[time-nuts] PPS offset between GPS receivers

David J Taylor david-taylor at blueyonder.co.uk
Tue Dec 11 09:55:43 UTC 2012

I'm now using the SKG25A1 as a PPS source for an NTP server. Aside from
the offset, I noticed a large offset jump in the NTP loopstats
(attached) occurring about once a day. This is not the server oscillator
drifting since the frequency graph looks good at this point and this
behavior can sometimes be triggered by issuing an NMEA hot start

I saw this report showing large differences between the PPS outputs of
different receivers:
I'm planning on getting two receivers for comparison, an iLotus M12M and
a Synergy SSR-6T (preferably with the discount). No replies from both


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).


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.

SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-taylor at blueyonder.co.uk 

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