Using GPS for Telemetry System Clock Generation and Synchronization On-Board Sounding Rockets

Steven P. Powell and Paul M. Kintner

Abstract:	A clock generation and synchronization scheme has been developed to rapidly phase-lock a sounding rocket telemetry system clock at 8.96 MHz to a standard 1 pulse-per second (1 PPS) GPS receiver output. The design employs a digital phase-locked loop system incorporating a novel algorithm which achieves frequency-lock and phase-lock in less than four 1 PPS cycles. This is an improvement of nearly an order of magnitude compared to traditional phase-locked techniques. Often science objectives require synchronized measurements made between multiple sounding rocket payloads. The placement of a GPS receiver on-board each payload permits the independent telemetry links to be synchronized to a common clock. All on-board data sampling is derived from this clock. Compared to post-reception synchronization on the ground, the sample timing errors decrease by about a factor of 103. This increases the value of data acquired from multiple payloads especially for multisensor signal analysis. In addition to phase-locking the frequency of the telemetry system clock to the GPS clock, the PCM telemetry frame is also locked in phase to the GPS 1 PPS. This is done without disrupting the PCM telemetry data stream or frame synchronization, by making fine adjustments in the PCM telemetry clock frequency with the aid of the digital phase- locked loop system. Furthermore, the design is inherently fail-safe in that it continues to generate a clock signal even if the GPS receiver fails. The GPS to telemetry system synchronization and phase- locking technique will be illustrated with a design example from the GPS receiver interface used on the upcoming NASA PI-LAZE-2 sounding rocket mission. Synchronization and rapid phase-locking of the on-board PCM telemetry system clock at 8.96 MHz to a standard 1 PPS GPS receiver output is accomplished in less than 4 seconds.
Published in:	Proceedings of the 9th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1996) September 17 - 20, 1996 Kansas City, MO
Pages:	1341 - 1348
Cite this article:	Powell, Steven P., Kintner, Paul M., "Using GPS for Telemetry System Clock Generation and Synchronization On-Board Sounding Rockets," Proceedings of the 9th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1996), Kansas City, MO, September 1996, pp. 1341-1348.
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