[time-nuts] GPS message jitter (preliminary results)

Gary E. Miller gem at rellim.com
Mon Jul 18 15:20:09 EDT 2016

Yo Mark!

On Mon, 18 Jul 2016 18:30:08 +0000
Mark Sims <holrum at hotmail.com> wrote:

> A few generalizations:   most receivers get the time messages out
> within a window less than 50 milliseconds wide with a standard
> deviation of less than 10 milliseconds.

How long did you run your tests?  My experience is that the GPS may be
stable for hours, then have a sudden shift in offset to a new stable

I would hope your tests were at lesat 24 hours long.

> The Navspark Venus based receivers have the greatest differences..  

Yes, they can be horrible.  But there is a soultion.  There is
a configuration option to tell the Navspark GPS to try to output
near the top of the second.  Then the results are much better.

> Surprisingly, the communications channel type is not that
> important.   I've seen little difference between hardware serial
> ports and a USB / RS-232 adapter.

How long did you run your tests?  I have also found that USB 1.1 NMEA
can be very stable. Maybe +/-100 microSeconds or better instead of the
expected +/- 500 microSeconds.  This is because the 1024/samples per
second on the USB port is linked to the system clock.  But after a few
days of solid performence the USB serial time may shift 1/1024th of a
second and stay at this new stable point.

You will find when mearusing the PPS that the Serial/USB difference is
much larger.  Maybe 1,000x or more.

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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