[time-nuts] Thunderbolt reception problems

[Magnus Danielson]

Warren,

WarrenS wrote:
>> If you needed to lower your AMU limit from 4 to 2, you are feeding the 
>> Thunderbolt a signal level below the intended levels. 
> 
> True, You MAY be feeding it a signal that is below what the cell site recommended High gain outdoor antenna will give it.
> But any conclusion past that sounds like pure speculation. And most of use are not using them in cell sites.

True, but lowering this value should be a warning-sign that you may not 
get the performance of the spec-sheet. There where some debate on wither 
the signal level was an issue or not. Lowering the AMU limit only lowers 
the acceptance level of signal strength for a sat in view to be accepted 
for tracking. Regardless how AMU is cooked up (an issue we could ponder 
over as a side-track), it remains a signal strength measure.

> I Can not comment on the other units Magnus referred to that don't work if set too low, 
> and I can not say what AMU is in the Tbolt
> BUT I can say with certainty that setting it to a value of 1 or 2  with 'Tboltmon.exe' version 1.2 works fine 
> and 1 works MUCH BETTER than 4 in some setup.
> Looks like the Tbolt software is not so dumb as to use sat signals that will screw it up.

It uses the AMU limit to select among the available sats. It then uses 
the T-RAIM to cancel out any outliners among that subset of sats to 
churn out which sats is being used for positioning/timing solution.

The AMU limit is nothing magic, it's only that we don't have a good 
reference to what the AMU value is in detail, but we do know that high 
values is good and low values is bad.

Lowering the AMU value as you did is good in the sense that you got more 
sats to actively track. It is bad that the signal levels the Thunderbolt 
is experience is so low that you need to take that action. Low signal 
values means more timing noise and thus more timing instabilty. Having a 
few sats is better than none.

So it is an indication that the unit would like some more gain... 10 dB 
or so.

> 'Trimble GPS Monitor V1-2.pdf'   instructions on page 13 shows the "signal level mask (AMU) set to 0.6

If the statement that AMU is a linear scale is correct, that would mean 
that the C/N limit is set 16,5 dB lower than normally, i.e. that C/N 
being 16,5 dB lower can be accepted.

It would be fun to play around with a variable damper to see what 
relationship to level the AMU value is on the Thunderbolt.

Cheers,
Magnus