[time-nuts] GPS message jitter (preliminary results)
Gary E. Miller
gem at rellim.com
Mon Jul 18 19:20:09 UTC 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
offset.
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.
RGDS
GARY
---------------------------------------------------------------------------
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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