[time-nuts] GPS for Nixie Clock

Bob Camp kb8tq at n1k.org
Fri Jul 15 14:40:16 EDT 2016


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.


> On Jul 15, 2016, at 2:23 PM, Gary E. Miller <gem at rellim.com> wrote:
> Yo Bob!
> On Fri, 15 Jul 2016 12:54:43 -0400
> Bob Camp <kb8tq at 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/
> ---------------------------------------------------------------------------
> Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
> 	gem at rellim.com  Tel:+1 541 382 8588

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