[ntp:questions] best gps receiver for time synchronization

Terje Mathisen "terje.mathisen at tmsw.no" at ntp.org
Thu May 28 07:10:02 UTC 2009

Unruh wrote:
> All I was saying was that just because the time routines are capable of
> recording time with nsec precision does not mean that that accuracy of
> the clock is anywhere near that, and I have great difficulty believing
> that the accuracy is any better than usec.

You are somewhat correct: Those ntp servers that do run at better than 
us precision/accuracy need some additional effort, i.e. phk's replacing 
the motherboard crystal with OCXO/TCXO/Rb oscillators, and then using an 
FPGA to measure the offset between the arrival of the PPS signal and the 
dispatch of the interrupt handler.

Even without the FPGA it is possible to do significantly better than 
1us, if you let the interrupt handler twiddle an output pin so that you 
can use an oscilloscope to measure the average latency, and then fudge 
that away.

NTPD with a PPS signal does not need all interrupts to be handled very 
quickly, only that a large fraction of them have short & relatively 
stable service time. Statistics will do the rest.

- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

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