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
David
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