[time-nuts] time-nuts Digest, Vol 21, Issue 22

Rob Kimberley time.bandit at btinternet.com
Thu Apr 20 03:34:45 EDT 2006

The current definition of the second is based on Caesium, as that was what
was the best then. However, with work on the newer optical standards, and
Rubidium Fountains, I'm pretty certain that in the next few years the second
will be re-defined. Attached document from UK national Physical Laboratory
(NPL) may be of interest.

Rob K

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Tom Clark, K3IO (ex W3IWI)
Sent: 20 April 2006 06:59
To: time-nuts at febo.com
Subject: Re: [time-nuts] time-nuts Digest, Vol 21, Issue 22

   VE2VM commented

But Isn't Cesium drift-free? Since the SI second is standardized as de
duration of 9192631770 oscillation of the hyperfine transition of the atom

If Cesium drifts, theren should be a more formal definition of the second
(Such as density, maximum C-field or level of purity). Does anyone here has

   The achieved frequency can be pulled by external factors. The official
   SI second definition is "9,192,631,770 cycles of the ground-state
   hyperfine splitting of the unperturbed cesium atom." The problem is
   making it be unperturbed by the effects of the finite microwave
   cavity, wall effects from the containment bulb, the length of time
   that the Cesium atom is "stored" in its excited state, etc.
   A good recent review is the paper by Diddams (Science, November 2004)
   available from the NIST web site at
   Getting the cesium atom unperturbed has led to larger and longer
   Cesium standards at NIST (for pictures see
   [2]http://tf.nist.gov/cesium/atomichistory.htm); with the longest of
   the Cs tubes (NIST-7), the interaction time (the length of time the
   Cesium atoms "live" in their excited state is ~10 msec. The newest
   generation of Cesium fountain clocks use Laser cooling to contain a
   cloud of cesium atoms at at temperature near 0º Kelvin to minimize the
   wall effects and to increase the storage time to ~1 second (see
   [3]http://tf.nist.gov/cesium/fountain.htm and

And also, something else i don't understand: Why do the newer GPS satellites
rely on Rb standards rather than Cs standards? Since Rubidium is known as
less precise than cesium? Is there a reliability issue there (Rb clocks are
more reliable / longer MTBF tha Cesium clocks). I don't know...

   The Cesium clocks in GPS have been less reliable (probably because
   they are more complicated) than the Rb clocks in early GPS satellites;
   [5]for some information see the FAQ on this USAF web site. The 16
   Boeing Block II & IIA satellites have 2 Cs and 2 Rb. The 7 LockMart
   (your One-Stop Defense Contractor!) Block IIR satellites launched
   (plus 8 more awaiting launch) have 3 Rb.
   The Rb/Cs mix will change will change again with the IIF & 3rd
   generation GPS satellites ([6]see Symmetricom propaganda here) and
   with the European Galileo series (Galileo is planning on H-Masers).
   There is an interesting article in [7]The Space Review on the clocks
   planned for these next generation navigation spacecraft.
   73, Tom


   1. http://tf.nist.gov/timefreq/general/generalpubs.htm
   2. http://tf.nist.gov/cesium/atomichistory.htm
   3. http://tf.nist.gov/cesium/fountain.htm
   4. http://tycho.usno.navy.mil/clockdev/cesium.html
   5. http://gps.losangeles.af.mil/jpo/gpsoverview.htm
   6. http://www.symmsda.com/about_us/index.asp
   7. http://www.thespacereview.com/article/534/1
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