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

unruh unruh at invalid.ca
Wed Feb 13 17:15:53 UTC 2013


On 2013-02-13, Ralph Aichinger <ralph at pangea.at> wrote:
> Terje Mathisen <"terje.mathisen at tmsw.no"> wrote:
>> Unless you replace the motherboard clock source, anything below ~us 
>> precision is wasted.
>
> Good to know. So basically the 1 or 2??s jitter I get after everything
> has had time to settle down is the maximum archievable unless you
> use rather exotic hardware.
>
>> 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?
>
> That is great, so basically jitter is a non-issue compared to standard
> computer hardware, and I am pleasantly surprised that the Sure board
> is that good. 
>
> Has anybody checked the PPS output for constant shift against
> UTC? Is it possible that the pulses are off with a constant offset,
> even if they have very little jitter? Or is this unlikely from what
> happens inside the receiver?

If it were out by 1us, it would put you out by almost a km on the
surface of the earth.
>
> /ralph -- I am waiting for the first tutorial how to build a 
>           rubidium-disciplined Raspberry Pi ;)



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