A timing system comprising an electronic master clock and a subsystem for distributing time signals from the master clock to end users is undergoing development to satisfy anticipated timing requirements of NASA's Deep Space Network (DSN) for the next 20 to 30 years. The developmental system is intended to supplant the aging DSN frequency and timing subsystem (FTS), which, while historically reliable, is complex, has limited distribution capacity and has become increasingly difficult to operate and sustain. This system has a modular, flexible, expandable architecture that is easier to operate and maintain than the present FTS. Replicas of this system could be useful in laboratories and other facilities in which there are stringent timing requirements that could include requirements to distribute precise time signals over long distances.

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The system [to be installed in each Deep Space Communications Complex (DSCC) in the original DSN application] includes three major hardware assemblies interconnected by an infrastructure of fiber-optic cables (see figure). One major hardware assembly is the master clock assembly (MCA), wherein time signals are generated in synchronism with a 100-MHz reference signal from an atomic frequency standard, denoted the "online" standard. The MCA is set to Universal Coordinated Time and generates a system time code (STC) for distribution of time-of-day and timing-rate information to the entire DSCC. The STC is sent, via fiberoptic cables, to a distribution assembly (DA). The DA contains 10 distribution modules (DMs), each of which reconstitutes the STC and transmits the signal, either to a second-stage DA for additional fan-out or to a time-code translator (TCT), which serves as a timing reference interface for an end user. The TCT compensates for transmission delays from the MCA and can generate a variety of time codes and pulse rates as required.

The MCA, the DAs, and the TCTs reside in standardized chassis that are hot-swappable and include dual redundant power supplies. The MCA and DA chassis are identical; the TCT chassis are different and match those of the TCTs of the FTS. The back of each TCT can accommodate four plug-in modules to provide different time-code and pulse rate outputs.

For high reliability, the system includes, from the perspective of each end user, two flywheel oscillators. One flywheel oscillator is part of the MCA. The main purpose of this flywheel oscillator is to maintain MCA time in the event of loss or interruption of the reference signal from the online standard. While the timing performance slowly deteriorates in the absence of the reference signal, the complex remains operational until the reference signal can be restored. A second flywheel oscillator is part of each TCT. This oscillator enables the TCT to continue to generate timecode and pulse-rate outputs in the event of interruption of time signals anywhere in the distribution infrastructure. The TCT flywheel oscillator is allowed to run for a holdover interval up to 12 hours — more than enough time for diagnosis and repair.

This system is designed to be user-friendly, requiring minimal expertise and minimal human intervention for clock setup and diagnosis of faults. In the original DSN application, operators already have an overabundance of status and fault information to analyze. In this system, the only status or fault information provided to operators is that which facilitates isolation of a failure to the module level. Local alarm indications in a TCT, visible as lighted front-panel light-emitting diodes (LEDs), are summed together and communicated back to the applicable DA by simply blanking one pulse of a 1-pulse-per-second monitor return signal. A missing-pulse detector circuit in each DM in each DA responds to a blanked pulse by turning on an LED in the DM. Each DA chassis contains one alarm representing the summed alarms of all 10 of its DMs. This alarm is passed further back up the hierarchy or collected by a status summary monitor computer visible to operators. With modularity and simple "go/no-go" monitoring and alarm information, operators can maintain operations with little understanding of the nuances of the precise timing system.

This work was done by Robert Tjoelker, Malcolm Calhoun, Paul Kuhnle, Richard Sydnor, and John Lauf of Caltech for NASA's Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Electronics/Computers category. NPO-40851


This Brief includes a Technical Support Package (TSP).
Master Clock and Time-Signal-Distribution System

(reference NPO-40851) is currently available for download from the TSP library.

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This article first appeared in the May, 2007 issue of NASA Tech Briefs Magazine.

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