[ntp:questions] NTP absolute accuracy?
unruh-spam at physics.ubc.ca
Tue Nov 3 02:36:21 UTC 2009
John Hasler <jhasler at newsguy.com> writes:
>Bill Unruh writes:
>> The bandwidth of whatever marks it has to be pretty narrow, or the
>> transmitter would interfer with everything around it. Ie, that means a
>> large time uncertainty.
>The 60KHz WWVB signal is synchronously amplitude-modulated at a 1Hz
>rate. The exact cycle on which the amplitude changes can be locked in
>by a digital phase-locked loop. A VCXO can be phase-locked to the
>carrier to within a small fraction of a cycle. Thus with a strong
>signal the PPS error should be dominated by variations in propagation
>delay. In practice accuracies of +-100usec are reportedly achievable,
>but my very casual research indicate that even the fancy receivers don't
>bother to include the hardware necessary to do so.
>Too bad NIST can't find the money for the PRNG phase modulation DCF77
This is of course all made totally redundant by the GPS time delivery--
even makes up for the transmission delay. It begins to look like like
Morse code in the days of cell phones. Quaint, but not really useful.
Now, perhaps if the whole GPS system gets knocked out by the Chinese,
those using such ancient techniques will be kings of the time hill, but
I suspect we all will have other problems then.
>jhasler at newsguy.com
>Dancing Horse Hill
>Elmwood, WI USA
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