[time-nuts] Thunderbolt reception problems

[WarrenS]

Some of the disagreement has to do with the fact that Two similar topics, each with a different answer are being mixed together here.

Magnus's point:  
1) How to make the Tbolt the best that it can be? 
Answer: Start with a good strong signal and a quiet environment.

Said's situation: 
2) How to make the Tbolt work the best that it can with a less than optimized existing setup.
Answer: Lower the AMU to 1, rise the elevation to 15or20, increase the TC setting to 500 sec.
(It will work better than when the factory defaults are use with the #1 case above)

Interesting enough, I have both cases with optimized setting running on my bench now and although the #1 is generally about 25% to 50% quieter, 
It is not always so.  About 25% of the time the #2 case is as quiet or quieter.  So the less than perfect #2 case is not really a big deal to most. 
There are much more important things that can be done if one likes to 'tweak & fiddle'.

concerning:
> that you may not get the performance of the spec-sheet. 
Not a problem, cause they seem careful not to include any specs concerning this except for the 1e-12 per day average.

ws

********************
----- Original Message ----- 
From: "Magnus Danielson" <magnus at rubidium.dyndns.org>
To: "WarrenS" <warrensjmail-one at yahoo.com>; "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Sunday, November 22, 2009 5:03 PM
Subject: Re: [time-nuts] Thunderbolt reception problems


> 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
> 
> 
>