The European space community has made a major commitment to the development of space crosslink technology and systems. The majority of effort is being spent on space optical communications. There is some work on 60 GHz technology but the investment is modest.
There are two components to the research on and development of space optical communications coordinated by ESA through ESTEC. The first component consists of the development of a flight terminal to be launched on SPOT-4 in 1996 and a similar package later on ARTEMIS. Figure 5.7 shows the operational profile of the system. The total planned cost of the SILEX program is close to $220 million. (An overview of the program is contained in the site report on Matra-Toulouse.) An evaluation of the status, maturity, and realities of the program is given in this section.
Figure 5.7. SILEX Operation Concept
The second component of the research and development program is advanced research to improve future terminal characteristics. A summary of this second component will also be given here.
The engineers encountered during the NASA/NSF panel's visit showed a good understanding of the technical problems of optical communications in space. They seemed to be more aware of publications and works of fellow researchers in Europe, U.S. and Japan than their counterparts in U.S. industry. The quality of the engineers also seemed to be more uniform across the many disciplines required to successfully complete such a project.
The SILEX industrial teaming arrangement differs in significant ways from any laser communications flight program run by U.S. industry. There are many subcontracts (20) and just about all the serious players participate. In the U.S., each company tends to have its own design and the bulk of the work is retained in-house. While having 20 subcontractors is scary, there is also improved chance of using the best available technologies. Keeping most of the work in-house is efficient administratively but most likely will result in compromises in technology choices and system design. Only time will tell which is better.
The design, development, fabrication and testing of a space laser communication system can be divided into the following major areas:
These areas are intimately related and an overly aggressive approach in one area usually puts much stress on the others. To that extent, the first and the last items, architecture and system engineering, are the most crucial ones.
The most significant architectural assumptions are the use of semiconductor lasers and direct detection receivers. This quickly limits the architectural scope, which is not necessarily bad. This choice of laser and receiver has the effect of limiting the maximum rate of the one-times-sync distance crosslink to about 50 Mbits/sec while using a reasonable size telescope (25 cm diameter). For the application on SPOT this data rate should be adequate. Scalability to higher rates is questionable; hence, in the advanced research program there are moves towards the possibility of heterodyne detection and higher power sources such as Nd:YAG and power semiconductor amplifiers. In order to concentrate the modest transmit power (50 mW), a diffraction-limited transmitter telescope is used. This is normal practice but it automatically implies that the spatial acquisition and tracking system must be very well designed. In general the architecture is rather regular with no significant innovations. The main characteristics of the system are given in Figure 5.8.
The prime choice for the transmitter laser is a 50 mW Spectra Diode Laboratory (U.S. company) laser which has been available commercially for a while. In SILEX, the laser is required to operate in a single spatial mode, but there are only loose requirements for spectral purity. This laser has been well characterized. Most lasers purchased off the shelf are single mode, but space qualification is yet to be completed. The most important concern is the stability of the modes under pulse operation and when the laser ages. The risk of not meeting the goals is modest. A Japanese laser is the back-up.
A space design laser source package was built with the requisite hardware for collimation. No data on the performance of this very important module are available. The beam quality over temperature, under mechanical disturbance and as the laser ages, is unknown. This is an area where most space laser communications programs have had troubles in the past. Until the final design is built and tested, this will remain to be an area of significant risk.
An EGG silicon avalanche photo detector (APD) is used for the direct detection receiver. This is by far the best APD available on the world market. Detection sensitivity on the order of 100 received photons per bit can be achieved. However, there is a concern about this device, which is hermetically sealed in a nitrogen environment. The leakage of nitrogen in vacuum may present conditions favorable for corona discharge to damage the device. There was no indication of whether or not this tricky packaging problem has been solved. A breadboard of the receiver module has been built and arriving at a space design will be routine. There is not the same type of risk as in the transmitter, since it does not have to be diffraction-limited and the f-number of the optics is large.
Figure 5.9 shows the acquisition and tracking design of the system, as well as their relations to the transmit and receive paths. Parallel search spatial acquisition is done by using a Thomson CSF 288 x 384 silicon charge-coupled device (CCD). Pixel size is about 30 microradians. The total uncertainty area is 8 milliradians, and the 8 W transmitter beacon is spread over that width. Judging from the fact that a 8 W beacon is used, the efficiency of the CCD must not be very high. Arraying 19half-watt lasers (incoherently) to get 8 W is rather inelegant and adds considerable mass. U.S. technology that resides in research laboratories can perform the same functions with 50 mW of laser power.
