[ntp:questions] NTP absolute accuracy?

Richard B. Gilbert rgilbert88 at comcast.net
Tue Nov 3 12:10:24 UTC 2009

John Hasler wrote:
> 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
> has.

Perhaps the "NAVSTAR" satellites and GPS provide more/better results 
than a VLF broadcast.

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