Optical Intersatellite Communication

IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 16, NO. 5, SEPTEMBER/OCTOBER 2010 1051 Optical Intersatellite Communication Zoran Sodnik, Bernhard Furch, and Hanspeter Lutz (Invited Paper) Abstract—This paper describes the achievements in optical in- satellite constellations based global telephone networks such tersatellite communication based on technology developments that as Iridium. Paired with an economic downturn this led to the started in Europe (European Space Agency) more than 30 years subsequent cancellation of the Celestri and Teledesic satellite ago. In 2001, the world-first optical intersatellite communication link was established (between the SPOT-4 and Advanced Relay constellations, which would have required several hundreds of and TEchnology MIssion Satellite (ARTEMIS) satellites), proving LCTs to establish high-speed data exchange between neighbor- that optical communication technologies can be reliably mastered ing satellites in the constellation. in space. In 2006, the Japanese Space Agency (JAXA) demon- The German Space Agency [Deutsche Gesellschaft f¨ur Luft- strated a bidirectional optical link between its Optical Inter-Orbit und Raumfahrt (DLR)] continued the development of laser com- Communications Engineering Test Satellite and ARTEMIS, and in 2008, the German Space Agency (DLR) established an intersatellite munication technology, realizing the strategic importance for link between the near-field infrared experiment and TerraSAR-X its industry. A second generation of terminals was developed, satellites already based on the second generation of laser commu- which are now operational in orbit, since 2008. They will form nication technology. the backbone of the new European Data Relay Satellite (EDRS) Index Terms—Coherent modulation, free-space laser communi- system to be deployed in 2013. cation technology, intersatellite communication. II. SILEX I. INTRODUCTION A. Early Years HIRTY years ago, in summer 1977, the European Space T Agency (ESA) placed a technological research contract for the assessment of modulators for high data rate laser links in When ESA started to consider optics for intersatellite commu- nications, virtually no component technology was available to support space-system development. The available laser sources space. This marked the beginning of a long and sustained ESA were rather bulky and primarily laboratory devices. Initially, involvement in space optical communications. A large number carbon dioxide (CO2 ) gas lasers were selected, because these of study contracts and preparatory hardware developments fol- were the most efficient and reliable lasers at that time and Europe lowed, conducted under various ESA R&D activities. In the mid had a considerable background in CO2 laser technology for in- 1980s, ESA took an ambitious step by embarking on the semi- dustrial applications [1]. A detailed design study of a CO2 LCT conductor laser intersatellite link experiment (SILEX) program, was undertaken and all critical subsystems were bread-boarded to demonstrate a preoperational optical communication link in and tested [2]. space. SILEX, which started routine operations in March 2003, This enabled ESA to get acquainted with the intricacies of has put ESA in a world leading position in optical intersatellite coherent, free-space optical communication, but very early on, it links. became evident that the 10.6-µm CO2 laser was not the winning In 1993, the Japanese Space Agency National Space Devel- technology for use in space because of weight, lifetime, and opment Agency (NASDA) and ESA agreed on a cooperation to operational problems. perform optical intersatellite communication experiments, and Toward the end of the 1970s, semiconductor diode lasers op- in 2006, communication links were established. erating at room temperature became available, providing a very After having made the investment into SILEX and demon- promising transmitter source for optical intersatellite links. In strating the feasibility of optical communication technology 1980, therefore, ESA placed the first studies to explore the po- ESA decided to leave the field to European industry pick up tential of using this new device for intersatellite links. At the on the lessons learned and to develop laser communication ter- same time, the French National Space Study Center (CNES) minal (LCT) for the commercial market. started to look into a laser-diode-based optical data-relay sys- This however turned out to be difficult because the GSM tele- tem. This resulted in the decision, in 1985, to embark on the phone network emerged, which became a strong competitor for SILEX [3]. SILEX consists of two optical communication payloads em- Manuscript received January 15, 2010; revised February 17, 2010; accepted barked on the ESA Advanced Relay and TEchnology MIssion March 7, 2010. Date of publication June 1, 2010; date of current version October Satellite (ARTEMIS) spacecraft and on the French Earth ob- 6, 2010. The authors are with the TEC-M, European Space Agency, Noordwijk 2200 servation spacecraft SPOT-4. It allows the transmission of the AG, The Netherlands (e-mail: [email protected]; [email protected]; maximum data rate the Earth observation camera on SPOT-4 can [email protected]). provide, namely 50 Mb/s, from low-earth orbit (LEO) to geo- Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. stationary orbit (GEO) using GaAlAs laser diodes and direct Digital Object Identifier 10.1109/JSTQE.2010.2047383 detection [4]. 1077-260X/$26.00 © 2010 IEEE 1052 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 16, NO. 5, SEPTEMBER/OCTOBER 2010 Fig. 1. Schematic depiction of the SILEX intersatellite link between SPOT-4 and ARTEMIS. In 1997, both terminals underwent a stringent environmental test program and the first host spacecraft (SPOT-4) was launched in March 1998. The preshipment review of ARTEMIS took Fig. 2. First image transmitted by the SILEX optical data-relay system on November 30, 2001. It shows the Southern part of the island of Lanzarote, place in ESA at the end of 1999, but the launch of ARTEMIS, Canary Islands, and Spain. which was initially scheduled on the Japanese launcher H2 A 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 Ariane-5 was negotiated. B. ARTEMIS and SPOT-4 ARTEMIS was eventually launched on July 12, 2001, but due to underperformance the third stage of its Ariane-5 launcher, the satellite was injected into a too low elliptical geostationary trans- fer orbit with apogee × perigee altitudes of 17 500 km × 590 km instead of 36 000 km × 860 km. Within 10 days, and by using most of its onboard propellant for a total of eight apogee motor firings, ARTEMIS was brought out of the radiation belts and Fig. 3. ARTEMIS orbit raising maneuvers performed by chemical thruster into a circular, however, below GEO (with 31 000 km altitude, firings (green) and by electrical propulsion (red). a 0.8◦ inclination and an orbital period of 20 h). In order to check out as early as possible the health of the C. SILEX Technology SILEX payload on ARTEMIS first tests with ESA’s optical ground station (OGS) on Tenerife were performed on November SILEX is based on ON–OFF keying modulation and direct de- 15, 2001 [5]. Pointing, acquisition, and tracking (PAT) parame- tection of laser beams in the 800 nm wavelength range. Both ters of the SILEX payload were optimized and two link sessions SILEX terminals on ARTEMIS and on SPOT-4 use wavelength of 20 min each, were performed. The PAT procedure is ex- discrimination (819 and 847 nm) to isolate their respective trans- plained in [6]. Five days later the first intersatellite link between mit and receive beams. ARTEMIS and SPOT-4 was established of which a schematic SILEX demonstrated for the first time that the stringent PAT is shown in Fig. 1. requirements associated with the extremely low divergence of Fig. 2 shows the first image, which was obtained on November optical communication beams can be reliably mastered in space. 30, 2001 by SPOT-4, optically transmitted to ARTEMIS and The attitude uncertainty of the ARTEMIS satellite platform relayed via Ka-band to a ground station in Toulouse. It shows is 0.1◦ = 1700 µrad (standard for telecommunication satel- the southern part of the island of Lanzarote [7]. lites), which makes pointing with microradian accuracy (the With help of its ion thrusters—initially foreseen for divergence of the SILEX communication laser beam is 7 µrad) north/south station keeping—ARTEMIS was spiraled out to- impossible. To establish contact the LCT on ARTEMIS first ward its nominal orbital position of 21.5◦ east in GEO. During scans the 1700 µrad uncertainty cone with a wide beacon laser the maneuver, which lasted from February 2002 until February (750 