Figure 5.8. SILEX System Characteristics
Figure 5.9. SILEX Acquisition, Trading, Transit and Receive Paths
Another CCD is used for the tracking sensor. It is a 14 x 14 Thomson CSF device that can be read out at rates as high as 8 kHz. At these high read-out rates, read-out noise is usually the limiting factor, contributing significantly to the tracking error budget of 0.2 microradians. The system can only track to within 1 degree of the sun. This will impose some operational constraints on the system. U.S. systems tend to use devices other than CCDs for tracking and generally achieve better performance.
The fine-steering mirror, a key component used to take out small but fast spacecraft mechanical disturbances, is made by TELDIX of Germany. It can move so as to meet a 300 Hz bandwidth, which is adequate but not robust. If the platform dynamics are quiet, it should do the job. In the U.S., 500 Hz mirrors are available and a 2 kHz mirror has been space-qualified in the laboratory. If the SPOT satellite is mechanically noisy, then the turnover frequency of only 300 Hz could be a concern. Even if a higher speed mirror were available, it is not clear that system performance could be any better because the CCD sensor is noisy and noise may be limiting sensor bandwidth.
There was very little exposure to the area of optical engineering during the visit. Judging from the fact that the SPOT satellite is operational and, from the bits and pieces of information inferred during the course of interchange, there is confidence that there is space optics expertise. The mass of the package is a rather aggressive one for bulk optics design. Achieving the mass goals may prove to be difficult and painful. Also, the collimation subsystem of the laser diodes has some very fast optics that have to perform at the diffraction limit over a wide range of environment. There are not enough data to believe this tough problem is totally solved. There is no indication of innovations that can surpass the best of U.S. space optics engineering.
The only completed end-to-end design is the breadboard. An artist's concept of SPOT-4 with SILEX is shown in Figure 5.10. Figure 5.11 shows the layout concept of the entire terminal on SPOT. Although there are subassemblies built to space design, the final space design was not presented. Until that design is available, it will be hard to assess the maturity of European technology and system design know-how. They seemed to be articulate about the use of finite element models and thermal models. However, there is no evidence of the combination of mechanical, thermal and optical modelling as an integrated design tool. The absence of such a well-developed tool has been the Achilles heel of many U.S. programs.
The optical bench of the SILEX terminal will be designed to have its first resonance above 130 Hz, so that the micro-disturbance of the spacecraft will to a large extent not excite any mechanical mode that would result in optical misalignment. The bulk optics design tends to be heavy, and the allocated mass is aggressive and may cause schedule slip and/or cost over-run. Meeting mass goals also has been a difficulty in U.S. industry. This usually occurs when modifications are made after the fact to fix some unforeseen mechanical and/or thermal problem.
For the precision optical assemblies, active thermal control is achieved by utilizing radiative cooling and active heating. Short term stability of 1 degree and long term stability of 2 degrees is achieved. This design is the lowest risk approach, albeit at the expense of heater power. Meeting the published power budget can be difficult.
A sound mechanical/thermal/optical design is a very difficult task for space laser communications. The SILEX program still has much to do in this area. It is an area in which U.S. industry has competence but still has had difficulty with in the past.
Figure 5.10. Artist's Concept of SPOT-4 with SILEX
Figure 5.11. SILEX Terminal Layout
System engineering is a discipline that touches all the areas mentioned above. A good system design is a balanced one with no significant uncertainty in any area. Nor will it let an aggressive assumption in one aspect of the design overly stress the requirements of another. To a great extent it is an art based on experience and analytic capabilities. The panel met European engineers who seemed to have the sophistication and awareness of all the right techniques (more so than engineers at some U.S. companies in this field). It is too early to tell whether the SILEX design is sound. It will be two years before space hardware is tested. A SILEX general schedule is given in Figure 5.12.
Finally, it should be noted that the SILEX mass and power goals are rather aggressive for the type of design. The program has just demonstrated a breadboard and is currently designing the space terminal. It will take an excellent effort to meet these goals and deliver the flight package to SPOT in two years at cost. The presence of many participants will further complicate the problem. If they can do it, the Europeans will definitely be ahead of U.S. industry, and the United States may lose its competitive edge, if it has any now.
In addition to the SILEX program, ESA is investing approximately 20 million U.S. dollars in advanced engineering development to improve the system characteristics of the laser communications terminal. The goals are:
ESA is looking at coherent systems to improve receiver sensitivity and higher power sources, such as crystalline lasers and semiconductor power amplifiers. There is also a move to develop a multiple access terminal to accommodate several simultaneous users. This effort tends to be coupled with internal research of companies and also has university participants.
In summary, the European space community has made a major investment in optical space communications. The SILEX program has the participation of many qualified European companies as well as a U.S. laser company. Technically, the program seems to be on sound footing, but the performance goals, schedule and cost targets could be a little too aggressive. If this program is successful, and time will tell, U.S. industry will have an established, strong competitor. The advanced technology program, although small by comparison to the flight program, is a sizable investment for the future.
Figure 5.12. SILEX General Schedule