[ntp:questions] How to determin hardware latency for PPS offset given simple tools.

Miguel Gonçalves miguel.barbosa.goncalves at gmail.com
Sun May 26 11:14:37 UTC 2013


Hi Paul!

On Fri, May 24, 2013 at 9:28 PM, Paul G <bodosom at gmail.com> wrote:
> I have four GPS receivers into four different computers.  I've assumed that I
> should be able to get small offsets between them but to do so I need
> to set time1.  If that's the correct approach how do I pick the "right"
> system to be the benchmark.  So far I've just been assuming that
> the purpose built M. Tharp server should have the lowest latency but I'm not sure -- in any case, at present, I can't adjust it.
>
> Each machine in the truncated table below (except 210) has a time1
> value derived in an ad hoc fashion.
>
> l     r    refid       st   delay   offset  jitter
> ==================================================
> 1   o22.0  .PPS.        0   0.000   -0.001   0.001
> 2   +244   .PPS.        1   0.068    0.000   0.006
> 2   *210   .GPS.        1   0.514   -0.008   0.136
> 2   +192   .PPS.        1   0.455    0.008   0.032
>
>
> I feel as if this topic should be clearly explained somewhere but I've been
> unable to find it.

My recent experience with 4 stratum 1 servers (one of them an
appliance from Meinberg - great box BTW) is that you will never have
every server in sync with each other due to the latency and delays of
the network (even LAN, same segment as is in this case). I don't depend
on external NTP servers.

My Meinberg NTP server:

$ ntpq -p 10.0.2.1
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 LOCAL(0)        .LOCL.          10 l    8   16  377    0.000    0.000   0.001
+GENERIC(0)      .GPS.            0 l   13   16  377    0.000    0.000   0.001
oPPS(0)          .PPS.            0 l    7   16  377    0.000   -0.001   0.001
-10.0.2.2        .PPS.            1 u   37   64  377    0.199    0.007   0.031
-10.0.2.3        .PPS.            1 u    6   64  377    0.171   -0.004   0.008
+10.0.2.4        .PPS.            1 u   51   64  377    0.157   -0.002   0.009

My Garmin 18 LVC on the roof:

$ ntpq -p 10.0.2.2
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
oGPS_NMEA(0)     .PPS.            0 l    5   16  377    0.000    0.000   0.004
+10.0.2.1        .PPS.            1 u    2   16  377    0.157    0.005   0.013
+10.0.2.3        .PPS.            1 u    1   16  377    0.144   -0.001   0.004
+10.0.2.4        .PPS.            1 u   16   16  377    0.140   -0.002   0.004

My Sure GPS board on the window sill:

$ ntpq -p 10.0.2.3
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
oGPS_NMEA(0)     .PPS.            0 l   13   16  377    0.000    0.001   0.004
+10.0.2.1        .PPS.            1 u   10   16  377    0.166    0.003   0.008
+10.0.2.2        .PPS.            1 u    5   16  377    0.138    0.004   0.004
+10.0.2.4        .PPS.            1 u    8   16  377    0.140    0.001   0.004

My 2nd Sure GPS board on the same window sill:

$ ntpq -p 10.0.2.4
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
oGPS_NMEA(0)     .PPS.            0 l    7   16  377    0.000    0.000   0.004
+10.0.2.1        .PPS.            1 u    4   16  377    0.201    0.000   0.011
+10.0.2.2        .PPS.            1 u    3   16  377    0.136    0.002   0.004
+10.0.2.3        .PPS.            1 u    2   16  377    0.140    0.001   0.004

For me < 10 us offsets is OK and acceptable between stratum 1 machines.

I believe this is what is expected.

Regards,
Miguel


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