AN INTERFEROMETER-BASED PHASE CONTROL SYSTEM James H. Ott and James S. Rice Novar Electronics Corporation, Barberton, Ohio ABSTRACT An interferometer-based phase control system for focusing and pointing the SPS power beam is discussed. The system is ground based and closed loop. One receiving antenna is required on earth. A conventional uplink data channel transmits an 8-bit phase error correction back to the SPS for sequential calibration of each power module. Beam pointing resolution is better than 140 meters at the Rectenna. INTRODUCTION Key to focusing and pointing the SPS power beam is the maintenance of precise phase relationships among the transmitted signals of each Space- tenna subarray. Specifically, the signals transmitted by each power module must arrive at the center of the Rectenna in phase. This results in a power beam having a planar wavefront pointed at the center of the Rectenna. However, structural deformations in the Spacetenna can, if not compensated for, alter the phases of the power module signals at the Rectenna by altering the path lengths of the signals between the power modules and the Rectenna. In addition, variations within the Spacetenna circuitry can also alter the phases of the signals. Novar Electronics Corporation has developed an interferometer-based phase control systemJ This approach, which we call Interferometric Phase Control (IPC), has three significant characteristics which differentiate it from the Reference System retrodirective approach. 1• Interferometric Phase Control is a ground based closed loop system. Unlike in the retrodirective approach, the phase correction information is obtained on earth by measuring the resultant power transmission of the Spacetenna power modules and comparing them against a reference. 2. The Spacetenna's power modules are calibrated sequentially. A signal from a reference transmitter near the center of the Spacetenna is sequentially phase compared with a calibration transmission of each of the power modules. 3. During normal power transmission, the frequency of each power module is shifted slightly during phase calibration. Maintenance of a properly focused and pointed power beam can be accomplished concurrently with the normal transmission of power from the SPS by using frequencies for calibration which are different from the power beam frequency.
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