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

gonzo . cadbloke at hotmail.com
Fri Jul 2 07:17:58 EDT 2010



The GPS16 is not a timing grade GPS and only claims 1uS on the 1PPS.
Given this, I think 0.5uS jitter and +/- 1.4uS wander is not bad.

cheers,
ian

http://www8.garmin.com/products/gps16/spec.html





> 
> Message: 1
> Date: Thu, 01 Jul 2010 10:15:16 +0000
> From: Paul Nicholson <vlf0308 at abelian.org>
> Subject: [time-nuts] Long period variation of GPS PPS timing?
> To: time-nuts at febo.com
> Message-ID: <4C2C6AB4.9030404 at abelian.org>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> 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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