[time-nuts] Long period variation of GPS PPS timing?

Paul Nicholson vlf0308 at abelian.org
Fri Jul 2 04:05:17 EDT 2010

This morning I waited for the VLF timing signals to settle
into their steady daytime phase - still showing the slow
cycle of phase variation.

Then I power-cycled the GPS, leaving everything else running.

During the 25 seconds it takes the GPS to begin sending PPS
again, my software continues to time the VLF signal using the
average PPS pulse interval measured over the previous 100 or
so seconds.

See the phase plot of DCF 77.5kHz for the result,


GPS rebooted at 16686 seconds, just after the slow cycle started
its upward ramp.  It seems to have caused a phase jump in the
slow cycle - it switched to starting a downward ramp.

To confirm, I rebooted the GPS again at 24915 seconds, as it was
about half way down a downward ramp.  Again a step change of
phase, this time it restarted at the bottom of an upward ramp.

Just to eliminate any effect in my software, I unplugged the
PPS signal from the soundcard for 30 seconds, leaving the GPS
running.   My software is starved of PPS for the same length
of time as during the GPS reboots.   This occurred at 27440
seconds, halfway along a downward ramp.  The ramp continued

Just to 'shake the bag' of temperature effects, I opened the
door of the cabinet containing the PCs and electronics, this
quickly dropped the temperature by 10 C.  Also took out
the unit containing the RC pulse forming, took its lid off
and left it in the open air.  Result: no noticeable change to
the slow cycle.

Well now, this sure looks to me like a GPS effect.
I think I'd better order another GPS, a different type,
maybe a GlobalSat MR-350P, or something, for comparison.

Will also try to move the GPS16 so that it can see more
of the sky - only about 50% visible at the moment, some
trees block the other half.

Paul Nicholson

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