OPTICAL COMMUNICATIONS AND INTERSATELLITE LINKS

Introduction

Space-based, free-space optical communications is a concept that has been around for many years. In the last few years, however, there has been impressive activity to bring the concept to fruition in civilian and government non-classified projects. Today's market for space-based optical communications is primarily intersatellite links (ISLs) which are the main focus of this chapter. There is also a place for high data rate (many Gbps) space-earth links, though propagation effects due to the atmosphere and weather make this a much more difficult link. Some activity in space-earth optical communications will also be covered here.

The usual parameters that system designers want to optimize drive the desire to utilize optical communications onboard a satellite: size, weight and power—and of course, cost. Under ideal assumptions about equivalent efficiency of signal power generation, detectors, and receiving surfaces, link equations show that optical communications systems with telescope aperture equivalent to that of the antenna of a radio frequency (rf) system could potentially provide tens of dBs of link efficiency improvement, e.g., data rate, margin, etc. This results strictly from the wavelength difference. These tens of dBs can be traded off against reduced optical aperture size, hence reduced size and weight, and the inefficiencies of optical signal generation/detection, and yet still support increased data rates relative to an rf system.

One significant factor in this trade-off is that the optical system will typically have a much narrower beamwidth than the rf system. This has both a positive and negative side. On the positive side, a narrower beamwidth means that the potential for interference to or from adjacent satellites will be reduced. This is particularly important in large LEO constellations. On the negative side, the requirements for more accurate pointing, acquisition and tracking (PAT) and the impact that this may have on the spacecraft could impose an unwelcome burden. Accurate PAT is critical to the acceptance of optical ISLs.

A secondary, though not unimportant, fact about optical communications is that, unlike the rf spectrum which is regulated by national and international agencies, the optical spectrum is currently unregulated.

Finally, reliability of optical communications systems, particularly their lasers, has been a concern in the past. This issue is being overcome by advances in optical and laser technology but needs documented space validation for wider acceptance.

Applications

Intersatellite communications is used primarily for "networking" a constellation of satellites at data rates up to many Gbps or for data relay purposes from tens of Mbps up to Gbps. These ISLs can be between all the various orbits that one might consider: low earth orbit (LEO), medium earth orbit (MEO), highly elliptical orbit (HEO), and geosynchronous earth orbit (GEO). There are currently systems like Iridium and NASA's Tracking and Data Relay Satellite System (TDRSS) that are using rf ISLs for these purposes. The ill-fated Japanese COMETS was to use rf ISLs. There are planned systems like ESA's ARTEMIS that will use rf and optical ISLs in the future. It is safe to say, however, that for many of the reasons outlined above, the future belongs to the optical ISL. This is evidenced by the fact that most, if not all, of the commercial satellite constellations now being announced, such as Teledesic, will be using optical ISLs. Iridium considered an optical ISL, but did not fly it primarily for business reasons, i.e., the risk perceived by investors.

Space-Earth links have been, and continue to be, primarily rf. Because of the advantages of optical systems related earlier, Japanese, European and U.S. researchers are investigating optical space-earth links from LEO as well as the far reaches of outer space. Optical links face a severe disadvantage due to the effects of the atmosphere and weather. Solutions include adaptive optics, spatial diversity, and onboard storage with burst transmission under good conditions. The first applications are likely to be in scientific satellites but as operational methodologies are developed, space-earth optical links will work their way into commercial systems.

As will be shown below, space-based optical communications development around the world has been primarily supported by government agencies. The European Space Agency, the Japanese government, and NASA and the DOD in the United States have been the main funding agencies. This is changing as the commercial satellite world integrates optical ISLs, and companies will be willing to form partnerships and invest more of their own independent research and development funds.

