of the subject study — this however still incorporated excessive numbers of DC-RF converters, unnecessary beam steering features, etc.) The tremendous amount of RF-equipment assumed in the lunar model also adversely impacted the transportation requirements since the assumption was made that Earth manufactured requirements would amount to 10% of the SPS equivalent per unit aperture area or 10 times the freight load per lunar base. Also implicit in the model selection and cost comparison methodology of the subject study is operation of LPS at the same RF frequency as for the reference SPS (2.45 GHz). There is no rational basis for making such an assumption! While the impact of a shift in nominal operating frequency on diffraction beam width, radiating and receiving aperture dimensions, peak allowable beam intensities through the ionosphere and propagation characteristics should apply in more or less similar proportion to both SPS and LPS for those system elements whose size, capacity and cost depend on frequency, the overall cost optimization cannot be expected to be the same for both LPS and SPS. During the period of intense engineering study of SPS, tradeoff studies for operating frequencies of 2.45 GHz and 5.8 GHz were made (NASA, 1977) (22). Use of the higher frequency permitted a fivefold reduction in the product of transmission antenna area times rectenna area but required about a 10% increase in SPS mass or area to compensate for efficiency reductions and the system was operationally subject to more frequent interruption during severe weather. Since the rectenna represented only about 20% of overall SPS costs versus 80% for space-related costs there was little or no potential for net cost reduction using higher frequencies. It will be shown later in our results that over half of the total system cost for LPS represents rectenna costs (for an unoptimized model using the SPS reference frequency — 2.45 GHz). There is, accordingly a powerful incentive to explore the use of higher transmission frequencies for LPS beam propagation. Unit Cost Criteria The major flaws in logic applied to unit cost criteria involved failure to properly distinguish unit element costs which are predominantly controlled by area, peak power, mass or other factors. Part of this is probably due to too rigid an adherence of a one-to-one correspondence between subelements of a LPS and SPS model. For example, high efficiency, high technology solar cell costs tend to be proportional to cell area rather than mass. Solar cell formation by large area thin film evaporation are usually rate limited by evaporation energy and tend to be proportional to thickness or mass rates per unit time or area. DC-RF power conversion costs tend to be proportional to power processed rather than number of units as chosen in subject study. (Comparable SPS-only trade-off studies between high power klystrons and magnetrons with 10% or less peak power showed little or no adverse cost impact of substitution.) Cost Comparison Factors Of all the subject study steps, the selection of cost comparison factors is subject to the greatest uncertainty based on the current state of model refinement even if a satisfactory model and realistic unit cost criteria were used. This is true since nearly all space operations for LPS are of a fundamentally different nature than for SPS. Some of the LPS operations do have counterparts in Earth steps required for SPS however.
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