µrad) and high laser power (>10 W). Scanning is done 2003, no data-relay operations were possible, because the ion- in a spiral fashion and upon detection of the ARTEMIS beacon engines thrust direction required a spacecraft attitude different by SPOT-4 within fractions of a second it sends a communica- from its nominal Nadir pointing. The spiraling out by electric tion beam back to stop the beacon scan. The two terminals then propulsion is indicated in Fig. 3 by the red band. track each other beams and optimize their angular alignment, SODNIK et al.: OPTICAL INTERSATELLITE COMMUNICATION 1053 Fig. 4. ARTEMIS (left) and SPOT-4 (right) LCTs during assembly at Astrium SAS (former Matra Marconi Space) France. after which the ARTEMIS communication beam is switched ON and the beacon OFF and data transmission begins. A sophis- ticated high-frequency beam steering mechanism ensures that mutual tracking on the incoming beams takes place. The perfor- mance data for the LCTs on ARTEMIS and SPOT-4, as well as some orbital data is given in the Appendix. The two terminals are shown in Fig. 4. Fig. 5. LUCE LCT on top of the OICETS spacecraft. E. SILEX Link Statistics D. Optical Intersatellite Communication Engineering Test Satellite The SPOT-4/ARTEMIS intersatellite link statistics since March 2003 counts 1862 sessions of which 73 failed with In 1993, the Japanese Space Agency NASDA and ESA agreed an accumulated link duration of 377 h and 39 min, while the on a cooperation to perform optical intersatellite communication OICETS/ARTEMIS intersatellite link statistics counts 83 ses- experiments and the preliminary design of Optical Intersatellite sions of which two failed with accumulated link duration of 14 h Communication Engineering Test Satellite (OICETS) and its and 21 min. LCT called laser utilizing communication equipment (LUCE) was finished in 1994. In September 2003, JAXA validated the F. Toward Smaller Terminals performance of the engineering model of its LUCE terminal with ARTEMIS in a space to ground link experimental campaign SILEX has been a vital development step in Europe as it pro- from ESA’s OGS in September 2003 [8]. vided in-flight testing of a preoperational optical link in space. OICETS was launched by a Dniepr launcher from Baikonur The program stimulated the development of many new space- (Kazakhstan) on August 23, 2005 into a circular sun- qualified optical, electronic, and mechanical equipments and synchronous 610-km orbit and first laser communication exper- technologies, which can now form a core for future optical ter- iments with ARTEMIS were performed on December 9, 2005. minals. However, with its mass of 157 kg and 50-Mb/s data rate, Unlike SPOT-4, OICETS is able to receive and transmit data, SILEX was hardly an attractive alternative to a RF terminal of and thus, it demonstrated the world-first bidirectional optical comparable transmission capability. intersatellite communication link receiving data at 2 Mb/s and One must bear in mind that the SILEX terminal had to be transmitting at 50 Mb/s [9]. dimensioned by using the limited laser diode power available at Fig. 5 shows the LUCE terminal on top of the OICETS satel- the end of the 1980s, namely 60 mW average power at 830 nm. lite. The LUCE terminal was built by a Japanese consortium The result was a 25 cm telescope aperture, both on the LEO and of NEC and Toshiba (NTSpace). The technical parameters are the GEO terminal (see Fig. 4). identical to the ones of the terminal on SPOT-4 with the follow- For an inter-orbit link (IOL) user terminal to be attractive, ing exceptions: the aperture diameter is 260 mm, the transmit it is important to keep mass, interface requirements to the host beam diameter is 130 mm (1/e2 ), the laser power is 100 mW, spacecraft, and cost to a minimum. Realizing this and antici- and the LUCE terminal weight is 170 kg. pating the need for small data-relay LEO user terminals, ESA When the intersatellite communication link campaign with launched several activities to develop lightweight LCTs. ARTEMIS was completed successfully end of 2006, intersatel- In the search for smaller and more efficient laser terminals, lite operations were stopped. Only space-to-ground links were ESA continued to investigate other advanced system concepts continued until September 2009. and technologies. Optically preamplified direct detection sys- 1054 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 