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
