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 <[email protected]> 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 >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