16, NO. 5, SEPTEMBER/OCTOBER 2010 tems operating at a wavelength of 1550 nm and return-to-zero (RZ) modulation were investigated and a test system was devel- oped [10]. However, technology tradeoffs, which were performed at the time by ESA’s industrial partners, awarded higher marks to coherent communication systems based on Nd:YAG laser radi- ation. The main reasons were the maturity of the existing Eu- ropean developments, the lower risk of radiation darkening of amplifiers based on fibers and a coherent system’s false light im- munity, the capability to operate with the Sun in the field of view. III. COHERENT LASER COMMUNICATION SYSTEMS Since 1989, ESA has placed strong emphasis on the develop- ment of Nd-YAG laser-based coherent communication system 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 Nd-YAG laser, a mul- tichannel coherent optical receiver and an electrooptic phase modulator. Germany continued the activities under the German National solid-state laser communications in space (SOLACOS) Fig. 6. Engineering model of the SROIL terminal program. The coherent Nd-YAG laser communication effort also stim- ulated the investigation of advanced concepts, such as optical at baseband, which simplifies considerably the communications amplifiers in fiber and/or semiconductor technology and the pos- electronics design. sibility of synthesizing the input/output aperture of the terminal On February 24, 1998 Oerlikon-Contraves Space (CH, with the help of an array of smaller subapertures, coherently Zurich, Switzerland) and Motorola (Schaumburg, IL) an- coupled among each other. Optical-phased arrays provide laser nounced that they had signed a Strategic Alliance Agreement communication systems with inertia-free, hence ultrafast, beam for the development and production of optical intersatellite link scanning ability needed for accurate beam pointing, efficient (OISL) terminals for the Celestri broadband satellite communi- area scanning, and reliable link tracking in presence of space- cation network in LEO. craft attitude jitter [11]. After signature of this agreement, Motorola approached the Upto the early 1990s, ESA’s optical communication activi- U.S. authorities to obtain a Technology Assistance Agreement ties were dominated by the data-relay scenario. Over the time, (TAA), which is the legal precondition to enter high-technology however, some potential future users of a data-relay service ventures with non-U.S. partners. Unfortunately, the U.S. State disappeared and the interest in a near-term development of sec- Department refused to grant such a TAA with Oerlikon– ond generation user terminals dropped considerably. However, a Contraves, while it had no objections to authorize dealings with new class of potential users of optical intersatellite links emerged the other European partners of Oerlikon–Contraves in the OISL with the intended deployment of extensive satellite networks for industrial team, namely Bosch Telecom and Carl Zeiss. mobile communications and interactive multimedia services. Subsequently, Bosch Telecom took over the prime contractor- In April 1996, ESA placed a contract with an industrial team ship for the Celestri OISL terminal development from Oerlikon– led by Oerlikon-Contraves Space (now Ruag Space AG) for the Contraves with Carl Zeiss and Ball Aerospace as subcontractors. design, realization, and test of a demonstrator of a compact and However, very shortly, thereafter the Celestri program was lightweight optical terminal for short-range optical intersatel- cancelled, but the German Space Agency (DLR) continued fund- lite links (SROIL). To achieve ultimate system miniaturization, ing of coherent LCTs under its LCTSX and TSX-LCT programs. highest transmit data rates and sufficient growth potential to Two LCTs were built, one to be flown on TerraSAR-X, a comply also with extended link ranges, the SROIL terminal was German Earth observation satellite with a synthetic aperture designed using a laser-diode pumped Nd:YAG laser transmitter radar payload operating in X-band, and a second one to be used together with a coherent detection receiver. The pointing sys- as spare. Fortunately, a flight opportunity came up on the near- tem of the SROIL terminal was based upon a periscope-type field infrared experiment (NFIRE) satellite, developed by the pointing assembly in front of a 35 mm diameter