[time-nuts] GNSS Disciplined Clock

Chris Albertson albertson.chris at gmail.com
Thu May 25 13:31:38 EDT 2017

The long term stability of GPS is very good.  Some one here will point
out exactly how one measures it.   But roughly when speaking of
accuracy you always need to specify a time interval.     For example
if the 1PPS is "off" by 15ns that is not bad and yes there are much
better systems if you need to measure time intervals on the order of
one second.   But if the signal is "off" by 15 ns over 100,000 seconds
that is well, 100,000 time better.

This is a basic reference and for some specialized end use case you
might couple it with other equipment.  Many of the concerns you had,
such as effects of the atmosphere get averaged out because the unit is
looking at satellites from all over the sky.  (averaging over space)
And you do git better results with better antenna locations that are
away from multi-path and have a 360 degree view of the horizon.  But
notice the unit has an temperature stabilized crystal oscillator that
is stable over many seconds. an is much more stable in the short term
then is a GPS receiver.  Trimble uses this crystal to average over

You also have to ask where is the tine data going to be used.  Are you
synchronizing a computer's internal clock or trying to measure the
frequency of a microwave transmitter

SO it falls back to the old thing about there being no "better" only
better for a specific use case.

Some of use were lucky enough to buy Trimble Thunderbolts, a previous
version of this unit when they were on eBay for $100 each.   For those
without 5 digits budget they ar pretty much the Gold Standard.  I have
mine installed with a good filtered DC power supply and an outdoor
antenna on mast well above the roofs of surrounding buildings.     I
get long term stability of about one part in 10E13.   Yes 13 digits
over long periods.   (I think?)  It is really hard to know because my
measurement system is a little circular referenced

On Thu, May 25, 2017 at 6:27 AM, Ebrahim Roghanizad
<e.roghanizad at gmail.com> wrote:
> Dear members
> I am a new amateur member in your group. Maybe my question has been asked.
> Recently I found Trimble Mini-T GG, whose data sheet is attached, as a good
> GNSS disciplined time reference. I would like to know if there exists a
> more accurate one, since it does not employ dual frequencies to compensate
> ionospheric delay, though it handles both GPS and GLONASS. Besides, could
> anyone guide me about the presented accuracy in the datasheet? There, it is
> stated that "When operating in Over Determined Timing Mode, the accuracy of
> pulse per second (PPS) is within 15 nanoseconds of GNSS/UTC." Does it mean
> that it includes both bias and the noise? In other words, is it true to say
> that "The time-synchronization error between two of them with a long
> distance is less than 2*15 ns"?
> Best Regards
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Chris Albertson
Redondo Beach, California

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