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

jmfranke jmfranke at cox.net
Thu Jul 1 09:50:58 EDT 2010

I am not sure about the Garmin GPS16, but all of the GPS receivers I use do 
not rigidly define the falling edge of the 1PPS signal.  Perhaps you should 
use only the leading edges for the pulse-to-pulse interval measurement.

John  WA4WDL

From: "Paul Nicholson" <vlf0308 at abelian.org>
Sent: Thursday, July 01, 2010 6:15 AM
To: <time-nuts at febo.com>
Subject: [time-nuts] Long period variation of GPS PPS timing?

> 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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