Geodetic survey of NIST Clock Laboratories, Boulder, Colorado
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Geodetic survey of NIST Clock Laboratories, Boulder, Colorado

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    "Einstein's theory of general relativity indicates that when any clock is operated at a location 'higher' than another ('up' is measured away from the mass that generates the local gravity field) it will be observed to run faster. That is, it will appear to 'tick' at a higher frequency to those observers 'below.' In our everyday experience this effect is unnoticeable, but the Time and Frequency Division at NIST-Boulder is developing atomic clocks (so-called 'optical' clocks) with accuracies approaching a few parts in 1018. [1,2] At this level, changes in a clock's height of even a few centimeters will cause a noticeable difference in its output frequency and must be accounted for. Technically, it is changes in the potential of the Earth's gravity field -- known as 'geopotential' -- to which the clocks are sensitive. To facilitate comparisons amongst clocks within a laboratory and/or between different laboratories, NOAA's National Geodetic Survey (NGS) agreed to measure geopotential differences at various locations throughout the NIST-Boulder campus. This includes both the laboratory for the existing time standard as well as the new, experimental clock laboratories. With geodetically-determined geopotential differences in hand, a prediction of the expected frequency difference between any two NIST-Boulder laboratories will be possible immediately. Looking forward, the hope is that one day such clocks can then be linked across continental or even global scales. Once the difference in local geopotential values is taken into account, it will allow for the direct comparisons of clocks for metrological, time distribution purposes. Further, the process can then also be reversed: observed differences in the frequencies of clocks operating at far-flung locations can be used to infer geopotential differences directly. This real-time 'geo potentiometer' would revolutionize the field of geodesy"--Introduction and motivation. [doi:10.7289/V5/TM-NGS-73 (]
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