[ntp:questions] how to have offset < 1ms

unruh unruh at wormhole.physics.ubc.ca
Thu Apr 15 04:58:51 UTC 2010

On 2010-04-15, nemo_outis <abc at xyz.com> wrote:
> John Hasler <jhasler at newsguy.com> wrote in 
> news:874ojdmx12.fsf at thumper.dhh.gt.org:
>> Richard B. Gilbert writes:
>>> One possible solution is using radio controlled clocks.  I have a
>>> wristwatch that uses a radio signal to correct itself.  I also have a
>>> wall clock that does the same.  Both work very well.
>> They won't work very well inside a steel ship or far out to sea.  They
>> use VLF (60kHz/77kHz) signals that only propagate a thousand miles or
>> so and not at all through steel.
>> In any case it is clear that the ACEB box can provide the performance he
>> needs if he can only communicate with it.  He has a networking problem,
>> not a timekeeping problem.
> Spot on!
> Regards,
> PS  Which means we can now dispense with much of the speculation regarding 
> GPS and its putative implementation, howsoever configured.

Not clear what you thought was spot on. Radio control is not GPS. It is
true that the equipment ( at a cost of maybe a 1000 times that of GPS)
can keep time, but so far the evidence is that it is useless as it
cannot actually report that time at all accurately. Whether it is
network, or the actual ntp card in the box we do not know ( the pings
indicate it is not a network problem) Again, putting a laptop on the
same network as the box, and doing a tcpdump on the ntp packets could
tell you if the delay is in the network or in the box. (If t3-t2 is
29ms, it is in the box. If t3-t2 is 1usec, it is in the network)


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