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Optical Communications in Space
Instrument Technology Division, ESA Technical and Operational Support Directorate, ESTEC, Noordwijk, The Netherlands
In summer 1977, ESA placed the first technological study contract in the domain of intersatellite optical links. Now, twenty years later, a major milestone has been reached with the SILEX laser terminals having been flight tested for integration with their host spacecraft. At the same time, ESA is preparing itself for a new challenge: the potential massive use of optical cross links in satellite constellations for mobile communications and global multimedia services. This is an opportune moment to look back at the past twenty years of ESA effort in laser communications, to take stock of the results achieved and to reflect on ways to face the challenges of the future.
Twenty 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 development followed, conducted under various ESA R&D and support technology programmes. In the mid- 1980’s, ESA took an ambitious step by embarking on the SILEX (Semiconductor laser Intersatellite Link Experiment) programme, to demonstrate a pre-operational optical link in space.
SILEX, which will be in operation in the year 2000, has put ESA in a world-leading position in civilian optical intersatellite links. While SILEX formed the backbone of ESA’s optical communications activities in the recent past, additional R&D activities were undertaken to develop attractive second-generation systems, particularly for the commercial satellite market. Indeed, at the turn of the century, literally thousands of intersatellite links – radio-frequency (RF) and optical – are expected to be in operation in commercial multi-satellite constellations providing mobile communications, video conferencing and multimedia services. The race is on for the European laser communication industry to enter this lucrative market. Optical technology offers too many advantages in terms of mass, power, system flexibility and cost, to leave the field entirely to RF. With the heritage of twenty years of technological preparation, European industry is well positioned to face this burgeoning demand for commercial laser terminals.
The early days
When ESA started to consider optics for intersatellite communications, virtually no component technology was available to support space system development. The available laser sources were rather bulky and primarily laboratory devices. ESA selected the CO2 gas laser for its initial work. This laser was the most efficient and reliable laser available at the time and Europe had a considerable background in CO2 laser technology for industrial applications. ESA undertook a detailed design study of a CO2 laser communication terminal and proceeded with the breadboarding of all critical subsystems which were integrated and tested in a complete laboratory breadboard transceiver model (Fig. 1).
This laboratory system breadboarding enabled ESA to get acquainted with the intricacies of coherent, free-space optical communication. However, it soon became evident that the 10 micron 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 intersatellite links. In 1980, therefore, ESA placed the first studies to explore the potential of using this new device for intersatellite links. At the same time, the French national space agency, CNES, started to look into a laser-diode-based optical data-relay system called Pastel. This line of development was consequently followed and resulted in the decision, in 1985, to embark on the SILEX pre-operational, in-orbit optical link experiment.
SILEX is a free-space optical communication system which consists of two optical communication payloads to be embarked on the ESA Artemis (Advanced Relay and TEchnology MIssion Satellite) spacecraft and on the French Earth-observation spacecraft SPOT-4. It will allow data transmission at 50Mbps from low Earth orbit (LEO) to geostationary orbit (GEO) using GaAlAs laser-diodes and direct detection.
The SILEX Phase A and B studies were conducted around 1985, followed by technology breadboarding and predevelopment of the main critical elements which were tested on the so-called ‘System Test Bed’ to verify the feasibility of SILEX. A detailed design phase was carried out in parallel with the System Test Bed activities up to July 1989. At that time, the development of SPOT-4 Phase C/D was agreed with an optical terminal as passenger. This was an important decision since it made a suitable partner satellite available for the ESA data-relay satellite project; the stage was therefore set to start the main SILEX development effort in October 1989.
In March 1997, a major milestone was reached in the SILEX programme: both terminals underwent a stringent environmental test programme and are now ready for integration with their host spacecraft. However, due to the agreed SPOT-4 and Artemis launch dates, it is likely that the in-orbit demonstration of the overall system will not start before mid-2000. Consequently, the GEO terminal will need to be stored after the completion of the spacecraft testing. The first host spacecraft (SPOT-4) is planned for launch in February 1998. The launch of Artemis on a Japanese H2A is delayed for non-technical reasons until February 2000. Apart from launching Artemis, Japan is participating in the SILEX programme with its own laser terminal, LUCE (Laser Utilizing Communications Equipment), to be carried onboard the Japanese OICETS satellite (Optical Inter-orbit Communications Engin-eering Test Satellite), set for launch in summer 2000.
