[time-nuts] Timing between two GPS PPS outputs.

Said Jackson saidjack at aol.com
Fri Dec 28 20:52:04 UTC 2012


Good place to ask indeed!

First, the spec is usually done in 1-sigma (rms) meaning about 68% of the pulses will be better than that. Nothing is usually specified about the remaining 32% of the pulses, so they could be 200ns apart, and still meet spec. Its all a question of statistics.

Second, yes - the error should not change based on antenna position especially if they are that close (say less than 100km apart). That is the beauty of GPS. 

Third, the ublox part allows you to do antenna delay compensation in nanoseconds, so you could put the two systems next to each other and use a scope to calibrate them to be within a couple of ns of each other on average. Average would mean over a day or two to get the best performance due to diurnal changes/errors.

Lastly, make sure to enable auto position survey and position hold mode, it will greatly increase the 1PPS stability and accuracy on those units.

Hope that helps,

Sent From iPhone

On Dec 28, 2012, at 11:49, Dan Kemppainen <dan at irtelemetrics.com> wrote:

> Hi all,
> I've been following this list with some interest, and have a question about two individual timing GPS units PPS outputs. Let's assume that the timing GPS units are identical units, say two SSR-6t's for example.
> If I take these two units and fire them up next to each other off of the same antenna one would expect the PPS signals to be very similar to each other. If the unit specifies 20nS, I would expect at most, say, 40nS Error between the two PPS signals. (Maybe this is not correct, by all means discussion is welcome).
> Now, lets say we fire these units up on different antennas at the same location. For the purpose of this discussion let's assume the antennas are identical or as identical as reasonably possible. (Same make, mode, cable length, etc). Will they still be within the specified errors or each other?
> Now, lets say we move them some distance apart say 500meters (or yards if you prefer). Do they still maintain the same relative error between them?
> The reason for this question is two fold.  I have a personal project in mind where I'd like to time stamp two events within several tens of hundreds of nS if possible, over some distance without cabling. Also, as I started thinking of this application, I realized my understanding of the details of GPS timing is a little weak. I figured this would be a good place to ask.
> Thanks,
> Dan
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