Japan

The Japanese have a strong program in optical communications. The Science and Technology Agency has designated the Communications Research Laboratory (CRL) of the Ministry of Posts and Telecommunications as a Center of Excellence for Optical Communications and Sensing. Thus the government has determined that optical communications and optical technologies, including sensing, are extremely important issues for Japan. As a Center of Excellence, the CRL has gathered researchers from around the world and devoted a lot of money for developments in this area. An overview of the types of links and systems being considered, from ISLs to space-earth links, is shown in Figure 3.9. A comment was made during the site visit to CRL that all ISLs of the future would be optical.


Fig. 3.9. Japanese Optical Communications System Plan (CRL).

It is a fairly broad-ranging program with increasing goals as shown in Figure 3.10 from CRL. Current plans call for investigation of multichannel medium bit rate (300 Mbps) systems using 0.8 µm wavelength technology while simultaneously developing high rate (1.2 Gbps) systems using 1.5 µm technology, which is more commonly available, due to terrestrial fiber systems development. In a ten year time frame, the plan is for operational 10 Gbps/channel systems.


Fig. 3.10. Performance targets.

The main players in the Japanese space-based optical communications world are NASDA and CRL from the government side and NEC on the industry side. NEC has been the main contractor on most of the payloads so far, although a number of companies-Toshiba and others-are involved in making the parts for these payloads.

Engineering Test Satellite VI (ETS-VI)

ETS-VI was intended to go into GEO. It did not achieve this, however, and lasted from 1994 to 1996, its lifespan a result of the effects of being in the wrong orbit. CRL and NASA's Jet Propulsion Laboratory (JPL) were able to do some space-earth experiments during the life of the spacecraft. It provided a bi-directional link at 1.024 Mbps using intensity modulation and direct detection (IM/DD). The spacecraft used a 7.5 cm diameter telescope. The downlink used a 0.83 µm, 13.8 mW AlGaAs laser diode. The uplink was at 0.51 µm using an argon laser from a 1.5 m telescope in Tokyo. The Laser Communications Experiment (LCE) is shown in Figure 3.11. Its mass was 22.4 kg and it consumed 90 W max.


Fig. 3.11. ETS-VI LCE.

Optical Inter-Orbit Communications Engineering Test Satellite (OICETS)

OICETS, which will be launched into LEO in 2000 carrying an optical terminal will be compatible with the European SILEX terminal and will communicate with the ESA ARTEMIS satellite in GEO. The Laser Utilizing Communications Experiment (LUCE) will have a 26 cm telescope with a 50 Mbps intensity modulated 0.847 µm, 200 mW laser diode link to ARTEMIS and a 2.048 Mbps direct detection link at 0.819 µm from ARTEMIS.

Japanese Engineering Module (JEM) on the International Space Station

An optical communications package will be constructed for the JEM. It will consist of a 1.5 cm aperture telescope and use 1.5 µm technology to provide 2.4 Gbps links from JEM to earth and to other satellites.

Europe

In Europe, ESA has been a primary driver in the development of optical communications although there have been a number of national efforts also. ESA is developing the ARTEMIS satellite (Figure 3.12) which is going to launch on a Japanese H-2 rocket in the year 2000. It will be used for data relay type applications from LEO satellites to GEO. One of the ISL capabilities will be optical. It will also have the capability of communicating to an earth terminal in the Canary Islands using the same ISL terminal.

The main players in Europe are ESA and the national governments, particularly the U.K., France, and Germany on the government side and Matra Marconi Space (U.K. and France) and Oerlikon-Contraves on the industry side.


Fig. 3.12. ARTEMIS.

Semiconductor Intersatellite Link Experiment (SILEX)

Both LEO and GEO SILEX terminals (Figure 3.13) built by Matra Marconi Space in France are complete and ready for integration into ARTEMIS and the SPOT-4 earth observing satellite. SPOT-4 was successfully launched in 1998. The SILEX terminal has a 25 cm aperture telescope with characteristics similar to those reported above for the OICETS terminal. The SILEX terminal has been tested and is performing in accordance with specifications. The maximum range is 45,000 km.


