[time-nuts] GPS accuracy specs
Richard H McCorkle
mccorkle at ptialaska.net
Sun Feb 16 16:31:04 EST 2014
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.
> 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
> 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.
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