[time-nuts] Tuning a Trimble Thunderbolt

Bob Camp kb8tq at n1k.org
Wed Apr 22 18:33:09 EDT 2015


Looking at that screen shot, something is *very* wrong with your GPS reception. Your GPS
is 10X worse than it should be. 

> On Apr 22, 2015, at 9:37 AM, Pete Stephenson <pete at heypete.com> wrote:
> On Wed, Apr 22, 2015 at 1:47 PM, Bob Camp <kb8tq at n1k.org> wrote:
>> Hi
>> Backing up a bit to “getting a TBolt running”.
>> 1) instal Lady Heather and get it connected to the TBolt
>> 2)  does it fire up and find any sats?
> Yes. It had been working consistently for several days prior to my
> first message.
>> 3) are the power supplies holding regulation?
> Yes.
>> 4) nail down the antenna in the best fixed location you can find
> Done.
>> 5) run the auto-calibration feature in LH
> Done. This changed the gain from -5.0Hz/V to -3.132Hz/V and changed
> the initial voltage to 0.347V. I switched the time constant and
> damping values back to their defaults of 100 seconds and 1.200,
> respectively.
>> 6) run a 48 hour survey with LH and write the location to ee memory
> Done. The location matches the averaged location surveyed from my
> Motorola Oncore UT+ (the antenna for which was about 10cm away from
> that for the Tbolt, some no-name mushroom-type antenna) and a handheld
> Garmin eTrex 20 (with GPS+GLONASS+WAAS) within a few meters. It also
> matches with Google Maps.
> In the attached screenshot you can clearly see the field of view from
> the antenna's current location over the last ~20 hours.
> Interestingly, the Oncore antenna (a cheap patch antenna from eBay)
> seems to be causing some intermittent issues with the Tbolt: if the
> antennas are too close there appears to be some sort of interference
> emitted by the Oncore antenna that makes it difficult for the Tbolt to
> lock onto the GPS signal and the Tbolt goes into holdover. Oddly, this
> is not consistent: the Tbolt and Oncore had coexisted for a few days
> with no problems but today some of the problems started up again.

I would bet that the amp on the “Oncore” antenna is oscillating. It may do it
intermittently. The frequency may swing back and forth through the GPS band. It
may be the source of your GPS problem. 


> The same issue occured if the Oncore antenna was too close to my
> Garmin GPS 18x LVC. I have since moved the Oncore antenna further away
> (it's now about 50cm) and signal reception for the Tbolt is much
> better. Weird, but distance seems to resolve the issue, so not really
> a problem anymore.
>> 7) Then check the EFC voltage, it should be fairly close to 0V, and not over 2.5
>> If you are > 2.5, that’s probably a broken unit.
> Doesn't seem to be a problem.
>> 8) Now start watching the EFC voltage for a few days and see that it’s leveling
>> out and not spiking. Again spikes = something broke.
> See the attached screenshot. There's a few small EFC voltage spikes
> when the unit enters or leaves holdover, but otherwise it seems
> reasonably smooth in my (admittedly untrained) view.
>> Until that’s all done, I would not dig to deep into the workings of the gizmo. It
>> needs to be set up first.
> Other than the intermittent issues with the Oncore antenna, everything
> seems to be working reasonably well -- there's no obvious failures
> that I can spot.
> Cheers!
> -Pete
>> Bob
>>> On Apr 21, 2015, at 4:30 AM, Pete Stephenson <pete at heypete.com> wrote:
>>> On Tue, Apr 21, 2015 at 4:25 AM, Charles Steinmetz
>>> <csteinmetz at yandex.com> wrote:
>>>> Pete wrote:
>>>>> On a related note, is it possible to extract any data regarding the
>>>>> training from the unit?
>>>> Not as far as the time-nuts community knows, no (other than looking at the
>>>> DAC voltage and temperature reporting during holdover and attempting to
>>>> reverse engineer the prediction algorithm by correlating those with the
>>>> long-term DAC voltage -- good luck).
>>> Finishing my PhD is enough work already. I don't think I'll try
>>> reverse-engineering the prediction algorithm quite yet. Perhaps later
>>> in my Copious Free Time(tm)?
>>>>> Are the training parameters saved periodically to non-volatile memory,
>>>>> or are they purely stored in RAM and so will be lost if powered down?
>>>>> If the latter, does the RAM have any provisions for backup power
>>>> I doubt it -- mine always act as if they are training from zero if they have
>>>> been powered down.  Because of the lack of precise retrace of quartz
>>>> crystals, I don't think you'd want old (pre-power-down) data, anyway.  Some
>>>> crystals will even come up drifting in the opposite direction after being
>>>> powered down, and they all take some time (days, at least) to settle down
>>>> after any disturbance (including power interruptions, however brief).
>>> Ok. It'd be nice if there was some way to keep the crystal going
>>> through power interruptions, even if the oven itself cooled off. I
>>> suspect Trimble (correctly) assumed that the vast majority of these
>>> units were to be installed in cell sites with reliable power so that
>>> wouldn't be an issue.
>>>>> Alas, the location for the antenna is suboptimal: in the best location
>>>>> available to me (an outdoor balcony) I have a clear view of the
>>>>> southern sky from 150-300 degrees (az) and from horizon to zenith with
>>>>> only a few low-elevation obstructions. However, this is only
>>>>> accessible in warm months
>>>>>    *   *   *
>>>>> The surveyed position is within about 10 meters of the actual location
>>>>> according to Google Maps and local building information.
>>>> That's a problem.  Every meter is approximately 3.3nS, so 10m introduces a
>>>> +/- 33nS error in the raw data (as much as 33nS closer to some satellites
>>>> and 33nS farther from others).  Add in the uncertainty due to noise, and you
>>>> get easily hundreds on nS of error in the computed solution.
>>> Indeed. I'm running a 48-hour survey with Lady Heather now to see if
>>> that can improve things a bit more.
>>>> Unfortunately, you are unlikely to do any better than this with the antenna
>>>> location you described.  Time to buy a house, with no tall trees nearby.
>>>> (You may already have heard that time-nuttiness can be expensive....  ;-)
>>> I won't be looking for a house for at least a few years, but when I do
>>> the skyview is definitely one of the criteria, as is the friendliness
>>> of the community to radio masts.
>>> Cheers!
>>> -Pete
>>> --
>>> Pete Stephenson
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> -- 
> Pete Stephenson
> <tbolt2.gif>_______________________________________________
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