Fig. 3.13. SILEX.

Figure 3.14 shows a notional view of a number of optical ISL terminals that have been under development under either ESA or national funding in Europe. There is a plan in this vigorous development of optical technologies. ESA has looked at various applications-LEO to GEO, GEO to GEO-and has been developing a wide set of terminals to satisfy these needs and in a lot of different places.


Fig. 3.14. European optical terminals (from ESTEC).

Small Optical User Terminal (SOUT)

Matra Marconi Space (U.K.) developed SOUT which is compatible with SILEX, though in a much smaller package, for LEO-GEO applications (Figure 3.15). This is what they call the elegant breadboard (prototype) that was completed in 1995 but is not space-qualified. It has a 7 cm aperture and is capable of 2-10 Mbps using IM/DD and a 0.8 µm AlGaAs laser diode. The package mass is 25 kg and it consumes 40 W.


Fig. 3.15. Small optical user terminal (Matra Marconi).

Small Optical Telecommunications Terminal (SOTT)

SOTT is a GEO-GEO terminal capable of 1 Gbps. The terminal definition was completed in 1996 by Matra Marconi Space (U.K.) for ESA. It was based upon a 20 cm aperture 0.85 µm 2 W laser and used IM/DD. The package had a mass of 45 kg and required 100 W of power.

Solid State Laser Communications in Space (SOLACOS)

SOLACOS was a German government funded project at Dornier Satellitensysteme GmbH. It is somewhat different from the other terminals presented in that it uses a solid state laser and uses coherent reception. It was developed for GEO-GEO applications with a bit rate of 650 Mbps. It has a 15 cm aperture and uses a 1.604 µm 1 W pumped Nd:YAG laser. Coherent reception uses the "SyncBit" method. It is a relatively large unit at 70 kg. The terminal breadboard was completed in 1997.

Short Range Optical Intersatellite Link (SROIL)

The latest ESA development is the SROIL (Figure 3.16) under development at Oerlikon-Contraves. The initial version is designed for LEO constellation-type applications with ranges up to 6,000 km. It has a 4 cm aperture, is capable of up to 1.2 Gbps, uses BPSK with homodyne detection ("SyncBit"). This version has a mass of 15 kg and uses 40 W of power. Contraves advertises other versions of the SROIL that can be used even up to GEO-type ranges.


Fig. 3.16. Short range optical intersatellite link (Contraves).

United States

The United States has a long history in space-based optical communications development as evidenced by the Ball Aerospace chart (Figure 3.17) from Ball's web site. Until recently, the U.S. effort has been primarily directed towards military/government endeavors. Unlike the European and Japanese programs, much of the information about these systems has been classified or at least dated if available. Recently, with the realization that optical ISLs are an excellent business line, the U.S. companies involved have begun marketing their products more openly and aggressively, and in fact reworking them to fit the more aggressive cost targets of the commercial world.

Even within the military, sponsors like BMDO and SSDC have lately funded optical terminal development that has been available in the open literature to some extent. ThermoTrex developed an airborne terminal for SSDC. Astroterra is currently building a system for BMDO that is going to fly on the STRV-2 spacecraft that will launch in 1998 and is currently going through integration and testing (Figure 3.18). The terminal characteristics are shown in the figure.


Fig. 3.17. Ball Aerospace activities since the 1970s.


Fig. 3.18. Astrolink-1000 terminal.

Recently (fall '97), Ball Aerospace and COM DEV of Canada have announced a new joint venture, Laser Communications International, to compete in the optical ISL market. This merger capitalizes on Ball's long history in optical communications and COM DEV's commercial space experience with the Iridium rf ISLs. The joint venture has chosen a 1.55 µm terminal based upon the investment already being made in terrestrial fiber-based systems at this wavelength and after a study by Ball, SDL, Lucent and USAF Phillips Laboratory showed that this fiber-based technology was space qualifiable. A prototype 1.55 µm terminal is shown in Figure 3.19. On-Off Keying modulation is the selected method though DPSK is also still considered as a possibility.


