[time-nuts] GPS accuracy specs

[Jim Lux]

On 2/16/14 11:55 AM, Jimmy D. Burrell wrote:
> I've looked at several different manufacturer GPS datasheets now
> regarding the 1 PPS output in an attempt to compare apples to apples.
> Some of them rate their 1 PPS output as something on the order of
> "PPS signals have an accuracy ranging 10ns" which seems ambiguous.
> Does that mean the leading edge of their 1PPS is within 10ns of the
> GPS clock? Or simply that the stability of their 1 PPS is within
> 10ns? Or both?
>
> Perhaps there's an industry standard for these specs of which I'm
> unaware?

Hah..
It's read the specs carefully and ask questions.

>
> The datasheet for my (presumably much older) Globalsat ER-102 seems,
> to me at least, to be much more clear stating "time reference at the
> pulse leading edge aligned to GPS sec., +/- 1 us". Which I interpret
> as the leading edge of my receiver's 1PPS is aligned with the GPS's
> clock to within +/- 1 us.


And is that "always" within 1 us? or 90% or 2 sigma or ??

ANd is that spec assuming some accuracy of the underlying GPS.  At 1 us, 
it probably doesn't matter, but at 10ns, you start worrying about 
position uncertainty.