[ntp:questions] NTP.conf using Dave Hart code.

Richard B. Gilbert rgilbert88 at comcast.net
Wed Feb 10 01:07:00 UTC 2010

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.

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