The groundrules for sizing the new microwave systems included using the present SPS antenna error parameters, i.e., 10° phase error, +1% amplitude error, 2% tube failures, +1 mTn antenna tilt, +3 min subarray tilt, .25" mechanical spacing between~subarrays, etc., and"the rectenna was sized to receive 88% of the transmit power. The relative antenna/rectenna sizes for 2.45 GHz and 5.8 GHz operation are shown in Figure 1. A detailed analysis of subsystem costs and masses for the reference 5 GW solar power satellite with silicon solar cells is given in the Boeing Aerospace Final Report DI80-25461-2, November 1979. These values are used as a baseline for computing costs for the different antenna/rectenna configurations. Since the purpose of this study is to determine the relative or differential costs for the various configurations, any future changes in the absolute costs for the reference system should not have a great impact upon the conclusions herein stated. The principal elements in the SPS recurring costs are satellite hardware, transportation, space construction and support, rectenna, program management and integration, and cost allowance for mass growth. These cost calculations also included the following guidelines: 30 year operating lifetime, plant factors of .92 and .90 for 2.45 GHz and 5.8 GHz operation respectively, 15% rate of return on investment capital, 22% mass growth factor to cover potential risks in solar array and microwave system performance estimates, 17% of net SPS hardware cost factor to account for mass growth, and 10 GW per year power installation. The cost and mass for each of twelve satellite subsystems were varied according to total power, antenna size, frequency, efficiency, etc, of the candidate antenna/rectenna systems. The electricity costs in mills per KWH and the differential cost increases for 2.45 GHz and 5.8 GHz systems are summarized in Figures 2 and 3, respectively. The data indicates costs for the 2.45 GHz systems are heavily dependent upon ionospheric power density limitations. The 2.45 GHz and 5.8 GHz alternate configurations can provide smaller rectenna sizes at the expense of added electricity costs. Summary The satellite and associated microwave system have been reoptimized with larger antennas (at 2.45 GHz), reduced output powers, and smaller rectennas. Four constraints were considered: (1) the 23 mw/cm2 ionospheric limit (2) a higher (54 mW/cm2) ionospheric limit (3) the 23 KW/m2 thermal limit in the antenna, and (4) an improved thermal design allowing 33% additional waste heat. The differential costs in electricity for seven antenna/rectenna configurations operating at 2.45 GHz and five satellite systems operating at 5.8 GHz have been calculated. The conclusions are: o Larger antenna/smaller rectenna configurations are economically feasible under certain conditions. o Transmit antenna diameters should be limited to 1-1.5 Km for 2.45 GHz operation and .75-1.0 Km for 5.8 GHz. o Three configurations were selected for minimum impact on electricity costs (See next page) o The present ionospheric limit of 23 mw/cm2 is probably too low and should be raised after the ionospheric heating tests and studies are completed. For SPS cost considerations, it is very important to ascertain the true upper limit. o The 5.8 GHz configurations are constrainted by antenna thermal limitations, rather than ionospheric limits. Potential utility grid impacts of 5.8 GHz
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