On 2010-02-10, Richard B. Gilbert <[email protected]> wrote: > David Lord wrote: >> BK wrote: >>> Details: >>> >>> First I installed NTP4.2.4p8 >>> Then I updated the binaries with your updates labeled 4.2.7p8 >>> Then I installed the serial PPS driver 20091228 >>> >>> Although for NTP purposes, you would like only one output string, I >>> have to output two NMEA strings because there is another device >>> looking at the serial stream also. I am outputting GGA and RMC >>> messages. According to the GPS manufacturer (I am using a Garmin >>> GPS15H) the PPS signal is applied just before outputting the NMEA >>> sentences that would be for that time period. I have the PPS signal >>> set to 80ms width. One oddity about my configuration is that the NTP >>> server will not be up 24x7. The machine will be booted, and I would >>> like the ntpd to discipline the local clock to a reasonable (+-10ms) >>> accuracy within 10 minutes. I have another machine that I will then >>> synchronize to the computer with the GPS. >> >> 10 minutes might be difficult from a cold start. How long from >> bootup to ntpd starting? How far out will clock be after bootup? >> I've been using "ntpd -q" before starting ntpd. That takes >> around 5 minutes before time is set to usually well within 10ms. >> After that ntpd is started and it's another few minutes before >> it's serving time from nmea and another few minutes before it's >> using pps to condition the clock. >> > > Ten minutes is not quite "asking for the impossible" but it comes very > close. NTPD needs about thirty minutes to get a reasonable > approximation of the correct time and ten to twelve hours to achieve the > accuracy of which it is capable! > > If you want the accuracy that the system is capable of, you run it 24x7! > > With a GPS timing receiver and a computer running 24x7 you can stay > within 100 microseconds or less! The GPS receiver is accurate to about > 50 nanoseconds; the difficulty is getting that time into a computer > while preserving accuracy.
Actually if the temp does not vary in the room and the computer does not have large spikes of heating ( due to wirking) you can get 1-2usec. I used to be able to get that easily. Ssomething has happened recently that I get a 10usec oscillation. Either periodic heating or something is going wrong with the system. If you were not on Windows, I would advise chrony, which has much faster convergence, but that does not help with Windows. _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
