The best solution may be the transmission line combined with the subarray-to-subarray transfer approach to exploit the advantages of each; i.e. , use subarray-to-subarray transfer approach locally and use the transmission (reference) line approach to reach out to the edges of the antenna. Other approaches that have been discussed for phase reference distribution or direct mechanical antenna distortion measurement involve laser techniques. These approaches are thought to produce added complexity and questionable reliability, especially in the light of a 30-year goal. The direct measurement of subarray physical displacement from a reference line, such as relative to a matrix of taut cables across the antenna, or comparison of the overall surface with a stored hologram pattern lack the mechanical simplicity of a direct rf measurement translated into an rf phase-adjusted power beam output. The latter seems best suited at this time to an active phased array and has the ability to correct both electrically and mechanically produced errors in the phase front, which the former does not. Figure 7-7 shows a detailed block diagram for implementing the adaptive approach. Feedthrough nulling is shown as a means of preventing high power coupling into the receiver to reduce receiver saturation problems. The ground based pilot beam would have the following characteristics: The error budget for the adaptive control system is given in Figure 7-8 where it is seen to produce a two percent power loss relative to a perfect implementation. The effect of this error on beam pointing accuracy is of great interest from control and safety standpoint, and this can be evaluated from the following relation:
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