavelength discrimination (819nm and 847nm) to isolate their respective transmit and receive beams.

SILEX demonstrated for the first time that the stringent pointing, acquisition and tracking requirements associated with the extremely low-divergence of optical communication beams (7µrad) can be mastered in space. As an example, the attitude of the ARTEMIS satellite is only stabilized the standard telecommunication requirements (±1700µrad) and to establish contact with SPOT-4 the laser communication terminal the ARTEMIS terminal needs to scans this uncertainty cone with a wide beacon laser (750µrad) and high power. Scanning is done in a spiral fashion and upon detection of the ARTEMIS beacon by SPOT-4 within fractions of a second it sends a communication beam back to stop the beacon scan. The two terminals then track each others beams and optimize their angular alignment, after which the ARTEMIS communication beam is switched on and the beacon off and data transmission begins. A sophisticated high-frequency beam steering mechanism ensures that mutual tracking on the incoming beams takes place. The performance data for the laser communication terminals on ARTEMIS and SPOT-4, as well as their orbital data have been published recently [10]. The two terminals are shown in Fig. 2.

Fig. 2: ARTEMIS (left) and SPOT-4 (right) optical laser communication terminals during assembly at Astrium SAS (former

Matra Marconi Space) France.

2.3 The optical ground station (OGS)

The idea of establishing a general purpose optical ground station facility, equipped with a large telescope, was already alive in the late 1980s when preliminary design concepts were elaborated. The main stumbling block, however, was the cost associated with such an installation. In this situation, “history” gave a helpful hand. After the fall of the Berlin wall, ESA engineers visited companies in the former DDR in search for industrial capabilities and came across a 1m telescope produced by Carl Zeiss Jena. The telescope, the 13th and last of a production series, should have been sold to Russia. However, the deal collapsed after the German reunification and the telescope was intended to be scrapped. The German Space Agency (DLR) procured the telescope, dome and control electronics as part of their policy to support industry in the “Neue Bundesländer”.

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After the acquisition of the telescope, the search started for a suitable location for the envisaged optical ground station. The dominant criterion was to find a place at high altitude in one of ESA’s southern member states. The observatories on the Canary Islands were therefore first choice. The location of the Teide observatory on Tenerife was chosen, because of its excellent seeing conditions at 2400m altitude, its proximity to the Earth’s equator, which minimizes the atmospheric path length to a geostationary satellite (ARTEMIS), and the excellent infrastructure provided by the Instituto de Astrofisica de Canarias (IAC). After signing a memorandum of understanding with the IAC, which gave ESA the rights to construct a building on IAC territory and to get access to its infrastructure in exchange of 25% of observation time, the actual construction of the Optical Ground Station (OGS) started in 1994. On 30 June 1996 the OGS was inaugurated together with the French “Themis” solar observatory by his majesty, King Juan Carlos of Spain. Final acceptance testing of the OGS telescope and of its control system took place in 1997, when “first light” was declared.

Although the OGS had seen first light in 1997, the real test as check-out station for SILEX had to wait a couple of years due to launch delays of ARTEMIS.

Fig 3: Location of the OGS at the Teide observatory in Tenerife (left) and the OGS building (right)

In order to check out as early as possible the health of the SILEX payload on ARTEMIS, it was decided not to wait until ARTEMIS had reached its final position, but to make first tests with ARTEMIS in its parking orbit using the ESA Optical Ground Station on Tenerife. On 15 November 2001 at 01:00 UTC, the beacon signal of ARTEMIS was acquired in the OGS for the first time, and - fully according to schedule - 27seconds later the communication beam was tracked by the OGS. Pointing, acquisition and tracking of the satellite were demonstrated and two communication sessions of 20 minutes each were performed. This event is a cornerstone in the long and successful history of the development of optical space communications in Europe, because it marked the first demonstration of such a link in space. Several days later, on 20 November 2001, SILEX, the first inter-satellite link between SPOT-4 and ARTEMIS was successfully performed and since February 2003 the optical data relay service has been used on a daily basis.

