system consisting of 60 satellites would cost $600 billion to $1.2 trillion in 1977 dollars, or $2,000 to $4,000 per kW of capacity. This is far more than any other currently economical technology. Committing such enormous resources to an unproven technology would not be prudent. There is not yet any reliable estimate of STS cost to compare this with. The factors discussed above make it seem reasonable that STS cost would be less than SPS cost. There are some factors which will adversely affect STS cost as discussed below. Transmission of hydroelectric power via STS requires two power conversion steps not normally required in hydroelectric power utilization - conversion of generator output to microwaves and reconversion of these microwaves to alternating current for distribution. Since STS is unproven technology, there is some risk that it simply will not work - a risk that will increase the cost of financing. This risk can be minimized by a long-term systematic approach to the project. Cost and risk factors clearly justify exploring the potential of STS further. Development of STS Obviously a study and research programme devoted to STS development would be required if an STS were to be constructed. I have given an outline of the motivation for and the implementation of such a programme. An STS development programme would require cooperative work between many different specialties, and would require a very disciplined approach to the problem. The first thing any programme must address is feasibility analysis. Since the only novel part of the system is the microwave link, preliminary design studies of this would be the first order of business. The sizes of the transmitting and receiving antennae and the reflecting surface must be traded off to find the optimum design configuration. To first approximation, we can assume that the transmitting station antenna should be similar in size to that on an SPS, or about 1 km in diameter. A very tight transmitting beam is desirable to minimize required reflector area in geostationary orbit. By analogy with SPS, the receiving area can be expected to be about 10 by 13 km (at 35° latitude) - though a reduction in this area is desirable. One way to reduce this area is to use a higher frequency. We must note that a frequency other than 2450 Mhz will interfere with a variety of microwave communication and tracking systems, and that weather effects become more intense as the frequency increases. One must note that if highly efficient and powerful lasers are developed for military use they would be ideal for STS power transmission. Use of lasers would lead to much smaller reflectors and receivers, as well as eliminating any interference with communications. Of course, weather interferes more strongly with lasers than with microwaves, and substantial work on this problem would be required. The modular reflectors would be the most unusual part of the system. While the absence of weight simplifies control problems, the extreme aiming accuracy required would present significant challenges. Presumably the modules would be independently aimed by feedback from the ground. Some other examples of areas requiring expert evaluation are environmental impact and potential hazards of reflector misalignment; beam dispersion due to transmission up through the atmosphere; optimal frequency selection to minimize interference while maximizing systems economy; the dangers posed by the high power density uplinks to birds and aircraft; and innovative techniques for keeping the modular reflector rigid.
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