[time-nuts] Setting loop TC using thunderbolt internal data.

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Jan 21 14:41:40 EST 2009

Hej Magnus

Magnus Danielson wrote:
> Tom,
> Tom Van Baak skrev:
>> Ah, OK. So you keep the antenna connected and you keep the
>> GPS receiver in position hold mode, still receiving fixes.
>> All you're doing is disabling the software disciplining algorithm.
>> That sounds like a good test. Let me try this too and see if I
>> agree with your conclusions.
> We discussed this a little while back. I proposed a three-cornered hat 
> using two receivers and one counter... as the Thunderbolts has a builtin 
> TIC to GPS which can be logged. We also discussed just running a single 
> Thunderbolt in holdover. Others had already tried that approach. Bruce 
> and I discussed to some degree the effect of steering on the result.

The 3 cornered hat technique only works well (even in the extended form
allowing finite correlations between the 3 sources) when the ADEV of the
sources being compared aren't too disparate.
Consequently this comparison will only work well for tau values in the
vicinity of the Allan intercepts which in turn will need to be not too

>> I'll try to simultaneously measure the 1PPS or 10 MHz outputs
>> as you suggest.
>> Has anyone hacked a TBolt yet to find which internal pin has
>> a raw 1PPS from the GPS engine (as opposed to the 1PPS
>> divided down from the OCXO)?
> Hmm... I have not even "repaired" (in the sense, that they are broken 
> until opened and "repaired", i.e. just looking under the hood for sake 
> of curiosity). any of mine...
> Cheers,
> Magnus


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