William T. Brandon
Vincent W.S. Chan
Robert K. Kwan
The Japanese government has made a major commitment in the development of intersatellite link technology. Most of the effort is concentrated on optical ISLs. It is felt that ISLs will enable the interconnection of space assets for readout of low earth orbit satellites and provide networking of geosynchronous earth orbit satellites. This will allow the Japanese space community's vision of a global space communication network supporting television broadcasting, mobile communications and computer connections.
There are two components to the optical ISL research and development in Japan. The first component consists of the flight demonstration of optical ISLs; the second, general research and development of optical technology to improve future optical ISL performance. The flight demonstration component includes the laser communication package called the Laser Communication Experiment (LCE) on Engineering Test Satellite VI (ETS-VI), and a contemplated flight demonstration called OICETS linking a LEO satellite with the European experimental communication satellite ARTEMIS. Meanwhile, various government laboratories and commercial companies are doing R&D on higher data rate systems and better performing spatial subsystems.
LCE is a modest space demonstration of optical ISL technology conducted by the Communications Research Laboratory (CRL) of the Ministry of Posts and Telecommunications (MPT) of the Japanese government. The flight demonstration is a `modest' effort because the data rate supported (1 Mbits/sec) is low, mostly due to the use of a small optical antenna and non-diffraction-limited optics on the spacecraft. Figure 6.1 shows the operations concept of LCE on ETS-VI. The downlink uses GaAlAs semiconductor lasers at 0.83 microns and the uplink uses an argon laser at 0.51 microns. Currently, the planned experiment is between ETS-VI and a ground station at CRL in Tokyo. Figure 6.2 shows a block diagram of the LCE and Table 6.1 shows the major specifications ofthe design. The package weighs 21.8 kg and consumes 81 W when operational. The 7.5 cm telescope is fixed with the optical beam being pointed by a steering flat mirror. The telescope is not diffraction-limited, with a beam divergence of 30/60 microradians. This design feature substantially reduces the risk of optical/thermal/mechanical engineering as well as the design/fabrication difficulty of the acquisition, tracking and pointing system at the expense of substantially reduced data rate handling capability. It will support 1 Mbits/sec from synchronous orbit to ground with a ground telescope diameter of 1.5 m and a 14 mW transmitter. The spatial acquisition and tracking system is conventional. The space package acquires the ground beacon spatially using a charge-coupled device (CCD) with a field of view of 8 milliradians. Fine tracking is done using a four-quadrant avalanche photo detector (APD) and two single-axis fine-steering mirrors. The transmitter laser of 14 Mw is Manchester coded and the receiver uses direct detection with an APD. The package has been integrated onto the ETS-VI spacecraft. A booster delay has caused the launch date to slip from l993 to l994. The flight package will demonstrate laser communications in space, but its performance characteristics are not good enough to demonstrate an unambiguous advantage over RF systems.
The European SILEX (Semiconductor Laser Intersatellite Link Experiment) system is of considerably higher performance than LCE on ETS-VI. It will support around 50 Mbits/sec using a 25 cm telescope on both ends. OICETS is a Japanese initiative with a LEO to GEO optical ISL compatible with the SILEX design so it will talk to ARTEMIS. Figure 6.3 shows the operations concept of the OICETS package. As indicated, a later connection with Communications and Broadcast Engineering and Test Satellite (COMETS) is also being considered. Figure 6.4 shows a candidate configuration of OICETS. As indicated in Figure 6.5, the entire optical train will be gimballed for optical design simplicity. A block diagram of a preliminary optical design is given in Figure 6.6. Again, the design is quot;conventional" with, except for minor variations, features that are almost mirror images of the SILEX design. The performance characteristics are given in Table 6.2. The telescope will be 30 cm and will be close to diffraction-limited, a more difficult optical design. The spatial acquisition and tracking systems are upgraded from the LCE design to accommodate the higher accuracies required. The system will support approximately 60 Mbits/sec with a 50 to 100 mW transmitter laser. The mass of 100 kg and power consumption of 140 W is comparable to that of SILEX. The program is currently at the breadboard stage. The decision to build the flight package will be made in l993. The anticipated launch date is in 1997.
Figure 6.1. LCE Operations Concept
Figure 6.2. LCE System Block Diagram
Major Specifications of LCE
Figure 6.3. Operations Concept of OICETS and ARTEMIS
Figure 6.4. Candidate Configurations of OICETS
Figure 6.5. OICETS Candidate Package Layout
Figure 6.6. Block Diagram of OICETS Package Preliminary Optical Design
Performance Characteristics of OICETS
The two-flight program will allow Japan to play a important role in the field of optical ISLs. However, the characteristics of these designs are modest and will not fully realize the advantage of optical over RF systems. Just like the ESA program, there are R&D activities in both Japanese government laboratories and commercial companies to improve optical ISL technologies. Foremost is the increase in data rate while keeping telescope size constant by using higher power transmitters and more sensitive detection schemes such as coherent detection. At ATR, a semiconductor laser amplifier is used to boost transmitter power to 1 W and coherent detection of phase-shift-keyed signals and polarization-modulated signals are being explored. Figure 6.7 shows the achieved characteristics of a 2.5 Gbits/sec transmitter breadboard. The output beam is diffraction-limited, a necessary characteristic for increasing the energy density at the receiving satellite. In addition to higher-rate transmitter and receiver development, there is substantial interest in better-performing spatial tracking and pointing systems. These improved subsystems will be necessary to point very narrow beams (of a few microradians) in a very high data rate system (>Gbits/sec).
The Japanese have advanced state-of-the-art device technologies that are as good as or better than those available to the European and U.S. communities. The panel did not hear enough about their system engineering to accurately assess its state of development. Although the panel did not see any radically new system designs, from the high-level description of the flight systems characteristics, one would draw the conclusion that their system engineering is adequate. The panel saw only a little hardware that is of space design. From what was seen, it is clear that both mass and performance are being traded off to arrive at a low-risk design. This is a wise and pragmatic approach.
The Japanese are serious about optical ISL and have put together a broad long-term development program. The simple LCE package may be the first working laser communication system in space. The follow-on OICETS package will place them on a par with the Europeans. The Japanese definitely have the advantage of better component technology, especially for high rate systems. It is hard to assess their space hardware fabrication capability, but such things will develop with experience.
Figure 6.7. Gbits/sec Breadboard