Several hundred laser communication sessions have been performed from the OGS with the laser communication terminal on ARTEMIS, mainly to characterize the laser beam propagation and the optical communication channel through atmospheric turbulence. The OGS is also monitoring the performance of the optical terminal on ARTEMIS in regular intervals [4].

Apart from the regular health monitoring of the SILEX system, the OGS has been used frequently to support other, national programs of optical space communications. In September 2003, the infrastructure of the OGS was made available to a team of engineers of the Japanese Space Agency (JAXA) to test the performance of the engineering model of the LUCE laser terminal, particularly in situations when the LUCE laser transmitter was not powerful enough to sustain the link through atmosphere [5]. LUCE (Laser Utilizing Communications Equipment) is a SILEX compatible laser communication terminal developed by JAXA under a cooperation agreement with ESA. It was launched into a LEO orbit in August 2005 on the Japanese OICETS satellite, after which JAXA performed inter-satellite laser communication with ARTEMIS. Similarly, the OGS helped in the preparation of the French LOLA (Liaison Optique Laser Aéroportée) experiment by providing measurement data to optimize the link performance. As part of the LOLA project, Astrium France developed a SILEX compatible laser communication terminal for use on an aircraft, with which optical communication with ARTEMIS was established.

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Fig 4: The Zeiss 1m telescope of the OGS

The OGS has also been playing a key role in the development and verification of second generation laser communications terminals. While SILEX has been a vital development step for Europe, providing in-flight testing of a pre-operational optical communication link in space and stimulating the development of many new space-qualified optical, electronic and mechanical equipment technologies, it has its limitations. Direct-detection, semiconductor laser diode technology, as applied in SILEX, LUCE and LOLA, is appropriate only for moderate data-rate systems, because of physical limits to the achievable laser power and detector sensitivity. On the other hand, coherent systems, based, for instance, on Nd:YAG laser radiation, are highly promising for high data-rate systems, because of the much higher achievable laser power and receiver sensitivity in case of coherent detection.

For instance, extensive use of the OGS was made during the development and early in-orbit testing of the coherent Nd:YAG laser communication terminals (LCT) developed by Tesat Spacecom (former ANT and Bosch Telecom) under DLR-German national funding. The first pair of these terminals has been launched in 24 April 2007 on the American DoD NFIRE (Near Field Infra-Red experiment) satellite, and the second one in 15 June 2007 on TerraSAR-X, a German Earth-observation satellite using synthetic aperture radar in X-band. The first in-orbit tests of the terminal onboard TerraSAR-X have been performed from the OGS in August 2007, where the correct pointing, acquisition and tracking performance was demonstrated. For this test, it was sufficient to use the 18cm diameter Maksutov telescope of the OGS wide field camera. The 1m telescope itself and its equatorial mount acted only as a stable tracking platform for the pulsed Nd:YAG laser transmitter. Since the equatorially mounted 1m telescope of the OGS has not been designed for fast tracking of satellites in LEO, sophisticated tracking software upgrades were necessary. With a laser output power of 10Watt it was possible to operate the laser with a beam divergence of 1mrad, such that the tracking accuracy of the telescope was sufficient to point the laser beam accurately towards the satellite. The same tests have been performed in October 2007 with the laser communication terminal on NFIRE before inter-satellite communication links using data rates of 5.6Gbps were successfully demonstrated in February 2008.

The OGS is now being prepared for the commissioning and testing of coherent laser communication terminals on the Sentinel Earth observation satellite and ESA’s next generation telecommunication satellite Alphasat [10].

