On Fri, May 25, 2012 at 2:51 PM, unruh <[email protected]> wrote: > On 2012-05-25, Chris Albertson <[email protected]> wrote: > > On Fri, May 25, 2012 at 12:29 AM, Terje Mathisen <"terje.mathisen at > > tmsw.no"@ntp.org> wrote: > > > >> Chris Albertson wrote: > >> > >>> Next how to get the PPS to be at the top of the second. Two methods > >>> > >>> 1) don't bother. Linux PPS will log the time of both the GPS's PPS > >>> and the Rb PPS. Read the log file to determine the offset. This give > >>> offset with uSec precision. To get better you need a hardware time > >>> interval counter. I happen to have a few of those and can read > >>> sub-nanoseconds but this is gross overkill. Microseconds are good > >>> enough for NTP...... > >>> > >> > >> Much simpler would be to use a fudge command to specify the current > phase > >> offset between the Rb and GPS PPS signals, i.e. if the Db happened to > start > >> out 367 ms off, just let ntpd know that these pulses are supposed to > have > >> such an offset. > >> > >> > > Yes, correct the "fudge" method is what I was getting at in #1 above. I > > assumed that every on here would know to use the measured difference in a > > "fudge" but the key is measuring the offset. Get that from the log > file. > > or a TIC if you have one. > > > > But there is a problem in the even with the best Rb the offset will drift > > over time. > > Yes. but slowly (probably about 1 sec per millenium or less) >
You are close. My envelope back says 30,000 years to gain a second. But this also means you can run a $40 Rb oscillator in cave, disconnected from the Internet or GPS for 30 years and only see a 1 ms error. The unit is about the exact size standard computer hard drive. When I said "it drifts" I was thinking of using this as a frequency standard, not so much for a GPS backup. In that case I'm concerned with micro second level errors so it needs to be corrected daily for drift. > Chris Albertson > Redondo Beach, California _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions -- Chris Albertson Redondo Beach, California _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
