Hi If the objective is convergence to < 1 ms any timing optimized gps receiver will do just fine. Non-timing receivers are going to do all sorts of bizarre things every so often. I don't think we can blame this all on the GPS.
The NTP setup he's running in the article is broken. Setting the proper time offset for the GPS you have is part of basic configuration. He alludes to several other setup issues in his distribution. NTP is deliberately damped when things are messed up. Looking at the data, Timekeeper is going to do some really strange things when varying asymmetric delays are involved. That's what NTP is trying to *not* do.... Bob -----Original Message----- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Hal Murray Sent: Monday, February 22, 2010 5:03 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] TimeKeeper? > That said NTP is very conservative in validating the stability of > clock sources. I have not delved into the code, but it is obvious > that even a refclock like a GPS receiver doesn't get any favours. Why > should it? Who knows whether the clock is dodgy or not? The NMEA strings from low cost GPS units have a lot of noise/jitter. In particular, the SiRF units are horrible. (They are also low cost and widely available.) The time offset has a sawtooth pattern with a long time constant that would be nasty to filter out. Think of hanging bridges. http://www.megapathdsl.net/~hmurray/ntp/GPSSiRF-off.gif > However the times he was reporting for the offsets to drop to less > than 1ms did look excessive. I've seen lots of comments about ntpd being slow to converge. I haven't investigated carefully, but they seem credible. One way to get in trouble is to have a bad drift file. You can get that if you have a warm system, shut it down, wait for it to cool off, then restart it. -- These are my opinions, not necessarily my employer's. I hate spam. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.