2.4 OICETS

In 1993 the Japanese space agency NASDA and ESA agreed on a cooperation to perform optical inter-satellite communication experiments and the preliminary design of Optical Inter-satellite Communication Engineering Test Satellite (OICETS) and its laser communication terminal called LUCE (Laser Utilizing Communication Equipment) was finished in 1994. In September 2003 JAXA validated the performance of the engineering model of its LUCE terminal with ARTEMIS in a space to ground link experimental campaign from ESA’s OGS in September 2003 [5].

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OICETS was launched by a Dniepr launcher from Baikonur (Kazakhstan) on August 23rd, 2005 into a circular sun-synchronous 610km orbit and first laser communication experiments with ARTEMIS were performed on December 9th, 2005. Unlike SPOT-4, OICETS is able to receive and transmit data and thus it demonstrated the world-first bidirectional optical inter-satellite communication link receiving data at 2Mbps and transmitting at 50Mbps [6].

The LUCE terminal was built by a Japanese consortium of NEC and Toshiba (NTSpace). The technical parameters are identical to the ones of the terminal on SPOT-4 with the following exceptions: The aperture diameter is 260mm, the transmit beam diameter is 130mm (1/e2), the laser power is 100mW and the LUCE terminal weight is 170kg.

When the inter-satellite communication link campaign with ARTEMIS was completed successfully end of 2006, inter-satellite operations were stopped. Only space-to-ground links were continued until September 2009.

2.5 SILEX Link Statistics

The SPOT-4/ARTEMIS inter-satellite link statistics since March 2003 counts 1862 sessions of which 73 failed with an accumulated link duration of 377 hours, while the OICETS/ARTEMIS inter-satellite link statistics counts 83 sessions of which 2 failed with accumulated link duration of 14 hours. The OGS/ARTEMIS bi-directional space to ground link statistics counts 393 sessions of which 34 failed with accumulated link duration of 78 hours.

3. COHERENT LASER COMMUNICATION SYSTEMS SILEX has been a vital development step in Europe as it provided in-flight testing of a pre-operational optical link in space. The program stimulated the development of many new space-qualified optical, electronic and mechanical equipments and technologies, which can now form a core for future optical terminals. However, with its mass of 157kg and 50Mbps data rate, SILEX was hardly an attractive alternative to a RF terminal of comparable transmission capability. One must bear in mind that the SILEX terminal had to be dimensioned by using the limited laser diode power available at the end of the 1980’s, namely 60mW average power at 830nm. The result was a 25cm telescope aperture, both on the LEO and the GEO. For an Inter-Orbit Link (IOL) user terminal, to be attractive, it is important to keep mass, interface requirements to the host spacecraft and cost to a minimum. Realizing this and anticipating the need for small data-relay LEO user terminals, ESA launched several activities to develop lightweight laser communication terminals.

In the search for smaller and more efficient laser terminals ESA continued to investigate other advanced system concepts and technologies. Direct-detection, semiconductor laser diode technology, as applied in SILEX, is appropriate for moderate data-rate systems; however, there are physical limits to the achievable laser power and detector sensitivity. Optical direct detection receivers using state-of-the-art Avalanche Photo Diodes (APD) require about 50 photons/bit to achieve a Bit Error Rate (BER) better than 10-6. On the other hand, coherent systems, based, for instance, on Nd:YAG laser radiation, are highly promising for high data-rate systems. There is no principle restriction to the achievable laser power and detector sensitivity can almost reach the theoretical quantum limit. In view of these perspectives, ESA has since 1989 placed strong emphasis on the development of Nd:YAG laser-based coherent laser communication systems and related hardware technologies.

As part of this effort, two parallel system design studies were placed in 1989 for the "Design of a Diode-Pumped Nd:Host Laser Communication System”. Funding difficulties prevented a full hardware implementation of such terminals, but a number of critical technology elements were bread-boarded and tested, including a diode-pumped NdP:YAG laser, a multi-channel coherent optical receiver and an electro-optic phase modulator. Germany continued the activities under the German national SOLACOS (Solid State Laser Communications in Space) program.

