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

David Lord snews at lordynet.org
Wed Feb 10 02:39:04 UTC 2010

Richard B. Gilbert 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.

Target was given above as +/- 10ms so just using
"ntpd -q" will get there but here that seems to take
between 5-8 minutes. Then after starting ntpd it's
about another 4 minutes before ntpd is giving out
time. As you say, it takes a few hours to reach
offsets in low us. If an ntp server is configured
with iburst, the startup time can be even faster.

These are from a restart yesterday morning:

09:54:40 clock PPS(0) 'clk_noreply' # ntpd started
09:58:57 synchronized to GPS_NMEA(0), stratum 0
10:00:18 synchronized to PPS(0), stratum 0

                                 offset  jitter

10:00:00   PPS(0)      .PPSb.    1.538   1.100
10:06:00  oPPS(0)      .PPSb.    1.847   0.091
10:12:00  oPPS(0)      .PPSb.    1.487   0.064
10:18:00  oPPS(0)      .PPSb.    1.241   0.043
10:24:00  oPPS(0)      .PPSb.    0.456   0.131
12:54:00  oPPS(0)      .PPSb.    0.054   0.002
15:54:00  oPPS(0)      .PPSb.    0.020   0.002


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