On Sat, 21 Mar 2020 14:24:48 +0100 Frank Stellmach <[email protected]> wrote:
> you're welcome to pose any questions here. Definitely! We all started as beginners and learned through asking questions. > Concerning your question, there are basically two different parameters > called 'stability' and 'uncertainty'. > I personally don't use the term 'accuracy' any more, because that's > somewhat misleading. I recommend here reading [1] and [2]. The terms used in metrology are confusing at first and one needs time to digest them. A quite a few things that we are used to do in "normal" life do not work for metrology anymore. Mostly because we assume that our standard (e.g. the calipers, the gauges, weights, etc) do not change. Once you enter metrology, your standard isn't stable anymore and you need to evaluate not only how accurate it is, but how stable. This all then ends up in an uncertainty of your measurement, parametrized by a specific set of conditions (temperature, how long you are measuring, etc). John Vig's Tutorial is also a good source of information to get started: http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=02_GPS_Timing/John_Vig_Tutorials_on_Crystal_Oscillators > Btw.: The GPS system delivers an in-official uncertainty, because the > D.O.D. clock is not participating in the S.I. representation of the UTC. This is not true. While BIPM only allows a single NMI per country to contribute to TAI/EAL these days, this wasn't case in the past. And for historic reasons there are a few countries where two entities contribute to TAI/EAL. The USNO, master over GPS time, is one of those non-NMI entities contributing. They also used to be in the past the one single organization that had the most atomic clocks running, though that's slowly changing now. They still are one of of the organisations that have the most stable clock ensambles contributing to EAL, though, and will stay so for the forseeable future. As for how far they are off, have a look at their circular T entry: https://webtai.bipm.org/database/canvas.html?utclab=ok&lab=usno&mjd1=57078&mjd2=58919 And compare it to, e.g. PTB: https://webtai.bipm.org/database/canvas.html?utclab=ok&lab=ptb&mjd1=57078&mjd2=58919 Attila Kinali [1] "Characterization of Clocks and Oscillators" NIST Technical Note 1337, by Sullivan, Allan, Howe, Walls, 1990 http://tf.nist.gov/general/pdf/868.pdf [2] "Handbook of Frequency Stability Analysis" NIST Special Publication 1065, by Riley, 2008 http://tf.nist.gov/timefreq/general/pdf/2220.pdf -- Science is made up of so many things that appear obvious after they are explained. -- Pardot Kynes _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