The coherent Nd:YAG laser communication effort also stimulated the investigation of advanced concepts, such as optical amplifiers in fiber and/or semi-conductor technology and the possibility of synthesizing the input/output aperture of the terminal with the help of an array of smaller sub-apertures, coherently coupled among each other. Optical phased arrays provide laser-communication systems with inertia-free, hence ultra fast, beam scanning ability needed for accurate beam pointing, efficient area scanning and reliable link tracking in presence of spacecraft attitude jitter.

In April 1996, ESA placed a contract with an industrial team led by Oerlikon-Contraves Space (now Ruag Space AG) for the design, realization and test of a demonstrator of a compact and light-weight optical terminal for Short-Range Optical Inter-satellite Links (SROIL). To achieve ultimate system miniaturization, highest transmit data rates and sufficient growth potential to comply also with extended link ranges, the SROIL terminal was designed using a laser-diode pumped

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Nd:YAG laser transmitter together with a coherent detection receiver. The pointing system of the SROIL terminal was based upon a periscope-type pointing assembly in front of a 35mm diameter aperture telescope, allowing almost full hemispherical pointing. The communication subsystem was designed as a BPSK (Binary Phase Shift Keying) homodyne system for a data rate of 1.5Gbps. Due to the homodyne detection scheme the communication signal is recovered at baseband, which simplifies considerably the communications electronics design.

On February 24th, 1998 Oerlikon-Contraves Space and Motorola announced that they had signed a Strategic Alliance Agreement for the development and production of Optical Inter-Satellite Link (OISL) Terminals for the CelestriTM

broadband satellite communication network in low earth orbit (LEO). After signature of this agreement, Motorola approached the US authorities to obtain a Technology Assistance Agreement (TAA), which is the legal precondition to enter high-technology ventures with non-US partners. Unfortunately, the US State Department refused to grant such a TAA with Oerlikon-Contraves, while it had no objections to authorize dealings with the other European partners of Oerlikon-Contraves in the OISL industrial team, namely Bosch Telecom and Carl Zeiss. Subsequently Bosch Telecom took over the prime contractor-ship for the CelestriTM OISL terminal development from Oerlikon-Contraves with Carl Zeiss and Ball Aerospace as subcontractors.

Fig. 5: The laser communication terminals mounted on the side panel of the TerraSAR-X satellite close to the star trackers (left) and on top of the NFIRE satellite already covered in MLI (right).

However, very shortly thereafter the CelestriTM program was cancelled, but the German Space Agency (DLR) continued funding of coherent laser communication terminals (LCT) under its LCTSX and TSX-LCT programs.

Two LCTs were built, one to be flown on TerraSAR-X, a German Earth observation satellite with a synthetic aperture radar payload operating in X-band, and a second one to be used as spare. Fortunately, a flight opportunity came up on the Near Field Infrared Experiment (NFIRE) satellite, developed by the American department of defense, when another NFIRE payload had been cancelled.

The laser communication terminals are based on BPSK modulation where the phase of a laser beam is used to transmit data instead of the intensity. BPSK requires some complicated receiver technology, such as a local oscillator, which needs to be phase-locked to the incoming light, but it offers the maximum detection sensitivity in terms of photons required per bit. To increase reliability the laser communication terminals a beaconless acquisition scheme is used, where the two terminals take turns to scan the uncertainty cones of their respective pointing directions. Wavelength discrimination to isolate their respective transmit and receive beams cannot be used because the wavelengths are identical, however polarization discrimination is applied [7].

The two laser communication terminals mounted onto the side panel of the TerraSAR-X satellite and on top of the NFIRE satellite are shown in Fig. 5.

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3.1 TerraSAR-X and NFIRE

The NFIRE satellite was launched on April 24, 2007 into a LEO orbit with 48.23° inclination and two month later, on June 15, 2007 the TerraSAR-X satellite was launch into a sun-synchronous LEO orbit with 510 km altitude and 97.45° inclination. After commissioning of both spacecraft the first successful inter-satellite communication link using coherent modulation techniques took place on February 21, 2008 [8].

