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Redefining time

According to reports published in the Physical Review Letter, the National Institute of Standards Technology (NIST) published a paper indicating that it had discovered a new type of atomic clock based on mercury-ion. The report said that the new atomic clock is so accurate it's nearly 6 times more precise than the current cesium-based atomic clock.

The report said that the NIST currently operates a cesium-based atomic clock called the NIST-F1, which is accurate for roughly 70 million years. If operated continuously, the NIST-F1 would only be off by 1 second after 70 million years. The new mercury-ion atomic clock on the other hand will take 400 million years.

The new experimental clock measures the atomic resonance frequency of a mercury atom. The atom itself is electrically charged and kept in an extremely cold suspension. Using the new mercury-ion atomic clock, scientists at the NIST say that they will be able to conduct more precise experiments and further develop applications that rely on atomic-time accuracy such as GPS systems. Currently, the international standard that defines what one second is relies on cesium-based atomic clocks -- 9,192,631,770 radiation cycles of the change between two energy levels of a cesium atom. The NIST says that it will be five to ten years before we see mercury-ion clocks replace cesium ones.



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By Shadowself on 8/1/2006 11:20:10 AM , Rating: 2
There is a LOT more to GPS accuracy than just changing the clock, as it were.

Until the most recent version of the GPS satellites they all had multiple clocks of two different kinds: Multiple clocks for redundancy and averaging of the times; Two different kinds of clocks (Rubidium clocks as well as and Cesium clocks) because there are two different aspects of timing accuracy: long term and short term accuracy/stability.

While this mercury-ion clock might be extremely accurate for long time scale measurements it may not be that accurate for short time scale measusrements. I have not read any papers which address this specific issue.

In order to have GPS satellite only (not using Differential GPS) accuracy, the system needs very good stability in both short term and long term time scales.

Finally, the note above states this is "kept in an extremely cold suspension" long term cryogenics on a satellite is extremely difficult and expensive (cryocoolers are notorious producers of vibration which, in general, is a very bad thing on satellites, typically requiring expensive vibration isolation techniques).

As a side note, Differential GPS can get you down to decimeter scale accuracies now. Once enough of the systems with the L-4 and L-5 freqencies are flying this will go down even further.


By DallasTexas on 8/2/2006 8:34:15 AM , Rating: 2
Good insight - thanks. It makes sense to me what your saying about the importance of low variance on the short term.


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