Fig. 3.19. Prototype ISL terminal from Laser Communications International.

Raytheon, a company with a long history in optical sensing systems, has also recently begun development of optical communications terminals for the ISL market. Its terminal is based upon a proprietary liquid crystal optical phased array (Figure 3.20) for beam steering. For most of the same reasons as above, Raytheon has chosen 1.55 µm for its terminal with a data rate greater than 1 Gbps. It has also chosen intensity modulation with direct detection because of its simplicity.

The MIT Lincoln Laboratory and NASA's JPL have programs for U.S. military and NASA optical communications needs respectively. Lincoln has long been developing 1 Gbps (and faster) communications terminals. Figure 3.21 is an example of a 1 Gbps DPSK testbed that has been a benchmark by which other developments have been measured. It is based upon a 1.55 µm wavelength and erbium-doped fiber amplifier technology. Lincoln also has developed a convolutional encoder and a decoder operating at these high bit rates for the free-space optical channel. Lincoln Laboratory will be responsible for the laser communications package to be carried on the National Reconnaissance Office's Geosynchronous Lightweight Technology Experiment (GeoLite) satellite. This package will be used to test space-earth optical communications links-particularly to assess atmospheric effects. TRW will be responsible for the satellite integration. Few technical details are publically available concerning the capabilities of this package. At JPL, the emphasis has been on space-earth communications at planetary distances which usually support only hundreds of kbps or a few Mbps, but the 10 cm aperture Optical Communications Demonstrator (OCD) shown in Figure 3.22 is capable of up to 250 Mbps and is being upgraded to 1 Gbps capability for near-earth experiments. It is based upon 0.86 µm technology and uses on-off keying at the higher bit rates. A version of the OCD being developed for outer planet missions will use pulse position modulation.


Fig. 3.20. Raytheon optical phased array.


Fig. 3.21. Lincoln Laboratory 1 Gbps testbed system.


Fig. 3.22. NASA/JPL Optical Communications Demonstrator.

In addition to those mentioned above, other potential suppliers of optical systems in the United States are Hughes, Boeing, TRW, and Lockheed Martin.

Trends

In general, smaller, better, and faster characterize the next generation systems. The single most identifiable trend is towards speed. This has been a dominating factor in keeping pace with terrestrial fiber systems. Ten Gbps systems will appear within the next few years. Higher power lasers and higher speed laser switching are aiding in achieving this, along with high speed electronics (ASIC and MMIC). There does not appear to be universal agreement concerning wavelength. A lot of the earlier work was done at 0.8 µm but there are now terminals at 1.06 µm. High-volume development associated with terrestrial fiber systems make components like Erbium-doped fiber amplifiers attractive for space-based optical communications, so many of the recent systems are focusing on the 1.55 µm range. Regarding smaller terminals, there is a coalescence of elements in the terminal, making use of the same detectors and a lot of the same electronics for doing multiple functions. Similarly, lighter components will be developed with new materials that will make these systems lighter in general.

Conclusion

In conclusion, Japan and Europe have had very vigorous and open development in optical communications terminals and systems. The U.S. providers have been somewhat hampered by previously classified programs but this is rapidly changing and many U.S. companies are competing with the European and Japanese companies for the growing ISL market. It should be clear to everyone that optical ISLs are coming. When? It should be soon since it is an important application. The first time a Teledesic or some other company deploys an optical ISL in a commercial system may well open the floodgate. Once these systems are in orbit and functioning many others will follow. Space to earth is a little trickier because of the atmospheric effects, and the fact that adaptive optics need to be developed, but there will be commercial applications of high data rate space-earth optical links in the near future.


Published: December 1998; WTEC Hyper-Librarian