Optical ground station on Tenerife
As part of the SILEX in-orbit check-out programme, ESA started to construct an optical ground station on the Canary Islands in 1993 (Fig. 2). This station, which will be completed by the end of 1997, simulates a LEO optical terminal using a 1 m telescope, allowing the performances of the GEO optical terminal on Artemis to be verified. The optical ground station will receive and evaluate the data transmitted from Artemis and will simultaneously transmit data at optical wavelengths towards Artemis. In addition to its primary objective as the SILEX in-orbit check-out facility, the optical ground station will also be used for space-debris tracking, lidar monitoring of the atmosphere and astronomical observations.
Towards smaller optical user terminals
SILEX has been a vital developmental step for Europe since it will provide inflight testing of a pre-operational optical link in space. The programme has stimulated the development of many new space-qualified optical, electronic and mechanical equipment items and technologies which can now form a core for future optical terminals. However, with its mass of 157 kg and electrical power consumption of 150 W, SILEX is hardly an attractive alternative to an RF terminal of comparable transmission capability.
One must bear in mind that the SILEX terminal had to be dimensioned using the limited laser diode power available at the end of the 1980’s, namely 60mW average power at 830 nm. The result was a 25 cm telescope aperture, both on the LEO and the GEO terminal (Fig. 3). Indeed, for reasons of design and equipment commonality, it was decided to have identical LEO and GEO terminals to save on overall programme cost. The large telescope diameter obviously puts very stringent requirements on the pointing, acquisition and tracking (PAT) subsystem and related components.
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. Realising this, and anticipating the need for small data-relay LEO user terminals, ESA launched a programme in January 1992 for the development and manufacturing of an elegant breadboard of a Small Optical User Terminal (SOUT). Working outside the SILEX programmatic constraints, the SOUT programme no longer had to consider identical LEO and GEO terminals but could concentrate on developing a truly asymmetric system using a smaller LEO transmitter with reduced constraints on the spacecraft. The SOUT terminal was specified for a data rate of 2 Mbps and was based on GaAlAs laser-diode technology with a SILEX-compatible wavelength and polarisation plan.
The SOUT terminal (Fig. 4) includes a number of innovative features. The mechanical interface to the spacecraft comprises an anti-vibration mount (soft-mount) which acts as a low-pass filter to the spacecraft microvibration spectrum. This reduces the bandwidth requirements for fine pointing, allowing a single Charge Coupled Device (CCD) sensor to perform both acquisition and tracking functions. The SOUT activities were successfully completed in December 1994, demonstrating that a compact terminal can be realised with a mass of about 25 kg.
After the successful completion of the SOUT programme, the UK supported the idea to adapt the SOUT terminal concept for low-data-rate cross links between two communication satellites in geostationary orbit. The name SOUT was changed to SOTT, the first ‘T’ standing for ‘Telecommunication’ instead of ‘User’. A contract was awarded to Matra Marconi Space (UK) at the beginning of 1995 under the ARTES-4 partnership programme, which is 50% co-funded by industry, to study the changes that would be necessary to convert the SOUT terminal to the SOTT terminal. The communication capacity was increased with the help of Maser-Oscillator-Power-Amplifier (MOPA) laser diodes. Presently, the SOTT programme is still continuing but directed towards new market needs requiring a 1 Gbps data rate and GEO-GEO link distances up to 83 000 km, such as in the Hughes Spaceway system.
Coherent laser communication systems
In its 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. Since 1989, therefore, ESA has 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 initiated in 1989 for the ‘Design of a Diode-Pumped Nd:Host Laser Communication System’. Funding difficulties within the ESA ASTP-4 programme prevented a full hardware implementation of such terminals, but a number of critical technology elements were breadboarded and tested, including a diode-pumped Nd-YAG laser, a multi-channel coherent optical receiver and an electro-optic phase modulator (Fig. 5). Initially, Germany and Italy were primarily supporting this work; Italy subsequently withdrew and Germany continued the activities under the German national SOLACOS (Solid State LAser Communications in Space) programme.
