Earlier I wrote:
> The "Explanatory Supplement to the Astronomical Almanac," mentions
> alternatives to atomic time under study that may offer
> improvements. (I don't have the book handy and cannot recall what they
> are. I will try to post a follow-up on Monday.) I believe that
> atomic timing technology is about fifty years old now...
[...]
> Jim
On pages 60-61 of the "Explanatory Supplement to the Astronomical
Almanac," is a discussion of modern timing technologies.
2.321 Quartz-Crystal Oscillators
... The performance characteristics may range from 10^-4 to 10^-13
in frequency stability per day. ...
2.322 Cesium Beam Standards
... laboratory cesium-beam frequency standards ... realize the
second with the utmost accuracy (currently, 1.5 x 10^-14) and are,
therefore, stable in the long term. ...
2.323 Hydrogen Masers
... The optimum stability reaches about 1 x 10^-15 for integration
times of 1000 to 10000 seconds. ... [S]ome hydrogen masers
equipped with automatic tuning of the cavity and kept in
temperature-controlled rooms have a long-term stability of the
same order as the best cesium standards. ...
2.324 Rubidium Vapor Cells
The rubidium clock is an appropriate device when a relatively
low-cost clock is needed that has better stability than a quartz
crystal clock. The rubidium clock can reach a stability of 1 x
10^-13 per day under the best conditions, but is subject to
temperature- and pressure-induced frequency variations. Ringer et
al. (1975) describe the design and performance of a clock for the
GPS satellites. This clock has achieved stability of 2 x 10^-13
per day.
2.325 Mercury Ion Frequency Standard
The mercury-ion frequency standard uses ions that are confined in a
small region of space by an electromagnetic field trap. Thus the
particles can be observed without having them collide with the
walls, which would disturb the atomic resonance. The mercury-ion
isotope Hg-199 has an extremely narrow microwave resonance line at
40507 MHz. Although this type of frequency standard should be a
large improvement over the cesium standard, it has an
unfortunately low signal-to-noise ratio, resulting in limited
short-term stability. However, the long-term stability is very
good, since integration can take place over a number of days
(Winkler, 1987).
I suppose I was incorrect to call these alternatives to atomic timing.
Quartz is molecular; the maser is ultimately atomic in nature (or
perhaps molecular, I'm not sure); and the others are all atomic. But
they are alternatives to atomic cesium clocks.
Jim
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