aperture tele- American department of defense, when another NFIRE payload scope, allowing almost full hemispherical pointing. The SROIL had been cancelled. terminal is shown in Fig. 6. The two LCTs mounted onto the side panel of the TerraSAR- The communication subsystem was designed as a BPSK ho- X satellite and on top of the NFIRE satellite are shown in Fig. 7. modyne system for a data rate of 1.5 Gb/s. Due to the homo- The LCTs are based on BPSK modulation, where the phase dyne detection scheme, the communication signal is recovered of a laser beam instead of the intensity is used to transmit data. SODNIK et al.: OPTICAL INTERSATELLITE COMMUNICATION 1055 Fig. 8. Alphasat spacecraft showing its large 11 m diameter L-band reflector. Since then, 55 LEO-to-LEO bidirectional intersatellite com- munication links have been performed demonstrating net data rates of 5.6 Gb/s over link distances of up to 4900 km. At this distance, the optical link breaks down because the laser beam passes the upper layers of the earth’s atmosphere. While in- tensity fluctuations (scintillation) caused by atmospheric turbu- lence is already detectable in altitudes of 80 km above the earth, at 30 km, the link can no longer be maintained. Communication link sessions lasted between 50 and 650 s with an accumulated time of about 16 000 s. The acquisition time has been reduced to around 30 s 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 [14]. B. Alphasat Fig. 7. (Top) LCTs mounted on the side panel of the TerraSAR-X satellite The data-relay scenario has reemerged as the most important and (bottom) on top of the NFIRE satellite already covered in MLI. application for optical communication technology, because it is the only way to retrieve the data generated by today’s Earth BPSK requires some complicated receiver technology, such as a observation satellites operating with synthetic aperture radars or local oscillator, which needs to be phase-locked to the incoming multispectral imagers. Despite offering extremely large band- light, but it offers the maximum detection sensitivity in terms of width LCTs require no license and their operation is interference photons required per bit. free. The German Space Agency (DLR) seized the opportunity To increase LCT reliability, a beaconless acquisition scheme to embark on ESA’s latest telecommunication satellite Alphasat, is used, where the two terminals take turns scanning the uncer- a data-relay technology demonstration payload (TDP#1), which tainty cones of their respective satellite platforms. Wavelength will consist of a LCT for intersatellite links and a Ka-band ter- discrimination to isolate their respective transmit and receive minal for space to ground links. The LCT will be an updated beams cannot be used because the wavelengths are identical, version of the ones flown on TerraSAR-X and NFIRE with in- however, polarization discrimination is applied [12]. More tech- creased telescope diameter and transmit laser power (see the nical information is given in Table I. Appendix for more information). This will increase the link distance to 45 000 km (to cover the LEO–GEO intersatellite distance) and enable a net data rate of 2.8 Gb/s. The Ka-band A. TerraSAR-X and NFIRE terminal will support 600 Mb/s on the satellite to ground link. The NFIRE satellite was launched on April 24, 2007 into The Alphasat satellite will be operated by Inmarsat Global a LEO orbit with 48.23◦ inclination and two month later, on Ltd. and will deliver new Broadband Global Area Network June 15, 2007, the TerraSAR-X satellite was launch into a sun- (BGAN) family of services, which provide a wide range of synchronous LEO orbit with 510 km altitude and 97.45◦ inclina- high-data rate applications to a new line of user terminals for tion. After commissioning of both spacecraft, the first success- aeronautical, land, and maritime markets. It will be positioned ful intersatellite communication link using coherent modulation at 25◦ east, covering Europe, Middle East, Africa, and parts of techniques took place on February 21, 2008 [13]. Asia (see Fig. 8). 1056 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 16, NO. 5, SEPTEMBER/OCTOBER 2010 TABLE I TECHNICAL DATA OF LCTS FOR SPACECRAFT DESCRIBED IN THIS PAPER C. EDRS System 3) Rescue teams that need Earth observation data within Despite the present telecommunication capabilities, there are disaster-struck areas. 