The coherent Nd-YAG laser communication effort also stimulated the investigation of advanced concepts such as optical amplifiers in fibre-optic and/or semiconductor technology and the possibility of synthesising the input/output aperture of the terminal with the help of an array of smaller sub-apertures, coherently coupled together. Optical phased arrays provide laser communication systems with the inertia-free and hence ultra-fast, beam scanning ability needed for accurate beam pointing, efficient area scanning and reliable link tracking in the presence of spacecraft attitude jitter. The feasibility and efficiency of this concept has been demonstrated, and an optical phased array with 16 telescope subapertures of 3-cm diameter is presently in development, along with an integrated-optics phase control unit (Fig. 6a,b).
Laser terminals for commercial systems
Up to the early 1990’s, ESA’s optical communication activities were dominated by the data-relay scenario. Over time, however, some potential future users of a data-relay service disappeared and the interest in a near-term development of second-generation user terminals dropped considerably. On the other hand, a new class of potential users of optical intersatellite links emerged with the intended deployment of extensive satellite networks for mobile communications and interactive multimedia services.
Driven by these new perspectives, ESA started internal studies in 1991 to investigate possible design solutions for the compact laser terminals potentially needed by such commercial satellite constellations. One of the initial results was MOMOT, the Monolithic Mini Optical Terminal. The MOMOT optical head is essentially a monolithic glass block which fully compensates for thermal expansion effects and supports all optical and electro-optical elements. By using novel diffractive optical technologies and advanced principles of microsystem design, a very compact and lightweight design became feasible (Fig. 7).
In April 1996, ESA placed a contract with an industrial team led by Oerlikon-Contraves Space (CH) for the design, realisation and testing of a demonstrator of a compact and lightweight optical terminal for short-range intersatellite links (SROIL). To be responsive to the projected market opportunities, the SROIL terminal was required to be capable of servicing the following mission classes:
- cross links between low-Earth-orbiting satellites in global satellite networks for mobile communication (IRIDIUM) and fixed-station data highways (Teledesic), such those depicted in Fig. 8
- cross links between co-located telecommunication satellites in geostationary orbit
- cross links between widely spaced geostationary satellites as proposed in ESA’s On-Board Processing System or in the Spaceway system from Hughes.
Considering the huge number of satellites involved in some of these configurations (e.g. 840 satellites for Teledesic, each carrying 8 ISL terminals), the cost issue becomes a dominant factor. To be attractive for these systems, optical ISL terminals, like SROIL, will have to be made very compact and robust, and be designed for mass production.
Following a Critical Design Review held in March 1997, go-ahead was given to produce a demonstration model of the SROIL terminal by summer 1998. To achieve ultimate system miniaturisation, highest transmit data rates and sufficient growth potential to also comply with extended link ranges, the SROIL terminal was designed using a laser-diode pumped Nd-YAG laser transmitter in conjunction with a coherent detection receiver. A two-axis pointing assembly in front of the 3.5 cm aperture telescope allows the SROIL terminal to achieve almost full hemispherical pointing (Fig. 9). The communication subsystem is designed as a BPSK (Binary Phase Shift Keying) homodyne system for a data rate of 1.5 Gbps. Due to the homodyne detection scheme the communication signal is recovered at baseband, which considerably simplifies the communications electronics design.
The terminal consists of two units, the optical head and the electronics unit. The optical head comprises the optics unit (Fig. 10) with beam forming optics, the laser unit, the actuators and sensors of the PAT subsystem, and some related electronics. The electronics unit comprises the PAT processor, the terminal controller, the communication electronics and the DC/DC converter.
Today, the problem of acceptance of optical free-space communication in the commercial payload market is not so much a technical one but rather the lack of its convincing in-orbit demonstration to the commercial satellite communications community. Consequently, various scenarios for a potential in-orbit demonstration are currently being studied as part of the SROIL contract.
Twenty years of technology endeavours, 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, the world’s first launch-ready civilian laser communication system. With this vast technological base at hand, European industry is well prepared to face the challenge of meeting the current demands for optical intersatellite links in the emerging multimedia, Global Information Infrastructure (GII) satcom market. The question that remains is primarily one of how space industry is able to adapt its practices and put the required resources into place. The shear number of terminals required, and the short times to market involved, will call for a paradigm shift in the way space products are manufactured. Old methods of space hardware design and qualification will have to be replaced by production-oriented, commercial manufacturing practices, with designed-in rather than tested-in quality and reliability. It is ESA’s hope that European industry will succeed in these endeavours and thereby be able to reap the fruits of the past twenty years of developmental effort.