one kilometer. There are several key issues associated with maintaining accurate control of the transmitted phase front of this array: 1. Obtaining an accurate reference phase at the subarray level. 2. Compensating for phase changes in the power amplifiers and feed system. 3. Compensating for subarray position errors and propagation effects. The first two effects can be compensated internal to the satellite by measuring the phase shifts or deducing them by range measurements. The third effect requires either transmitting a pilot beam up from the ground and measuring the phase received at each subarray or making a number of field measurements on the ground and deducing the subarray errors from them. The pilot beam approach is called retrodirective phase control and the latter is called ground-referenced phase control. These methods, along with the hardware implementation of them, were considered during the study. The retrodirective approach is very attractive because detailed field measurements on the ground are not required. Furthermore, the ground fields may not be related to the antenna aperture distribution by a simple Fourier transform because of the ionospheric and atmospheric perturbations between them. As long as the propagation effects are equivalent at the pilot frequency and the power frequency, the retrodirective approach gives a direct and precise measurement of the subarray phases required and also produces the correct phases by frequency conversion. The basic difference between the two concepts can be summarized as follows: • Ground referenced phase control — Simple; however, effects of ionosphere do not make the aperture field a simple inverse transform of the received field. • Retrodirective phase control — • Required pilot beam (one megawatt) • Phasers set at conjugate received beam position • Precise beam control, adapts to ionospheric perturbations • Complex electronic subsystem There are many ways that a retrodirective system can be designed. As an example, the retrodirective phasing system shown in Figure IV-C-4-5 of the JSC study report (which uses synthesizers to produce 10 MHz and then 2450 MHz from the 2440-MHz pilot) is typical. Rockwell's studies show that if the power frequency does not need to be exactly related to the pilot frequency, many alternate designs are possible. A similar alternate phasing concept as shown in Figure 2.3-20 is simpler and should have less overall phase noise due to the lack of frequency division or multiplication. The power frequency is exactly 2f2 - fl, where f2 is the internally generated pilot frequency and

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