Measuring Clocks and Watches

Measuring watches with GPS

A GPS reference can also be used to measure precision watches or marine chronometers. The measurement procedure is the same as it is for clocks. But difficulties arise when the beat of the watch drifts into phase with the GPS reference pulse. When the readings are recorded on a computer, a sawtooth shape develops as with clocks, but the jump in value is much smaller and more variable, and it’s more difficult for the software to accurately remove them.

It’s easier to measure watches or chronometers if you can manage to have them generate a signal once per second. I’ve been working with Jerry Walker to make this possible, and built a custom timer that divides the tick from a watch or chronometer by an arbitrary number. If you divide the half second chronometer tick by two, you get a signal every second. For a precision watch, you can divide by five to get one signal per second. Our standard GPS analysis software can then accommodate these readings.

The following graph shows the rate of a marine chronometer as measured with a GPS reference receiver. The graph spans 3-1/2 hours of time. You can see periodic fluctuations in the rate of the chronometer. These fluctuations are not caused by temperature changes, but are imperfections in the wheelwork of the timepiece.


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