matured, were recognized to be a potential solution to some remaining SPS problems. In general concept, the structure of the SPS transmitting antenna was composed of a large number of subarrays, each in turn composed of a large number of radiating modules consisting of antenna and microwave generator. The SPS subarray became a model for an earth based transmitter that could be used to beam energy to a balloon or airplane for its propulsion so that it would remain in position at high altitudes in the earth’s atmosphere for extended periods of time, performing useful communication and surveillance functions. When the necessary electron steering means was added to the basic subarray to track the balloon or aircraft, it was immediately evident that an electronically steerable subarray in the SPS would solve many of the problems associated with the antenna design of the SPS, as will be discussed. The NASA sponosred studies of microwave powered aircraft also Introduced a new light weight and flexible format for the rectenna, the device that captures and rectifies the microwave energy at the reception point?. It was soon recognized that this new format with its mass to power ratio of no more than two kilograms per kilowatt of DC power output, and its potentially easy deployment in space could become the low mass source of continuous power that the electric propulsion community has been searching for to make practical interorbital vehicles transferring material from low-earth orbit to geosynchronous orbit®. In effect, the microwave power transmission system for the SPS, greatly scaled down in size, could be reversed in direction to send power up into space from the earth to power the electrically propelled vehicles. Thus, the SPS concept set into motion the technology needed to drastically reduce the transportation costs associated with placing it in geosynchronous orbit, and in doing so also solved the more general problem of LEO to GEO transportation of bulk materials. This paper will discuss these two developments in more detail. All of the material to be presented in this paper has been previously presented in professional society symposia and in addition has either been published or accepted, reviewed, and type-set for publication. APPLICATION OF GROUND-BASED, ELECTRONICALLY-STEERABLE ARRAYS TO THE SPS SPACE-BASED MICROWAVE TRANSMITTING ANTENNA At the conclusion of the SPS study, the subarray unit in the antenna had been developed to consist of a number of identical radiating modules, all radiating the same phase®. The size of the subarray and the number of radiating modules in it were limited by the fact that efficient transmission to earth demanded that opposite edges of ideally flat subarrays could depart by no more than a twenthieth of a wavelength, or about 0.5 centimeters from a plane normal to a line drawn from the satellite to the center of the rectenna on the earth9. In addition, it was recognized that the subarrays under environmental conditions would depart from flatness, but a tolerable departure could be no more than 0.5 centimeters. It was further recognized that it would be difficult to predict the various distortions introduced into a truss type structure with one side much hotter than the other under steady state conditions of operation, and by the extreme temperature differences occasioned by the transient occultation of the satellite by the earth. The engineering responses to this situation were to limit the size of the array, utilize exotic and expensive materials with a low coefficient of thermal expansion, and to place mechanical jacks at the four corners of the array to realign them as necessary. The solution to the problem of misaligned and warped arrays is a direct application of the electronic steering format worked out for tracking microwave powered aircraft with the power beam. The application of this method to the SPS subarray is shown in Figure 1 and described in the following paragraphs®. At the center of the subarray there are two microwave interferometers which track the alignment of the subarray with the rectenna on the ground. One interferometer tracks angular alignment around the X axis; the other around the Y axis. For a reference the interferometers use the phase front of a microwave beam whose source is a beacon in the middle of the ground rectenna. The frequency of this beacon is sufficiently far removed from the frequency of the microwave power beam so that any interference can be eliminated by simple microwave filters. Any angular deviation from normal incidence is converted into two separate digital signals that are sent out through a row and column matrix to all of the radiating modules. A simple microprocessor in each radiating module multiplies these two signals by two multipliers, whose values correspond to the position of the module in the row and column matrix. The microprocessor then sums the two results and applies the sum to a digital phase shifter. The output of the digital phase shifter is then added to a reference phase which has been distributed at low power levels to each radiat- ?W.C. Brown, "Design definition of a microwave power reception and conversion system for use on a high altitude powered platform," NASA Contractor Rep. CR-15866, Contract NAS6-3006, Wallops Flight Facility. ®W.C. Brown and P.E. Glaser, "An electric propulsion transportation system for low-earth orbit utilizing beamed microwave power," Space Solar Power Rev., Vol. 4, pp. 119-129, 1983. 9For technical explanation see pages 2.6-2.8 of Reference 6.
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