[ntp:questions] How do I validate my PPS clocks?

unruh unruh at invalid.ca
Mon Feb 25 00:15:32 UTC 2013


On 2013-02-24, bodosom at gmail.com <bodosom at gmail.com> wrote:
> I can't find or figure out how to validate my ntp results.
>
> I (currently) have two Linux boxes with PPS via the NMEA driver from
> Garmins (18 & 18x) and a Sure board connected to a purpose built
> NTP server.  When I set up my first Garmin all of my remote offsets
> were negative which seemed unlikely so I set time1 to 5.3ms which
> also seems unlikely but  which brought me into congruence with some
> nearby stratum 1 servers.

They could certainly all be negative if the time from your computer to
the remote ones are much less than the time from the remote to the
local. 


>
> Below is the output* from an intermediate machine on my home network
> (3Mb ADSL) which is temporarily connected to several servers.
>
> 192.168.0.2 is running with time1 5.3ms.
> 192.168.0.244 is running with time1 132us.
> 192.168.0.210 is the appliance and has currently has no adjustments.**
>
> Should I expect /all/ (ignoring the high jitter ones) of the remote
> clocks to have negative offsets?

Perfectly possible. 

>
> Is there a sensible to way to make this assessment purely within the
> NTP system?

Sure, just as you did you put a local pps on your system which should
give you a time accurate to a few microseconds.
Your pps look like they are just what you would expect if the raw pps
were accurate. remove the time1 adjustments. 
(Note that it is possible that you are using the wrong side of the pulse
for the PPS.)



>
>      remote           refid      st t when poll reach   delay   offset  jitter
>==============================================================================
> +192.168.0.2     .GPS.            1 u   14   16  377    0.109   -5.429   0.006
> *192.168.0.244   .GPS.            1 u    1   16  377    0.112    0.000   0.008
> +192.168.0.210   .GPS.            1 u   15   16  377    0.567   -0.001   0.007
> -216.66.0.142    130.207.244.240  2 u   54   64  377   63.526   -7.131   0.192
> -38.229.71.1     204.123.2.72     2 u   25   64  377   67.175   -4.368   0.472
> -63.240.161.99   72.26.198.233    3 u   19   64  377   70.797   -3.284   0.320
> #204.235.61.9    128.105.201.11   2 u   17   64  177   73.331   -9.732   0.426
> -216.229.4.69    216.229.0.179    2 u    -   64  377   78.848   -4.987   1.337
> -199.7.177.206   64.147.116.229   2 u   13   64  377   86.072   -7.451   0.235
> #2604:180::8138: 164.244.221.197  2 u   56   64  377   91.521  -33.681  25.573
> -198.101.234.139 128.138.140.44   2 u   35   64  377   97.360   -2.065   0.584
> #2604:2880::870: 139.78.135.14    2 u   23   64  377  115.057  -12.785  22.738
> -207.7.148.214   204.123.2.5      2 u    2   64  377  123.982   -8.091   0.747
>
>
> *This billboard is sorted by delay.
> **You'd have to recompile the control software.



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