So Loran isn't really dead yet! John
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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On >Behalf Of Tom Clark, K3IO (ex W3IWI) >Sent: 20 April 2006 06:59 >To: [email protected] >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 >133Cs? > >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 >it? > > > 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 0ยบ 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 > >References > > 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 >_______________________________________________ >time-nuts mailing list >[email protected] >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > >_______________________________________________ >time-nuts mailing list >[email protected] >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
