[ntp:questions] How exact are the PPS pulses of common GPSses?

Terje Mathisen "terje.mathisen at tmsw.no" at ntp.org
Wed Feb 13 13:07:30 UTC 2013

Ralph Aichinger wrote:
> After having my Raspberry Pi with PPS running, with an
> offset and jitter value in the region of 1 to 5 microseconds,
> I am wondering: How precise is the PPS signal of typical
> consumer hardware (MTK chips, etc.)?
> Has anybody hooked up the PPS output of different consumer
> GPSes to an oscilloscope and compared it against eachother
> with a precision clock?
> Is the precision of the PPS pulse higher than what
> the computer/ntpd daemon can usefully handle, or is
> the PPS pulse the limiting factor?

Unless you replace the motherboard clock source, anything below ~us 
precision is wasted.

OTOH, if you do have a surplus Rb, plus a hw latency timer, then you can 
make use of the best of the cheap timing receivers, i.e. 15-50 ns.

Check out what Poul-Henning Kemp (phk (at) freebsd.org) has written 
about ntp servers!

I have seen reports that the original Motorola Oncore UT+ had something 
like 30 ns RMS jitter (after applying the sw saw-tooth correction), 
while the $35 Sure board has been tested by one of the time-nuts people 
at ~25 ns?


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

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