while the Reference System was designed as a future national electric power system of the industrialized country. To solve the problem of extremely high cost of space systems which is considered as the main obstacle against this purpose, the following points have been emphasized for the conceptual design. 1. To simplify the orbital segment, the solar array and antenna are fixed on the same structure to be stabilized by gravity gradient force. 2. To make the concept realistic, only existing technologies, including those yet to be qualified, will be used, although renewal of technologies will be considered positively. 3. To reduce the cost for transportation to space which is the dominant portion of the total project cost, the equatorial low earth orbit has been chosen rather than the geosynchronous orbit. As a result, a concept of a model designated as SPS 2000 is being developed. SPS 2000 is modularized by flight units to be carried by a commercial launch vehicle. Every subsystem is divided into equal portions and installed evenly in each module of flight units. Thus, each unit can be operated and tested as a small SPS, although the performance is not satisfactory for customer service. SPS 2000 will transmit the microwave power to rectennas located along the equator separated from each other by a distance of 1200 km for about three minutes in every orbit path. Unique features of an SPS project indicated by this conceptual study will be summarized in comparison with the Reference System, as follows: 1. SPS 2000 is an evolutionary system which can be started from the size of a payload of a launch vehicle. It can be a first milestone even for the Reference System 2. SPS 2000 is substantially international system. International aspects of SPS will be discussed more seriously for SPS 2000 than for the Reference System. 3. SPS 2000 can serve exclusively the equatorial zone, especially benefiting geographically isolated lands. This will be a new aspect of societal issues which was not discussed in the CDEP. These features suggest that the potential customers as well as electric utilities will play an important role in the next stage of the study. Their participation in the further study on the aspects of technology options, ground segment design and possibility of future evolution will be an essential step for SPS to be evaluated correctly without prejudice.
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