[time-nuts] Testing frequency using NTP Bruce GPS ps

Tom Van Baak tvb at LeapSecond.com
Mon Oct 13 03:19:42 UTC 2008

Comments below...

>  I have to learn more about how you do your measurements. A 53132A is
>  way out  of  my price range at the moment. But I  do  have  a 53310A
>  which should give comparable results.

Yes, if you can get a clean series of 1PPS sub-ns time interval
measurements over GPIB from your 53310A then you're all set.
What time/frequency reference do you have available against
which to compare your GPS receiver?

>  I am interested in learning more about these errors. Is there a list
>  somewhere that describes the sources of error, and what can  be done
>  to reduce it? I keep coming across hints of doing this  in software,
>  but it would seem that has to be done in the signal  processing part
>  of the receiver. But we don't have access to that. Is there anything
>  else we can do to help remove some of these errors?

This is a large topic. Google for words: gps error budget
and you'll begin your long journey. Also look for topics like
ionosphere multipath troposphere ephemeris. A typical OEM
GPS receiver board will get you within 100 ns of UTC without
much effort. Here's a couple of pages to get you started:


>  That is very interesting. What do you mean by granularity? I  have a
>  file somewhere  that  talks about the algorithm to  select  the next
>  1PPS pulse.  It added 500ps to the rounding, so the result  would be
>  on 1 ns boundaries. Is that what you meant?

Your GPS receiver outputs a correction value in fixed integer
units of ns; no less. Other words loosely used on this context
are resolution or quantum or precision.

>  That is getting very subtle. How much effect does it have, and where
>  on earth do you learn all this stuff?

When you write your software to correct sawtooth errors you'll
see where GPS or TIC sign errors and off-by-one sample errors
can bite you. Not all receivers do it the same way, either, so
make sure to study the manual(s).


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