Since then 55 LEO to LEO bidirectional inter-satellite communication links have been performed demonstrating net data rates of 5.6Gbps over link distances of up to 4900km. At that distance the optical link breaks down because the laser beam passes the upper layers of the Earth’s atmosphere. While intensity fluctuations (scintillation) caused by atmospheric turbulence is already detectable in altitudes of 80km above the Earth, at 30km the link can no longer be maintained. Communication link sessions lasted between 50 seconds and 650 seconds with an accumulated time of about 16000 seconds. The acquisition time has been reduced to around 30 second from the start of acquisition until communication takes place by carefully determining the attitude error and thus minimizing the uncertainty cone of the acquisition scan on both spacecraft [9].

3.2 Alphasat

The data relay scenario has re-emerged as the most important application for optical communication technology, because it’s the only way to retrieve the data generated by today’s Earth observation satellites operating with synthetic aperture radars or multi-spectral imagers. Despite offering extremely large bandwidth laser communication terminals require no license and their operation is interference free. The German Space Agency (DLR) seized the opportunity to embark on ESA’s latest telecommunication satellite Alphasat a data relay technology demonstration payload (TDP#1), which will consist of a laser communication terminal (LCT) for inter-satellite links and a Ka-band terminal for space to ground links. The LCT will be an updated version of the ones flown on TerraSAR-X and NFIRE with increased telescope diameter and transmit laser power.

Fig. 6: Artist’s impression of the Alphasat spacecraft showing its large 11m diameter L-band reflector.

This will increase the link distance to 45000km (to cover the LEO-GEO inter-satellite distance) and enable a net data rate of 2.8Gbps. The Ka-band terminal will support 600Mbps on the satellite to ground link.

The Alphasat satellite will be operated by Inmarsat Global Ltd and will deliver a new broadband global area network family of services, which provide a wide range of high data rate applications to a new line of user terminals for

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aeronautical, land and maritime markets. It will be positioned at 25° East, covering Europe, Middle East, Africa and parts of Asia.

3.3 European Data Relay Satellite (EDRS) System

Despite the present telecommunication capabilities, there are still a number of limitations that delay the delivery of time-critical data to users. With the implementation of the joint European Commission/ESA Global Monitoring for Environment and Security program, it is estimated that European space telecommunication infrastructure will need to transmit 6 terabytes of data every day from space to ground. The present telecom infrastructure is challenged to deliver such large data quantities within short delays and conventional means of communication may not be sufficient to satisfy the quality of service required by users of Earth observation data. In addition, Europe currently relies on the availability of non-European ground station antennas to receive data from Earth observation satellites. This poses a potential threat to the strategic independence of Europe, as these crucial space assets effectively may not be under European control. The EDRS System offers a solution to these challenges.

EDRS will consist of three GEO satellite, equipped with laser communication terminals for inter-satellite links and Ka-band terminals for the space to ground link. Its first customers will be the Sentinel 1 and 2 Earth observation satellites, which are being deployed within the Global Monitoring for Environment and Security (GMES), a European Union Initiative for the establishment of a European capacity for Earth Observation.

4. CONCLUSION Today, the problem of optical free space communication to enter the commercial payload market is not so much of technical nature but rather the need to convince commercial satellite operators that optical communication systems are reliable. This will be demonstrated by the deployment of the European Data-Relay Satellite (EDRS) system.

Thirty years of technology endeavors, sponsored by ESA and other European space agencies, has put Europe in a leading position in the domain of space laser communications. The most visible result of this effort is SILEX and the planned installation of laser communication terminals on the European Data Relay Satellite (EDRS) System.

ACKNOWLEDGMENT

The authors would like to thank Astrium SAS, Tesat Spacecom, Ruag Space, JAXA, NICT, NTSpace, IKN and DLR for their support and cooperation.

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