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Spare Time

Published: 02-10-2009
Source: JILA Scientific Communications

NIST’s WWV radio station and transmission towers located 12 miles northwest of Fort Collins, Colorado. Credit: Greg Kuebler

Using data brought in with a memory stick, Judah Levine compares the performance of NIST's primary time scale (in Boulder) with the remote time scale near Fort Collins. (lower left) Three atomic clocks (the empty space normally houses a fourth clock taken to Boulder for repair). (upper left) the time distribution system sits above the B measurement system. (top right) the A measurement system. (right) Judah Levine uses time-scale computers to compare the performance of the remote time scale .

Several hundred half-liter water bottles decrease temperature fluctuations in the remote time-scale room inside the WWV radio station. Credit: Greg Kuebler

In a rural northern Colorado landscape punctuated by plentiful corn fields, a tree farm, an abandoned feedlot, and a handful of McMansions, only one thing is certain: the exact time. The nation’s backup time scale, consisting of four atomic clocks, two measurement systems, and supporting hardware is tucked away inside radio station WWV's remote transmission station, located 12 miles northwest of Fort Collins. Fellow Judah Levine travels to the station site an average of once a week to check on the performance of the backup time scale, which he designed and built in 2005.

The remote time scale controls WWV’s 2.5–20 MHz transmissions of time, tones, and tics destined for the National Institute of Standards and Technology’s (NIST's) talking clock (303-499-7111) and other time services. WWV disperses its signals via five antennas mounted on towers ranging from 7.5–60 m tall.

The remote time scale also controls the transmissions of sister station WWVB, located just down the road from WWV. WWVB transmits 60 kHz signals to wall clocks, watches, and the JILA time display on the first floor of the tower. It employs eight 122 m towers (in two diamond configurations) to support two antennas, each linked to its own helix house containing two enormous air coils (transformers). On-site employees maintain the towers, antennas, and transmission stations.

Levine's responsibility is the remote time scale. He recently brought one of the time scale's atomic clocks back to Boulder for what he hopes is a simple repair. Meanwhile, the remote time scale is running on its minimum number of clocks since its measurement systems use a "majority rules" strategy for evaluating which clocks are performing best. The absence of the fourth clock means Levine has to check up on the time scale more often than usual. A check-up consists of comparing the remote time scale’s performance with that of the primary time scale at NIST.

Levine carries data from the primary time scale to the Fort Collins site on a memory stick. Levine takes advantage of his nearly 30 years experience building and maintaining the nation's time scales to perform the evaluation. About once a month, he decides to gradually "steer" one of the measurement systems back into sync with the primary time scale.

Levine also identifies and fixes environmental problems that affect the performance of the remote time scale. For instance, in the fall of 2007, he built a power system for the remote time scale after a power failure at the station broke the time scale. The new power system consists of two car batteries, a power supply, a trickle charger (to keep the batteries continuously charged), and two car headlights (for discharging the batteries). The system not only protects against power failures, but also smoothes out large daily fluctuations in the power supplied by the local rural electric association.

Levine has also shielded the time scale's room to isolate it from the radio station's radio transmissions and filled a shelf with hundreds of half-liter water bottles. The water bottles help keep the temperature constant in the room by absorbing heat when it’s warm and releasing warmth when it's cold. Despite the time scale room's being in the center of the building, it experiences serious temperature fluctuations because of the lack of insulation in the building.

The environmental and power enhancements have significantly improved the performance of the remote time scale. It is now working well. Levine routinely tests its performance with respect to the global positioning satellite (GPS) system. Four GPS antennas on the roof of the WWV building provide comparison data.   - Julie Phillips



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