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
command.
I saw this report showing large differences between the PPS outputs of
different receivers:
http://www.gmat.unsw.edu.au/snap/publications/mumford_2003a.pdf
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
yet.
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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: [email protected]
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