[time-nuts] Accuracy results with Trimble Thunderbolt?

[Frank Stellmach]

Hello Frank,

you're welcome to pose any questions here.

Concerning your question, there are basically two different parameters 
called 'stability' and 'uncertainty'.
I personally don't use the term 'accuracy' any more, because that's 
somewhat misleading.

The Stability is a characteristic of oscillators, let it be OCXOs, a 
naked GPS receiver, or a combined system of a GPS receiver which 
disciplines an OCXO. This stability statistics (Allan Deviation, or 
ADEV, e.g. inside LH) describes, how much fluctuation / jitter you 
encounter on different time scales (also of your counter) when you use 
your TB as a time base, on different Gate Times of your frequency 
measurements.

You might study such ADEV diagrams for different GPSDO, OCXO, Rb-, Cs- 
and MASER clocks on several time-nuts pages to get a better idea: 
http://www.ke5fx.com/gpscomp.htm

For the Trimble TB, these fluctuations  are on the order of 10^-10 .. 
10^-11 for a short Gate Time of maybe 10msec.. 1sec, and prevent that 
you get frequency measurements more precise ('accurate') than that.

If you use averaging, or a longer Gate Time, then these fluctuations go 
further down due to the good short term stability of the OCXO inside the 
TB, then increase at around the time constant you've chosen (500sec?) 
due to the big jitter of the GPS signals, and then go down again, into 
the 10^-12.. 10^-13 region at averaging times of hours or days, because 
the GPS satellite system is synchronized to a Cs master clock at the 
D.O.D. Therefore you can achieve an uncertainty (~ 'accuracy') of about 
10^-13 also, but that depends also on the oscillator / clock you have in 
your house.

Btw.: The GPS system delivers an  in-official uncertainty, because the 
D.O.D. clock is not participating in the S.I. representation of the UTC.

This LH parameter 'OSC' will give you an estimate, how close the 
internal OCXO is currently synchronized to GPS time, but you always have 
to take into account the typical ADEV jitter for your specific 
measurement setup.

Frank