Hi Brooke, > HP had some way around SA that improved the timekeeping.
HP called it the "Smartclock Algorithm" and you can find some very basic information about it here: http://www.hpl.hp.com/hpjournal/96dec/dec96a9.pdf I have been trying months to find a reference on how it REALLY works but it seems that this is one of the better kept secrets of HP. Best regards Ulrich > -----Ursprungliche Nachricht----- > Von: [email protected] > [mailto:[email protected]] Im Auftrag von Brooke Clarke > Gesendet: Donnerstag, 10. April 2014 22:56 > An: Tom Van Baak; Discussion of precise time and frequency measurement > Betreff: Re: [time-nuts] GPSDO & Crystal Aging > > > Hi Tom: > > That makes sense because the GPS was just coming on line and > not anywhere near a full compliment of satellites and SA > was on. > HP had some way around SA that improved the timekeeping. > Has that ever been disclosed? > > Have Fun, > > Brooke Clarke > http://www.PRC68.com > http://www.end2partygovernment.com/2012Issues.html > > Tom Van Baak wrote: > > Hi Brooke, > > > > True, except that in most cases the long-term frequency > drift rate is > > so tiny compared to all the short- and mid-term instability > that it is > > not worth worrying about. In other words, I agree it is > modeled as a > > "linear ramp", but the ramp, even at huge timescales, is so > close to > > flat, what's the point? > > > > Look at the output of a typical OCXO. Short-term the > frequency varies > > by tens or hundreds of ps/s; that's parts in 10^11 or 10^10. By > > contrast, you have wait an entire day or week before you get that > > level of frequency error due to drift. > > > > When you're in a rowboat outside SF bay, it's the 3 m waves > every 5 to > > 10 seconds that you need to steer against, not the 3 m tides that > > occur gradually over 12 hours. > > > > Can someone show me a counter-example? Why is it better to include > > aging rate into the PID. What quantitative improvement in > performance > > does this actually represent? I don't disbelieve it, I just > have never > > seen the numbers. > > > > One case where knowing the aging rate is important is during > > multi-hour or multi-day holdover. Perhaps that's why HP > included the > > 128-hour circular record of frequency/aging into their firmware. > > > > /tvb > > > >> Hi: > >> > >> AFAICR the HP GPSDOs included the idea of measuring the > aging rate of > >> the crystal and applying that correction during holdover. This was > >> also mentioned by Brooks Shera in relation to his GSPDO > (there was a > >> plot), but I don't think it was part of the firmware? > >> > >> So rather than just locking the control voltage to the last used > >> value it would be much better to add a linear ramp. > >> <http://www.rt66.com/%7Eshera/> > > > > > > _______________________________________________ > > time-nuts mailing list -- [email protected] > > To unsubscribe, go to > > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
