SECTION 7 PHASE FRONT CONTROL The MPTS subarray phase front control subsystem must provide high accuracy beam pointing and focusing of a high power microwave beam in the presence of a non-homogeneous, time varying atmosphere and ionosphere, thermal deformation of the array waveguide and structure, and phase variation of transmission lines, converters, and phase shifters. Safety considerations, to limit high power densities to acceptable values outside the receiving antenna area, require positive control over beam focusing and pointing during steady state and transient conditions. High overall efficiency requires both accurate beam pointing and proper focus. The effect of efficiency degradation with phase error is shown in Figure 7-1. Figure 7-2 illustrates two basic approaches to phase control: command and adaptive. The former relies upon measurements made at the receiving site to make connections via a telecommunications link to the transmitting array. The latter uses a reference beam sent from the receiving antenna site to enable measurements and corrections to be made at each subarray. The command scheme could be implemented in two ways: phase estimation and "bit wiggle." Phase estimation relies upon measurements made over a matrix of sensors covering the entire receiving site. These are processed by algorithms which interpret the errors in beam center location and power density distribution on the ground as distortion of the phase front at the transmitting antenna from the ideal. Corrections are then sent over the command link to a central distribution point at the transmitting antenna and/or directly to a subarray. The "bit wiggle” approach sequentially examines each subarray’s performance by commanding a distinctive (phase) modulation on its output. Corrections are then sent via the command link. The approaches are illustrated schematically in Figure 7-3 and a summary comparison is given in Figure 7-4.
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