[ntp:questions] using both a PPS source and a HBG (dcf-77-like) timesource?
Unruh
unruh-spam at physics.ubc.ca
Mon Jan 14 23:32:11 UTC 2008
folkert at vanheusden.com (Folkert van Heusden) writes:
>> > day, but I think that was for long distances, bouncing off the
>> > ionosphere which changed height at night. I'd expect it to be
>> > pretty stable if you are close to the transmitter - ground wave.
>>
>> HBG is in Switzerland if I remember correctly, and from your post, you are
>> in Netherlands,
>correct
>> so you can estimate propagation delay. However, I don't see
>> why you would want to use an LF reference of lesser accuracy and estimated
>> delay when you can use GPS.
>> Just a thought.
>Well if I remember correctly someone said to me once that the
>time-string returned by cheap gps device (like my garmin 18 lvc)
>sometimes is a bit off while its PPS signal is fine.
Yes, the pps is stated to be accurate to better than a microsecond. The
interrupt time on your system when quiet is a few microseconds. The NMEA
response string is maybe good to a few msec at best ( even subtracting out
the "average" delan due to the length of time it takes to read the string).
Ie, the fluctuation in the string read time is probably like 10ms, a factor
of about 10000 worst than the pps.
Yes, 185 ms is about how long it takes to read the nmea string (about 70
characters as 4800 baud is about 1/6 of a second) YOu can tell ntp to
subtract off say 180ms but that would still leave you with a sizeable
jitter.
>Currently I'm syncing against NMEA/PPS and seeing quiet a big offset:
For what? This looks exactly like what I would expect.
> remote refid st t when poll reach delay offset jitter
>==============================================================================
>xGPS_NMEA(0) .GPS. 0 l 7 16 377 0.000 -185.08 1.001 <---
> PPS(0) .PPS. 0 l - 16 0 0.000 0.000 0.001 <---
>-muur.intranet.v .DCFa. 1 u 5 64 77 0.072 -9.207 0.023
>-thegateap.intra .SHM. 1 u 46 64 37 0.873 0.931 0.045
> SHM(0) .SHM0. 2 l - 64 0 0.000 0.000 0.001
> SHM(1) .SHM1. 2 l - 64 0 0.000 0.000 0.001
> SHM(2) .SHM2. 2 l - 64 0 0.000 0.000 0.001
> SHM(3) .SHM3. 2 l - 64 0 0.000 0.000 0.001
> NTP.MCAST.NET .MCST. 16 u - 64 0 0.000 0.000 0.001
> 192.168.64.0 .BCST. 16 u - 64 0 0.000 0.000 0.001
>-auth1.xs4all.nl 194.109.22.19 3 u 48 64 77 7.945 1.514 1.158
>-auth2.xs4all.nl 193.67.79.202 2 u 46 64 77 9.450 -0.893 0.713
>+ntp1.nl.uu.net .GPS. 1 u 45 64 77 13.693 0.173 9.097
>*chime2.surfnet. .GPS. 1 u 111 64 76 12.262 0.387 1.092
>-superboer12.stu 193.79.237.14 2 u 45 64 77 13.012 4.401 2.248
>+ntp.networking4 193.190.230.65 2 u 46 64 77 9.242 0.444 0.494
>+mailer.zylom.co 193.190.230.65 2 u 44 64 77 9.040 0.430 0.538
>-aivd.xelerance. 193.0.0.228 2 u 41 64 77 8.881 -0.610 3.154
What are you doing? Why all of these sources?
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