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 <[email protected]>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 <[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. > -- Chris Albertson Redondo Beach, California _______________________________________________ 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.
