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

John Day johnday at wordsnimages.com
Thu Apr 20 15:21:23 EDT 2006

So Loran isn't really dead yet!


At 03:34 AM 4/20/2006, you wrote:
>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
>    [1]http://tf.nist.gov/timefreq/general/generalpubs.htm
>    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
>    [4]http://tycho.usno.navy.mil/clockdev/cesium.html).
>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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