[ntp:questions] NTP absolute accuracy?

John Hasler jhasler at newsguy.com
Tue Nov 3 01:40:37 UTC 2009


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.
-- 
John Hasler 
jhasler at newsguy.com
Dancing Horse Hill
Elmwood, WI USA




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