[time-nuts] PPS offset between GPS receivers

Gabs Ricalde gsricalde at gmail.com
Tue Dec 11 13:00:24 UTC 2012

On Tue, Dec 11, 2012 at 8:00 PM, David J Taylor
<david-taylor at blueyonder.co.uk> wrote:
> From: Gabs Ricalde
> []
> David,
> 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?
> ============================================
> Agreed, that if your antenna has a good view of the sky you should not be
> seeing these drop-outs, at least not on a regular basis.  There are
> atmospheric conditions which will affect the GPS signals, though, but they
> should be rare.  No chance you get a trucker with a GPS jammer driving by at
> the problem times, I suppose?
> You say LOCAL and PPS - no "seconds" reference such as GPS/NMEA?
> You might also want to check what's happening on the box at the time of the
> jump.  If it's regular, perhaps some scheduled task is the cause?
> Cheers,
> David
> --
> SatSignal Software - Quality software written to your requirements
> Web: http://www.satsignal.eu
> Email: david-taylor at blueyonder.co.uk

I'm not sure about the jammer but I'm running a timing receiver in
position hold several floors up, I haven't seen dropouts like this.

ntpd is running with a "noselect" NMEA source since I'm having problems
with ntpd marking the PPS and NMEA as falsetickers. The startup sequence
for the server is this:

* run ntpd -g -q with NMEA enabled
* run chronyd for 3 minutes to set the time and frequency offset
* copy the current frequency to ntpd's drift file, then run ntpd with
NMEA disabled

This hack seems to work everytime with ntpd ready in less than 4 minutes
after turning on. I just hope nothing would happen that changes the

I have seen 0.2 us spikes every hour from some unknown task but the
larger spikes are rare. Another device running the same OpenWrt firmware
but with a timing receiver has only the small periodic spikes.

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