This section addresses technologies or concepts that could be major departures or advances beyond today's satellite technology. Today's conventional definition of a communications satellite is a three-axis body-stabilized satellite that is positioned in GEO orbit and has a number of antenna beams designed to concentrate RF power and to re-use available frequencies. That definition is expanding to cover LEO satellites in global constellations, LEO store-and-forward satellites, higher powered DBS and mobile satellites, and satellites with ISLs. Increasingly, communications satellites are moving to higher and higher frequencies despite advances in frequency re-use. These are essentially evolutionary developments driven as much by new applications as new technology. The first question is thus whether totally new architectures or experimental concepts will evolve over the next 10 to 20 years which could redefine our understanding of the communications satellite. The second question is whether these new ideas are currently being developed, and significant progress being made, in the United States, Europe or Japan.
Possible "break-through" technologies or concepts have been discussed in popular, trade, and scientific journals around the world in the last few years. These ideas can be categorized as follows:
Use of ground-based power transmitters to help stabilize extremely low mass (i.e., 20 to 25 kg) space communications platforms in LEOs. These high capacity, low mass satellites would be artificially maintained in low-earth geostationary orbit.
All of these concepts -- which imply major departures from today's basic paradigm of what a satellite is, how it operates, and how much it costs to provide services -- could well be critical to the longer term viability of satellite communications and the mix of telecommunications services they provide. The advent of new terrestrial wireless technologies and fiber optic cable could indeed require a totally new approach for the 21st century.
The principal observation with regard to Europe is that limited work is being undertaken in this area of "radical new departures." It is significant to note that, to the extent such work is in fact being done in Europe, it is for the most part in different areas than in the United States.
The idea of a space station for telecommunications, an orbital antenna farm, is at least 20 years old. The idea is to deploy one or more large antenna reflectors that can be used with a multi-beam feed system to create a large number of high powered "pencil" beams and a significant amount of frequency re-use, perhaps up to 50 or even 100 times. Depending upon the frequency and power employed, such a space station might be used for fixed satellite services (FSS), television or radio direct broadcast services (DBS) or mobile or radio determination services. This technology might also be employed for new applications -- such as high definition television broadcasting, wristwatch transceivers (i.e., the "Dick Tracy" radio wristwatch), video on-demand, etc. This approach has not received a great deal of study by NASA and U.S. aerospace interests in recent years despite a great deal of investigation in the 1970s and 1980s. High costs, technical complexity, and other problems (e.g., thermal expansion with large structures, lifting large masses into GEO, and transmission delay from GEO) have tended to reduce U.S. interest is such projects.
ESA, however, has decided to explore the possibility of large scale antenna reflectors that might be deployed at low cost. If this critical objective were achieved then an entire space station concept for telecommunications might become feasible particularly for DBS or mobile applications which cannot be easily duplicated by terrestrial fiber systems. In particular, the feasibility of large scale inflatable antenna structures with aperture sizes of 12 to 18 m has been investigated and developments undertaken with ESA funding at ESTEC under the ASTP Research Program and in industry. Actual work on this inflatable antenna project was carried out by the European firm Contraves. Alenia Spazio has developed and qualified, under the ASTP program, a large solid reflector (4 m diameter) with foldable tips for space use. An alternative design for a large unfurlable mesh antenna proposed by MBB was not pursued when MBB withdrew its concept from consideration. Apparently a folding deployable large-scale antenna is also under active study. The most mature development of the inflatable antenna is with respect to the ARTEMIS Program which is scheduled for 1995 launch. It would seem that overall efforts in the area of large reflectors are well advanced in Europe and may well be ahead of U.S. efforts.
Dr. Rene Collette, Director of Telecommunications for ESA, noted that good progress was being made in developing large scale reflectors that could be designed, manufactured and deployed at relatively low cost. He indicated that proposals had been put forward for consideration of a joint development program with the Russian space agency to experiment with very large reflectors of more conventional design that could be launched by the Energia launch vehicle. He also suggested that this might be one of three potential cooperative projects between NASA and ESA in space communications research.
Another area of very long range development effort is mm wave applications. The need to move to higher frequencies where broad radio spectra can be used for advanced broadband applications seems quite obvious. Applications such as 3-D HDTV, video-on-demand, virtual reality imaging, etc. will, even with digital compression techniques, require a thousand times the bandwidth of fax, medium speed data or telephone services. Precipitation attenuation, the lack of SSPAs for operation above 20 GHz, and the lack of an active commercial demand for satellite communications in the mm wave bands has caused development efforts in this range to lag.
ESA has, however, begun development in frequencies above 30/20 GHz to look beyond the OLYMPUS and ARTEMIS projects into the true mm wave band. Several other independent efforts have been undertaken in Germany and Italy, suggesting that serious interest in these higher bands is currently being pursued in specific funded research programs.
In Germany, the German Aerospace Research Establishment (DLR) is pursuing the PROMETHEUS project which is examining short distance vehicle mobile communications. This is part of the EUREKA industrial research program for Europe and focuses on terrestrial radio communications at 60 GHz, but a satellite component is also being examined. Funding from RACE and RACE II may also be forthcoming to support this effort. Since precipitation attenuation can be very severe at 60 GHz, extensive data on these effects at different elevation angles associated with LEO satellites has been collected at look angles ranging from a low of 13 degrees and a high of 43 degrees. These have allowed the development of computerized propagation models over this range of conditions.
In Italy, the ITALSAT satellites will allow propagation experiments at 50/40 GHz. The first package was launched on the ITALSAT F1 satellite in January 1991. A similar package was originally planned to be launched on ITALSAT F2 in 1994, but was later replaced by the EMS payload sponsored by ESA. Alenia Spazio is currently manufacturing 50/40 GHz propagation measurement terminals for approximately eight sites in Europe. Further, Alenia has developed a 44/20 GHz space antenna using a flat dichroic reflector under military funding. Alenia's work covers space antenna development up to 90 GHz as well as large phase arrays up to 10 m in diameter.