Hi Well, if you actually sit down and measure the serial timing on a modern GPS module, you will find that it is very consistent. That degrades quickly if you enable a few dozen sentences. Sub 10 ms accuracy is not a tough target to hit when standing still.
You *will* need to work out the delay between your chosen character in the string and the signal on the clock. If you are talking about a Nixie based device, there are lots of “many ms” sort of delays to track down and calibrate out. You measure them, come up with a fudge factor, and stuff that into your code. Once you have the number, it’s not likely to change. Bob > On Jul 15, 2016, at 2:23 PM, Gary E. Miller <g...@rellim.com> wrote: > > Yo Bob! > > On Fri, 15 Jul 2016 12:54:43 -0400 > Bob Camp <kb...@n1k.org> wrote: > >> To get a time resolution of 10 ms (yes 10X 1 ms), you don’t really >> need the pps. The timing of the serial string is probably “good >> enough”. > > Very unlikely you can get 10 ms from the serial timing. And as long > as you are not overflowing your buffer with serial data the number > of sentences does not matter. > > You are looking at the time in the first sentence of the second to > grab a time stamp. > > In terms of absolute accuracy (offset), a few GPS start the first second > before the end of the second, some as late at 900 ms, or more, into the > second. > > In terms of jitter, the first GPS sentence may jitter up to +/- 400 ms > over a few hours. The GPS has a small CPU and a lot of floating point > math to do. As sats come and go the time to compute a fix, and output > the first sentence, varies a lot, in unpredictable ways. > > My guess is that with most consumer GPS, when you have the average delay > dialed in, you'll have a tough time getting much better than +/- 100 ms. > > I have pretty graphs of several weeks of data for several GPS types > if you want to see the actual data. > > For example, I have an Uputronics HAT here: > https://pi3.rellim.com/week/ > > The offset of the GPS time from PPS time is 80 ms. The jitter is just > under +/- 30 ms. > > On an Adafruit HAT the offset about 250 ms and jitter is almost +/- 400 ms: > https://pi2.rellim.com/week/ > > On an MR-350p the offset is 80 ms and jitter is just under +/- 150 ms: > https://rellim.com/graphs/week/ > > YMMV. > > RGDS > GARY > --------------------------------------------------------------------------- > Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703 > g...@rellim.com Tel:+1 541 382 8588 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.