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

David Taylor david-taylor at blueyonder.co.uk.invalid
Thu Feb 14 07:43:31 UTC 2013


On 13/02/2013 09:00, 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?
>
> /ralph

Ralph, I have compared a number of consumer-level timing and navigation 
GPS/PPS devices and they are usually with 100 ns, and typically quite a 
bit closer.  I haven't been able to compare against true UTC, but as 
they are that close on several different manufacturer' devices I have no 
reason to think that they are far from UTC.  At this level, BTW, the 
circuitry controlling the rise-times of the PPS output matters when 
comparing.

What the time is /inside/ the Raspberry Pi is another question.  How 
long does it take from PPS input to reading the counter?  I've seen 
figures for the Raspberry Pi in excess of 10 microseconds.  So although 
jitter may be reported as a few microseconds (or zero on one of my 
Raspberry Pis), the virtual clock inside the PC, the time which NTP has, 
may be ten microseconds or more away from "real" time.

For my applications it doesn't matter, so it doesn't concern me, but 
maybe it would concern others who may know more about this issue than I do.
-- 
Cheers,
David
Web: http://www.satsignal.eu



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