hertz frequency the transmitter and ionospheric thermal limits set 5 megawatts as the maximum power that could be delivered in a single beam; however, lower powers per beams are possible. It was assumed that obtaining the maximum power level for a given size of transmitter system would result in the most economical power plant. Though we have the example of the Goldstone test as one experiment related to the satellite power system, it is significant to realize that nearly all of the efforts on SPS have been studies based on assumptions and projections of where the technology may be in the future, and ought to be in the future, in order to achieve a satisfactory power system. Supporting the SPS studies, however, are broadly based NASA and DOD space research and technology programs covering nearly all the technical disciplines important to SPS. Studies conducted outside the SPS program showed that laser power transmission could possibly be feasible in space missions and for delivering power to Earth. An output of one of these early studies of laser power transmission is illustrated in this picture (Figure 3) which shows a system where the power plant is in low Earth orbit and laser relays are placed in geosynchronous orbit in order to achieve delivery of power to a single spot on Earth. With this background I believe you can see then that SPS has had a history of studies and relies heavily on R&T activities being conducted outside the SPS program, that are supportive of the Concept Development and Evaluation Program, which I will now discuss. In the systems definition effort, which has been the responsibility of NASA, the objective has been to define a baseline system concept or concepts to evaluate technical feasibility and to provide information required for environmental and socio-economic assessments. As time has gone on in the Concept Development and Evaluation Program, the initial objectives in systems definition were expanded to include studies of alternative approaches and to conduct critical supporting investigations where needed and as may be possible. One of the first requirements of the Concept Development and Evaluation Program was to establish a reference Satellite Power Systems concept for use in conducting evaluations of environmental impact, societal concerns, and comparative assessments. In order to establish a reference system, common guidelines (Figure 4) were established based upon the judgment derived from the earlier studies which I have just described. Many people can and have quarreled with these guidelines; however, it was necessary to establish some basis for conducting the studies. As further work is done on SPS, it will be necessary to examine the impact of changing these guidelines to determine whether more optimal or more favorable versions of SPS might be derived from different starting assumptions. From these guidelines and the studies which have been conducted over the years, the reference satellite power system concept was established. The general concept is outlined on this viewgraph (Figure 5). It shows a large flat structure (5 Km x 10 Km) holding solar arrays for generating DC power from the sun. The DC power is collected and delivered by means of a rotary joint to a transmitting antenna system. The transmitting antenna was set at 1 kilometer in diameter and it is formed of transmitting antenna subarrays which contain the DC-RF power
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