[ntp:questions] NTP absolute accuracy?

David Lord snews at lordynet.org
Mon Nov 2 10:23:48 UTC 2009


Unruh wrote:
> "David J Taylor" <david-taylor at blueyonder.not-this-bit.nor-this.co.uk.invalid> writes:
> 
>> "Unruh" <> wrote in message news:IEbHm.50125$Db2.41863 at edtnps83...
>> []
>>> MSF?
> 
>> See:
>>  http://www.npl.co.uk/science-technology/time-frequency/time/products-and-services/msf-radio-time-signal
> 
>> UK 60KHz radio time signal.
> 
> Ah, OK. Its accuracy is probably not much better than a few msec I assume. 
> GPS is a few usec. Since the network gives accuracies of better than a few ms, MSF
> is not a good way of testing the accuracy of network set clocks. 
> 

Hi

I expected to get away with it from trying upstairs:

hrs   % offset jitter   % offset jitter   % offset jitter
            (ms)  (ms)        (ms)  (ms)        (ms)  (ms)
100   50  0.185 0.086   95  0.642 0.255  99.5 1.111 0.488  frame aerial
  70   50  0.362 0.172   95  0.802 0.370  99.5 1.057 0.495  ferrite-rod

With PPS driver added
hrs   % offset jitter   % offset jitter   % offset jitter
            (ms)  (ms)        (ms)  (ms)        (ms)  (ms)
  26   50  0.165 0.070   95  0.621 0.246  99.5 0.673 0.333  frame aerial

The % refers to logging at 30min intervals so 50% of offsets were
at or under 165us.

Having moved the receiver with ferrite-rod downstairs it's not so
good, I'm not yet setup for PPS driver.

hrs   % offset jitter   % offset jitter   % offset jitter
            (ms)  (ms)        (ms)  (ms)        (ms)  (ms)
26    50  0.561             1.095             1.344

For same system using ntpd and internet from before using MSF:
hrs   % offset jitter   % offset jitter   % offset jitter
            (ms)  (ms)        (ms)  (ms)        (ms)  (ms)
96    50  1.535 0.467   95  4.404 1.672  99.5 4.850 4.889

the other two servers from internet are better.
96    50  0.642 0.610   95  3.726 2.258  99.5 4.959 2.916
96    50  0.507 0.520   95  1.329 0.811  99.5 2.588 2.030


I'll try swapping to frame aerial and PPS module tonight but
I think I'm in a leaky faraday cage with wire mesh under the
plaster on the walls so for anything better will have to wait
until I've finished the units that will be mounted on mast
at rear of house.

David




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