[time-nuts] GPS accuracy specs

Chris Albertson albertson.chris at gmail.com
Sun Feb 16 22:45:17 EST 2014


Yes,  "Sawtooth" is data sent over the serial port.    Actually it is not a
sawtooth until you plot its value over time.   When you are applying it, it
is just a single number.


On Sun, Feb 16, 2014 at 4:33 PM, Jimmy D. Burrell <jimmydburr at gmail.com>wrote:

> Richard,
>
> I confess, heretofore when I saw "sawtooth" conversations, and tried to
> follow the thread, my eyes would glaze over after one or two emails. I
> presumed they were strictly discussing some RC constant
> circuit-design/tradeoffs type "sawtooth".
>
> Your explanation 'lit the light bulb' for me on this topic.
>
> Thanks for your response.
>
> Jim...
> N5SPE
>
> On Feb 16, 2014, at 3:31 PM, Richard H McCorkle <mccorkle at ptialaska.net>
> wrote:
>
> > Jim,
> >
> > Generally navigation receivers don't include survey and position hold
> > features so the time solution accuracy is typically about +/- 1us.
> > Timing receivers survey their position over a large number of samples
> > (typically 10,000) and go into position hold mode once the survey
> > completes. The fixed position allows higher accuracy in determining
> > the time solution, typically to +/- 1ns. However the 1PPS output is
> > placed on the nearest GPS clock edge, typically derived from an XO,
> > so the pulse placement resolution is limited by the GPS clock period.
> >  The GPS XO clock drifts so the 1PPS placement also drifts over the
> > clock period, creating a "sawtooth" like displacement in time over
> > the GPS clock period. With a receiver like the M12+ the placement
> > varies roughly +/- 12ns for a 25ns 1 sigma 1PPS accuracy. For better
> > accuracy the M12+ also includes a message with the predicted 1PPS
> > placement error of the next pulse to the +/- 1ns time calculation
> > resolution. The combination of the 1PPS placement to the nearest
> > clock edge and the sawtooth correction message giving the placement
> > error allows resolution of the GPS time to +/- 1ns using either a
> > software correction of the sample data or hardware correction of
> > the 1PPS pulse using a variable delay.
> >
> > Richard
> >
> >
> >> 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?
> >>
> >> 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.
> >>
> >> Jim...
> >> N5SPE
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-- 

Chris Albertson
Redondo Beach, California



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