amplifiers and the antenna waveguides for forming the beam. The beam is controlled in the reference system by a retro-directive pilot beam system. In travelling from geosynchronous orbit to the Earth, the microwave power beam expands in area from a circle 1 kilometer in diameter to an ellipse that is 10x13 kilometers at the 35° reference latitude. It is now a low power density microwave beam with a peak intensity of 23 milliwatts per square centimeter in the center and 1 milliwatt per square centimeter at the edge of the rectenna. The rectenna is made of a series of panels which contain an open screen ground plane with half-wave dipole antennas and diode rectifiers. DC current is collected and delivered to the peripheral rectenna where it is processed for distribution to the electrical network. Artist illustrations of two versions of the reference system are shown on the next viewgraph (Figure 6). One version of the reference system uses silicon solar cells in a planar array without concentration of the solar energy. A silicon reference was included in the study because of the vast amount of experience available now and expected with silicon technology in the future. The gallium aluminum arsenide solar cell version was included as a reference system because the gallium aluminum arsenide solar cells have several advanced features which make them attractive for use in the satellite power system. The next chart (Figure 7) shows the comparison of the two reference systems in terms of their mass in millions of kilograms. We see that the total mass for the gallium aluminum arsenide array option is in the range of 34 million kilograms while that for the silicon option is in the range of 51 million kilograms. If transportation costs were the determining factor, obviously the gallium aluminum arsenide version would be chosen because of its much lower mass; however, the gallium aluminum arsenide solar cells are likely to be considerably more expensive than the silicon, at least in terms of current projections. Also there are some greater degrees of uncertainty on the achievement of performance desired in one solar cell option versus the other. In addition to the definition of the reference system, which will be covered in much greater detail in other sessions of this Symposium, the NASA activities in the Concept Development and Evaluation Program included studies in many of the critical areas of SPS and associated areas of critical supporting investigations. The next viewgraph (Figure 8) shows a table of funding of the NASA activities during FY 1977-1980, the years of the Concept Development and Evaluation Program, and shows how the funds were expended in the major areas of endeavor. As can be seen, approximately $7.9 million were expended for systems activities during the Concept Development and Evaluation Program. Of this total, $2.2 million was spent on systems definition activities which are those functions and studies which integrate all the other work into comprehensive system concepts. You can note that most of the effort was accomplished during the first two years in systems definition for the preparation of the reference system, as I have just discussed. The footnote also indicates that we have been able to put some small amount of money into additional studies of laser energy transmission as an option for the satellite power systems concept. Some results of the laser system studies will also be given later in this Symposium. Another major area of emphasis in the systems activity has been in the area of
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