[time-nuts] GPS 1PPS ultimate accuracy

Bob Camp kb8tq at n1k.org
Thu Jan 15 07:51:16 EST 2015

> On Jan 15, 2015, at 4:56 AM, Andrea Baldoni <erm1eaae7 at ermione.com> wrote:
> On Mon, Jan 12, 2015 at 06:16:01PM -0500, Bob Camp wrote:
>> Actually it’s a bit worse than you might expect. 
>> The uncorrected sawtooth will give you about 20 ns of wander. At the one day
>> level, GPS without some sort of ionosphere help (like a dual frequency
>> receiver) will add another 10 ns or so to that. Net, your pps is spread over
>> a 30 ns range. 
> Hello Bob.
> Thank you, now I have a better idea.
> I understand that the NTP is completely ruled out and also between GPS there
> is a strong difference.
>> With things like 5335’s running around for cheap prices, I would suggest
>> doing this with a counter. You are going to spend a lot of days getting very
>> much data. Your time’s got to be worth something …. 
> Actually I own a Racal 1995 that should be better than the 5335 with its 1ns
> single shot resolution.
> However, I don't still own a GPSDO to reference the counter so how do you
> suggest to use it?

Divide both of the things you are testing down to 1 pps. 

Trigger the start on one and the stop on the other. 

Read out the difference to the 1 or 2 ns resolution of the counter.

That’s going to be ~ 100 X better than measurement with the rolling counter.


> I should use total A over B with the DUT in A and the PPS in B?
> Best regards,
> Andrea Baldoni
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