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

Rob Kimberley rk at timing-consultants.com
Thu Jul 1 11:42:44 EDT 2010


Interesting. 

The period of variation is approx. 50 minutes. Trying to think what that may
be related to. 

Would like to see the results taking the leading edge of the 1PPS also. I'm
assuming your location is fixed, and you are tracking sufficient SVs, as
I've seen something similar when moving.

Rob Kimberley

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of jmfranke
Sent: 01 July 2010 2:51 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] Long period variation of GPS PPS timing?

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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