[ntp:questions] How do I validate my PPS clocks?
David Taylor
david-taylor at blueyonder.co.uk.invalid
Mon Feb 25 10:57:13 UTC 2013
On 24/02/2013 23:11, 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.
>
> 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?
>
> Is there a sensible to way to make this assessment purely within the
> NTP system?
>
> 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.
For comparison, my results with a number of different stratum-1 servers
are here:
http://www.satsignal.eu/ntp/ntp-offsets.txt
This includes FreeBSD, Linux, and Windows stratum-1 servers.
Unless those PCs are very slow, I would suggest that the time1 value
should be well under 1 millisecond, and likely under 50 microseconds.
My own results mostly show a positive offset for remote servers, which I
put down to asymmetry on my consumer Cable Modem connection (30 Mb/s
down, 3 Mb/s up).
The remote servers are a mixture of pool and other servers, some
stratum-2 and some stratum-3. Be aware that some of the "national"
stratum-1 servers can be rather overloaded, producing poorer performance
than the better stratum-2 servers. Equally, servers in the pool may
well be not as good as the best stratum-2 servers. So it's a little pot
luck!
Does that help?
--
Cheers,
David
Web: http://www.satsignal.eu
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