critical technology development and test program is presented. A ground test involving transmitting and receiving antennas is recommended to obtain data on beam controllability and radio frequency interference, which will provide design confidence for orbital tests. The planned orbital test program demonstrates operation of the open cathode dc to rf converter, satisfactory vacuum high voltage plasma interaction, orbital assembly techniques, and operations and maintenance development. The orbital test program for the microwave power transmission technologies assumes availability of the Shuttle transportation system and a power source presumed to be a part of its own orbital test program. 1. INTRODUCTION Microwaves can traverse the atmosphere with low attenuation, and advances in microwave power technology have been considerable since the first demonstration of appreciable power transfer by Brown (1963). The combination of a solar photovoltaic power source in geosynchronous orbit with microwave transmission to earth was first proposed by Glaser (1968). This Satellite Solar Power Station (SSPS) concept received increasing attention (J. Microwave Power, 1970; Brown, 1973) and led to a feasibility study conducted by a team consisting of Arthur D. Little, Inc., Grumman Aerospace Corp., Raytheon Co., and Textron Inc. under NASA sponsorship (Glaser, 1974). Results were sufficiently promising to warrant support of more detailed studies in the technologies involved. The study concentrated on the microwave power transmission system (MPTS) for transmitting energy from space to earth, and as such the results are independent of the power source selected. For examples, a solar thermal converter or a nuclear reactor in orbit could be considered in place of a solar photovolataic source. Nevertheless, the solar photovoltaic source remains the best known and studied of alternatives, and so was used for purposes of illustration where required. The study involved preliminary analysis, conceptual design, technical and economic evaluation, and planning for a technology development, a ground demonstration and an orbital test program. The concept of space to earth microwave power transmission is illustrated in Figure 1. A transmitting antenna in geosynchronous orbit beams microwave power to a ground antenna where it is rectified to de power. Functional blocks of such a power transmission system are shown in Figure 2. Efficiency is a prime consideration in any transmission system, and it is evident that elements must average over 90% if overall efficiency is to be a modest 60%. These efficiency considerations dictate that the antennas be extremely large scale, e.g., the transmitting antenna is on the order of 1 km in diameter and the receiving antenna is on the order of 10 km, because of the long transmission distance of 37000 km. This scale implies that large units of power, on the order of 5 GW-10 GW, must be transferred and that the power source in turn must be very large scale.
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