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

[Paul Nicholson]

I am using the PPS from a Garmin GPS16 to timestamp a VLF
signal received directly via a PC soundcard.  Signal into one
channel, PPS into the other.

Using pulse centroid timing, I'm seeing about 0.5uS jitter of
the pulse-to-pulse interval, and an exponential moving average
with time constant 100 seconds is applied to smooth this out.

What I have left is a slow cyclic variation of the signal
timestamping.

It shows up when I measure the phase of an off-air signal,
eg here is MSF 60kHz from Anthorn.

  http://abelian.org/vlf/live/pp100701msfb.png

The same variation appears (in unison) on other off-air timing
signals, eg DCF and HBG both show the same cycle, so I guess
this is variation of the GPS PPS itself, not the VLF signal phase.

The period is about 3000 seconds and the amplitude corresponds
to about +/- 1.4uS.

Having no prior experience of using GPS PPS timing, I was
expecting some pulse-to-pulse jitter, but this slow variation
has caught me by surprise.  Question: is this typical of GPS
timing signals, or have I got a problem in my software?
--
Paul Nicholson
http://abelian.org
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