At the PMA, simple mixing and filtering circuitry detects two difference frequency signals. One signal is due to S] and S£« The other, which is called a phase reference signal, is that due to Sj and sri. These two beat frequency signals are then phase compared to obtain the phase difference between them. The phase difference between the two beat frequency signals is a function of the z-axis deformation* of the Spacetenna at the location of the power module being phase tuned plus biases in the phase feed network of the SPS. Certain components of the phase difference change with a change in frequency, others do not. Since the power module being phase tuned is transmitting at a frequency different from the power beam frequency, it is necessary to distinguish between these frequency dependent and frequency independent components in order to determine the phase correction that will be correct at the power beam frequency. This is done by shifting srj and S2 to a different set of frequencies, according to a phase ambiguity error avoidance criterion, and making a second phase difference measurement. These two phase difference measurements are numerically adjusted by -2ir, 0, or +2ir according to a second phase ambiguity error avoidance criterion. These two numerically adjusted phase differences provide sufficient information to calculate the phase error correction^ transmitted back to the SPS power module being phase tuned. This phase error correction can be made with an 8-bit binary word sent to the SPS via a data channel. An 8-bit accuracy produces a phase resolution of 360° * 2$ = 1.4°. This is sufficient to give a power beam pointing resolution better than 140 meters at the Rectenna. A tradeoff exists between satellite bandwidth requirements and the power module updating rate which is limited by filter settling times. It is anticipated that the frequency separation between S] and S2, srj and the power beam will be on the order of 1 MHz. At these frequency separations, the update interval for an entire Spacetenna can be on the order of a few seconds. It is possible that this will be fast enough to correct for any changes that will occur at the Spacetenna due to deformations, thermal effects, etc. Phase Tuning During Startup It is also possible to use this interferometer technique to phase tune the power modules at the power beam frequency during initial startup or maintenance. This would be necessary to calibrate the phase tuning system used during normal power transmission for any phase vs. frequency nonlinearities. In this case, the measured phase difference is the phase error correction. *deformation in a direction toward or away from the Rectenna.
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