[time-nuts] GNSS Disciplined Clock

Ebrahim Roghanizad e.roghanizad at gmail.com
Sat May 27 15:26:45 EDT 2017

Dear Chris

As far as I know, atmospheric effect can not be compensated by looking at
satellites from all over the sky and averaging, since it does not have a
random nature, rather it introduces bias to the solution. For example, if
atmospheric effect is not removed, one can not get a relative position
accuracy of sub-meter in long distances even by employing the method of
RTK. All I said here is about position. Now, I would like to know about the
output of time in this condition. What is the utmost reachable accuracy for
a timing output from a GNSS receiver? I do not mean the precision that
reflects the noise behavior. I think that the best result is obtained when
the receiver supports dual frequency in order be able to deal with
ionospheric delay. Am I right? In that case, is there any GNSS receiver
with this ability?

Thanks a lot

On Thu, May 25, 2017 at 10:01 PM, Chris Albertson <albertson.chris at gmail.com
> wrote:

> 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
> time
> 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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