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

unruh unruh at invalid.ca
Sun May 26 21:25:28 UTC 2013


On 2013-05-26, Miguel Gon?alves <miguel.barbosa.goncalves at gmail.com> wrote:
> 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.

Lan will have delays of the order of .15ms (worse for gigabit) with
accuracy of the order of 10-20us. Direct PPS will be of order 2-5us, but
only if you have only one. If you have more than one, then they get in
the way of each other ( the interrupts all arrive at the same time, and
each has to wait for th other to finish which is about 10us. )


>
> 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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