[time-nuts] PPS offset between GPS receivers

Gabs Ricalde gsricalde at gmail.com
Tue Dec 11 11:38:33 UTC 2012

On Tue, Dec 11, 2012 at 5:55 PM, David J Taylor
<david-taylor at blueyonder.co.uk> wrote:
> Gabs,
> I've seen similar jumps, and it happens when the GPS/PPS signal drops out
> for a while.  In my case, the GPS receiver is sitting just in an upstairs
> room, not near a window or the root (as I normally have my other receivers).
>  http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html#u-blox
> Watching a low-current LED connected to the PPS signal (through an
> appropriate 680-ohm current-limiting resistor), I sometimes see the PPS stop
> flashing.  If it stops for long enough, NTP will revert to a different
> reference.  The hot-start may also result in a re-acquisition, and hence a
> break in the PPS pulses.
> I now need a 10-channel 'scope, as I have Rapco 1804M, Trimble Resolution
> SMT, a couple of Garmin GPS-18/x, Sure Electronics boards, and the u-blox
> (navigation) receivers shown above.  Comparing the PPS outputs they are all
> within about 100 ns, more or less.  <G>  Quite adequate for running NTP on
> my systems.
> Cheers,
> David
> --
> SatSignal Software - Quality software written to your requirements
> Web: http://www.satsignal.eu
> Email: david-taylor at blueyonder.co.uk


I forgot to thank you for your helpful site and NTP plotter.

I have the antenna outside with a 180 degree view of the sky, outages
should be rare. Looking at the loopstats, the outage during the 4 us
jump is about 12 seconds. This is a test server, I only have the LOCAL
and PPS refclocks configured so during the outage the clock just
flywheels. I agree these cheap GPS receivers are more than enough for a
stratum 1 NTP server.

How about a 10 channel TIC? I'm sure someone could suggest a way to do
it, probably several PICTIC II's?

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