Antenna Surface Tolerance Losses A loss factor that will occur depends on how accurately the surfaces of the transmitting antenna and the reflector are controlled. Maintaining the required mechanical (and/or microwave beam phase) tolerances over such large antenna areas is challenging, and inadequate tolerances will reduce overall PRS efficiency. Since the receiving antenna rectifies the power from each element separately, it is not critically dependent on mechanical or phase tolerances, as long as the beam fills the receiving antenna. Transmitting antenna tolerances will affect beaming efficiency. Phase errors from one microwave power source to another will similarly affect efficiency. However, because both mechanical and phase tolerances can occur on the transmitting antenna, the tolerances on each must be held tighter by the square root of two to maintain performance. A tight reflector mechanical tolerance is required because a reflector surface error affects the path length of a ray both arriving and departing, compared to the path length of a ray reflected off a neighboring point. To limit defocusing losses on each path to 5%, the following tolerances must be held: The highest antenna gains that have been achieved in radar and radio-astronomy antennas, are limited by surface accuracy and tolerance. At 2.45 GHz, the highest gain achieved is about 62 dB. At 5.8 GHz, the highest gain is about 67 dB. For comparison, the required gains of the antennas for PRS applications will be about 100 dB. To achieve these significantly higher gains, an active error-correction system will be required to reduce antenna surface tolerance to the required accuracy, in a fashion similar to the "adaptive optics" now widely used in astronomical telescopes. To accomplish this correction, the transmitting antenna and the reflector in orbit will each be divided into subarrays or subsections, within which the required tolerances can be met. Each of these subarrays or subsections will be actively aligned by trimming the microwave beam phase of mechanical position to maintain the overall antenna gain. The active correction system will be controlled by additional microwave signals sent back and forth between the reflector and the ground antennas. Even though this system is operating at microwave frequencies, this arrangement is equivalent to "adaptive optics." The reflector is divided into subarrays which will hold the required phase and mechanical tolerances. The retrodirective control technique trims the phase (or position) of each subarray significantly reducing the number of control loops. To handle
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