A space feed system developed by Rockwell offers excellent potential improvements over the previous concepts. It must be noted that a simple space feed, with small-sized pickup reflectors, can equal the performance of a complex coaxial system with high losses. The geometry of the space feed basic concept is shown in Figure 2.3-21. A feed, only 57 meters in diameter, sparsely populated with 148 radiating elements in a regular grid, will produce up to 873 ''grating lobes." When properly designed, each of the grating lobes will illuminate 873 pickup 2-meter reflectors on the array. One pickup reflector can feed nine subarrays connected by small-diameter coax. This approach allows efficient transmission of the reference signal with the pickup reflectors spaced 30 meters apart. The outputs of each 2-meter reflector feeds a nine-way power divider (see Figure 2.3-22). An ideal 1:7854 power divider system would have 39-dB loss, so the overall ideal feed introduces 20.6 dB of loss. Considering the loss of a coax system to receive +10 dB at each subarray, a total of 9.12 kw must be transmitted, or about 62 watts at each of the 148 feed elements. Solid-state sources can thus be used for the feed. The phase errors introduced by variations in the path length between the subarray and the space feed must be corrected to ensure that the subarray outputs are in the correct phase, because the retrodirective system does not account for this. The two tones at 2.42 and 2.47 GHz can be repeated back to the feed system with time division multiple access (TDMA) to separate the various returns. These two tones represent a double-sideband suppressed signal with 25-MHz modulation, so the phase of the absent carrier can be deduced within a 180-degree ambiguity. An Figure 2.3-21. SPS MPTS Transmit Array Geometry Using Space Feed Reference System

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