[time-nuts] GPS accuracy specs

Jimmy D. Burrell jimmydburr at gmail.com
Sun Feb 16 19:33:54 EST 2014


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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> 
> 
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