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

[Stephan Sandenbergh]

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.

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.

Regards,

Stephan.


On 19 November 2013 19:31, Graham / KE9H <timenut at austin.rr.com> wrote:

> Stephan:
>
> This sounds like some kind of antenna placement issue.
>
> What is your Latitude?
>
> Can you describe your antenna systems?
>
> Best case: Antennas above all nearby reflecting objects/buildings/trees,
> clear
> view of the sky for 360 degrees,
>
> Worst case: Indoors, patch antenna on the window edge of a North facing
> window, so that
> 100 percent of all signals received are via reflections and multi-path,
> bouncing from
> tall buildings in the area.
>
> When there are signals received via reflection, and the receiver changes
> the
> set of satellites it is using for the solution, you will see a time jump
> equal to
> the apparent change in location of the receiver, for the different
> solution.
>
> With heavy multi-path/reflections, antennas just a few cm apart will be
> looking
> at different solutions, and will jump time/location at different times.
>
> --- Graham / KE9H
>
> ==
>
>
> On 11/19/2013 10:45 AM, Chris Albertson wrote:
>
>> Two antennas near each other?   Could they interact under some conditions?
>>
>> Could you be seeing multi path in some satellite geometries?
>>
>>   Try spacing them apart or BETTER get a splitter and use only one antenna
>> but take the second antenna an far away, I wonder if it is acting like a
>> reflector or if the amplifier is radiating EMI.
>>
>>
>> On Tue, Nov 19, 2013 at 6:01 AM, Azelio Boriani <azelio.boriani at screen.it
>> >wrote:
>>
>>  One M12+ is the reference and the others are DUTs? Three times but in
>>> one direction only for a 300ns total or what? The offset returned to
>>> 0? Please, detail better what happened.
>>>
>>> On Tue, Nov 19, 2013 at 12:17 PM, Stephan Sandenbergh
>>> <ssandenbergh at gmail.com> wrote:
>>>
>>>> Hi,
>>>>
>>>> I've recently measured the 1PPS outputs of three Motorola M12+T GPS
>>>> receivers using two HP53131A TICs. Antennas are located next to one
>>>>
>>> another.
>>>
>>>> Now I notice the one M12+T has changed its time offset by 100ns three
>>>>
>>> times
>>>
>>>> over the period of 48hrs. The jitter remains the same, only the offset
>>>>
>>> that
>>>
>>>> changes.
>>>>
>>>> I'm currently having a re-run of the measurement. But, in meanwhile, has
>>>> anyone seen this kind of behaviour? A firmware issue perhaps?
>>>>
>>>> Regards,
>>>>
>>>> Stephan.
>>>> _______________________________________________
>>>>
>>>>
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