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
pgpiGR_dzzMMN.pgp
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