4) Security forces that transmit data to Earth observation still a number of limitations that delay the delivery of time- critical data to users. With the implementation of the joint satellites, aircraft, and unmanned aerial observation vehi- European Commission/ESA Global Monitoring for Environ- cles, to reconfigure such systems in real time. 5) Relief forces that operate among their units in the field ment and Security program, it is estimated that European space telecommunication infrastructure will need to transmit 6 TB of and require telecommunication support in cutoff areas. data every day from space to ground. The present telecom in- EDRS will consist of three GEO satellite, equipped with LCTs for intersatellite links and Ka-band terminals for the space frastructure is challenged to deliver such large data quantities within short delays, and conventional means of communication to ground link. Its first customers will be the Sentinel 1 and 2 Earth observation satellites, which are being deployed within may not be sufficient to satisfy the quality of service required by the GMES, a European initiative for the establishment of a Eu- users of Earth observation data. In addition, Europe currently relies on the availability of non-European ground station an- ropean capacity for Earth Observation. tennas to receive data from Earth observation satellites. This poses a potential threat to the strategic independence of Eu- IV. CONCLUSION rope, as these crucial space assets effectively may not be under Today, the problem of optical free-space communication to European control. The EDRS system offers a solution to these enter the commercial payload market is not so much of technical challenges. nature, but rather the need to convince commercial satellite There are a number of key services that will benefit from this operators that optical communication systems are cost efficient systems infrastructure right from the start. and reliable. This will be demonstrated by the deployment of 1) Earth observation applications in support of a multitude the EDRS system. of time-critical services, e.g., monitoring of land-surface Thirty years of technology endeavors, sponsored by ESA motion risks, forest fires, floods, and sea ice zones. and other European space agencies, has put Europe in a leading 2) Government and security services that need images from position in the domain of space laser communications. The most key European space systems, such as Global Monitoring visible result of this effort is SILEX and the planned installation for Environment and Security (GMES). of optical communication technology on the EDRS system. SODNIK et al.: OPTICAL INTERSATELLITE COMMUNICATION 1057 ACKNOWLEDGMENT Zoran Sodnik was born in Rijeka, Croatia, in 1957. He received the M.Sc. degree in technical cybernetics from the Technical University Berlin, Berlin, The authors would like to thank Astrium SAS, Tesat Space- Germany, and the Ph.D. degree in optical engineering from Stuttgart University, com, Ruag Space, JAXA, National Institute of Information and Stuttgart, Germany. He was an Assistant Professor at the Institute for Applied Optics, Stuttgart Communication Technology, NTSpace, and DLR for their sup- and was a Postdoctoral Researcher in optical metrology using differential and port and cooperation. two-wavelength interferometry. In 1993, he joined the European Space Agency (ESA), Space Research and Technology Centre (ESTEC), Noordwijk, The Netherlands, where he was involved in the development of ESA’s optical ground REFERENCES station (OGS) for testing laser communication terminals onboard satellites in [1] W. Reiland, W. Englisch, and M. Endemann, “Optical intersatellite com- space. He is supporting optical technology developments for future science projects of ESA, namely the Large Interferometer Space Antenna (LISA) and munication links: State of CO2 laser technology,” in Proc. SPIE, 1986, its precursor mission, LISA pathfinder, and is involved in the development of vol. 616, pp. 69–76. optical metrology systems for instrument alignment and formation flying in [2] P. Huber, W. Reiland, V. Klein, and A. Popescu, “Full scale laboratory breadboard model (LBM) of a free space laser transceiver package,” in space. Proc. SPIE, 1990, vol. 1218, pp. 467–477. [3] G. Oppenh¨auser, M. Wittig, and A. 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