[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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