[time-nuts] Testing frequency using NTP Bruce GPS ps
Mike Monett
XDE-L2G3 at myamail.com
Mon Oct 13 03:56:31 UTC 2008
"Tom Van Baak" <tvb at LeapSecond.com> wrote:
>Mike,
> 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.
No luck. It is very good for looking at PLL loop dynamics, which is
the reason I bought it.
But it doesn't seem to be useful for Allan variance, so I'll have to
make my own system. I'll use the digital mixer I posted earlier.
> What time/frequency reference do you have available against which
> to compare your GPS receiver?
I ordered a rubidium, two OCXO's, and two Oncore UT Plus GPS cards.
I plan on making a standard 1PPS phase/frequency detector to lock to
the 1PPS.
The second version will use the same model GPS card, but it will use
a new method of locking to the 1PPS.
I'll compare the results with the rubidium and OCXO's locked to
either GPS.
>> 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:
>http://edu-observatory.org/gps/gps_accuracy.html
>http://www.trimble.com/gps/howgps-error2.shtml
>http://www.cmtinc.com/gpsbook/chap6.html
>http://www.ias.ac.in/currsci/oct102007/927.pdf
>http://www.usace.army.mil/publications/eng-manuals/em1110-1-1003/c-4.pdf
>http://seismo.berkeley.edu/~battag/GAMITwrkshp/lecturenotes/unit1/unit1.html
Thanks very much for the links. That information will be very
helpful.
[...]
> 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.
Now I am confused. I thought it used the internal crystal clock for
the timing, and selected which pulse to output for the 1PPS, and
this is what causes the sawtooth. If so, the timing is determined by
the crystal.
[...]
> 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).
I don't use software to correct the sawtooth. It is a whole new way
of locking to the 1PPS.
A lot of the errors will simply vanish. But I'll still be vulnerable
to ionosphere delay, constellation changes, and pigeons sitting on
the antenna.
>/tvb
Thanks for your help!
Best Regards,
Mike Monett
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