Phased Array Control Phased array technology uses “electronic steering” instead of mechanical pointing to direct the output beam to the receiving array(s). One approach would use a pilot beam from the receiver(s), which is received and phase-conjugated by the transmitter to create an output beam precisely reversed from the incident pilot beam. This requires no overall processing capability except for the ability to maintain a synchronized clock to measure the phase. Each individual output element is required to adjust its phase to sum coherently into the output beam. Each element receives the pilot beam, compares the pilot beam phase with the reference (master) clock, and adjusts its output beam to conjugate the phase: i.e., the output oscillator phase is to be exactly as far behind the reference clock zero as the pilot beam phase is ahead of the reference clock zero. This phased array control approach thus has three parts: (1) maintaining the reference clock, (2) receiving the pilot beam, (3) setting the proper element phase. For a rigid array using a central clock, the local reference signal consists of the central signal compensated for the “fixed” transmission delays. The difficulty of synchronizing the local clock references increases if the array is not rigid, since the distance to the central clock, and thus transmission delays, may vary. In this case a means to measure the elements displacement, such as a laser interferometer, may be used to compensate the delay. It is also possible to provide each individual element with an oscillator to use as a clock to compare phases. To maintain coherence of the output beams, all of the clock oscillators must be synchronized in both frequency and phase. If the local oscillators drift from the correct phase with a characteristic drift time t (~1/Af, where Af is the error in frequency), their phase must be reset on a time scale short compared to the drift time. Clearly, the more stable the local oscillators, the less often they need to be reset. The pilot beam need not necessarily be at the same frequency as the transmitted beam unless a mixing technique (e.g., a four wave mixer) is used to generate the phase conjugation. A pilot beam at, or near, the same frequency as the power beam has the advantage of automatically correcting for atmospheric effects. A disadvantage of a pilot beam at the output frequency is the difficulty of distinguishing pilot beam from output (e.g., by polarization difference). Failure to adequately isolate the pilot beam from the output beam would result in undesirable self-stimulated oscillations of the transmitter. Any beam which can provide timing information with sufficient accuracy can be used; for example, the pilot beam can be a microwave beam at many times higher frequency. An interesting alternative is to use a pulsed laser as the pilot beam. Techniques now exist to make laser pulses with durations of picoseconds and shorter; sufficiently fast to use for microwave frequencies up to hundreds of gigahertz. Each laser pulse would be used to set the phase signal. Monolithic integrated circuits for control of a phased array based on laser-transmitted reference
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