[time-nuts] Strange 100ns jumps on Motorola M12+T

[Joseph Gwinn]

On Thu, 21 Nov 2013 03:55:57 -0500, time-nuts-request at febo.com wrote:
time-nuts Digest, Vol 112, Issue 64
> 
> Message: 6
> Date: Thu, 21 Nov 2013 10:49:43 +0200
> From: Stephan Sandenbergh <ssandenbergh at gmail.com>
> To: Discussion of precise time and frequency measurement
> 	<time-nuts at febo.com>
> Subject: Re: [time-nuts] Strange 100ns jumps on Motorola M12+T
> Message-ID:
> 	<CADbj3vZ4BQTbkuDqYKCQGxaQ1GLjOraksrL+d8TDp9Y7gkYxBw at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
> 
> Hi,
> 
> I'm using three Motorola GPS Timing 2000 Antennas. They are co-located on
> the roof of our building with a clear view of the skies. The view is almost
> 360deg but a mountain is blocking off a small bit of it to the South-East
> side. I'd assume some multipath is possible, but I'd thought that the deep
> valleys between the buildings would get rid of most of it, with only the
> reflections from the roof top etc remaining.
> 
> My latitude is South 33deg.
> 
> Yes I'm using one GPS as reference and the others are DUTs. Since, 
> the HP53131A
> TICs don't measure negative time (i.e. event on channel2 happens before
> event on channel1) I purposely offset the reference GPS by 100ns. Thus, I
> could measure negative time by later subtracting the 100ns in
> post-processing.
> 
> Below is a plot so you could see exactly what I measured. What is peculiar
> is that the time jumps by exactly 100ns to 200ns. Almost as if the GPS
> receiver decides to offset the time by twice the amount I set it to. Which
> is why I initially thought it might be a firmware thing. I suppose
> multipath is a good explanation, it is just odd that the time error is
> exactly 100ns.
> 
>  URL: 
<http://www.febo.com/pipermail/time-nuts/attachments/20131121/218b90fa/attachment.png>
>
> Using a single antenna and splitting it is an idea, but then I still need
> to devise something to inject the antenna DC power at the other end of the
> splitter etc. And, we are trying to measure the relative offset between the
> antenna/receiver pairs for calibration.
> 
> Thanks for the suggestions so far.

The 100ns is exactly one cycle at 10 MHz.  GPS receivers use this, or 
something very close, as an internal clock.

I bet that the receiver is going in and out of lock, or is hovering 
right on the edge.

What kind of signal strength and number of satellites tracked are you 
seeing?  Plotting this along with the offset plot may be instructive.  
By the way, it may be that a reference receiver is jumping, not the 
receiver under test.

Joe Gwinn