of less scattered power were greater than the additional receiver costs) if the phase control receivers were less than $600 each. A later Boeing Aerospace Company study indicates that these receivers can be built for less than $600 in high volume quantities. There is also an environmental advantage in phase controlling at the power module level in that the grating lobes incident upon the earth are reduced in amplitude and in quantity. Figures 1 and 2 show the locations and amplitudes of the grating lobes from a single 5 GW SPS system with phase control to the power module level. Recent simulation results indicate the off-axis grating lobes may be considerably reduced from the data shown in the figures. The location jitter or the error in path length from the pilot beam transmitter to each radiating slot in the antenna is reduced by going to the smaller antenna size associated with an individual tube rather than to the larger subarray. This location jitter, which appears as a phase error, scatters 6 MW of power at the tube level as compared to 87 MW at the subarray level. • A reduction in allowable amplitude jitter. The reference SPS system has a ±1 dB amplitude jitter across the surface of each subarray or power module. Analysis results indicate that power transfer efficiency (88% for the reference system) is relatively insensitive to amplitude jitter. However the voltage and amplitude regulations for the high efficiency, high gain klystron tubes have to be maintained to approximately 1% for satisfactory operation. Therefore a ±1% amplitude tolerance is recommended for the antenna error parameter. This change will not affect the microwave transmission efficiency budget. • Metal matrix waveguides. The SPS reference system has aluminum for the subarray distribution and radiating waveguides. Because of thermal distortion problems a graphite/aluminum metal matrix composite is now being developed as a possible replacement for the aluminum. The antenna structural members are composed of a high-temperature graphite plastic material for rigidity. The antenna primary structure has a 1040 meter x 1040 meter x 100 meter pentahedral truss configuration which suppors a secondary structure. This secondary structure provides a base for mounting and aligning the transmitter subarrays. Both the primary and secondary structures must maintain a high degree of stability over wide operating temperature fluctuations to preserve the three arc-minute flatness requirement, hence the need for low coefficient of thermal expansion materials. • Startup/Shutdown Procedure. The satellite will have to shut down 87 times per year due to solar eclipses by the earth. In addition there will be eclipses by the moon and other SPS, as well as scheduled shutdowns for maintenance. A number of possible sequences for energizing/deenergizing the microwave system were investigated. Three sequences provided satisfactory performance in that the resultant sidelobe levels during startup/shutdown were lower than the steady-state levels present during normal operations. These three sequences were: random, incoherent phasing, and concentric rings-center to edge. Thus no microwave radiation problems are anticipated during startup or shutdown operations, either scheduled or unscheduled. Shaped Beam Synthesis: Studies into reshaping the beam pattern to improve overall rectenna collection efficiency and to provide additional means of sidelobe control were undertaken. These studies included: (1) Adding phase reversal at the klystron input as a first step towards a continuously variable phase distribution across the antenna surface. The results showed that reshaped beam patterns
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