[time-nuts] ublox NEO-M8T improved by insulated chamber?
mlewis000 at rogers.com
Thu Nov 2 21:54:57 EDT 2017
Earlier this week, I put the breakout board with my NEO-M8T into an
aluminum can. The can is split into a lower half and an upper half. The
lower half was insulated on its sides internally, but open to the upper
half, which wasn't insulated. The lower area contains the NEO-M8T on its
breakout board and its matching com breakout board.
In the unusual skyview/RF environment described below:
- LH was typically showing two or three green sats, with a min of none
and a max of five for very brief periods.
- The average dBc of the green sats was 22 dBc, with a max of 29 dBc.
- Two screen shots of LH from this time period show an Accu of 12 ns and
This morning, I insulated the inside of the upper half of the can, and
added insulation to seal the top of the lower area into a chamber that
contained the GPS module board & its com board. Since then, its run for
around ten hours, same weather as yesterday except more rain, ambient
room temperature wasn't measured but is definitely warmer. Since after
around an hour of running:
- LH has been showing between two and eight green sats, typically three
- Their average dBc is 30 dBc, with a max of 37 dBc.
- LH Accu is showing as 6 ns.
I have no idea what the temperature is inside the chamber.
As I write this, LH is showing three green sats, at 33, 34 and 35 dBc.
I expected a more stable internal TCXO in the GPS module, but I didn't
expect stronger signals. Although perhaps I should have, as the block
diagram for the NEO-M8T does show its TCXO pointing at a "Fractional N
Synthesizer" inside the UBX-M8030's "RF Block". It also shows a RTC
Crystal for a RTC inside the "Digital Block".
Is this coincidence or can reception improve with:
- a higher temperature module?
- a more stable module temperature?
I'm tempted to add some thermal mass (block of Al) to the top of the M8T
and a chunk of insulation on top of that.
As I finish this, LH is showing five sats, 23, 30, 31, 32 & 34 dBc, Accu
On 01/11/2017 9:55 AM, MLewis wrote:
> I had anticipated reception issues, which is why I went with the M8T
> for its sensitivity, multi-constellation and it's a timing module so a
> good PPS on a single sat - only to get surprised that my version
> didn't have GAL enabled. But I didn't envision reception would be so
> bad that not having GAL would be material.
> I'm also too close to that tall building that is reflecting the sats
> over the Bering Strait at me. It's a military computer site, which I
> thought would be pretty tight on stray RF, but it has antennas. I
> asked a friend who works there about my GPS issues and if RF from the
> site may be influencing things. He hesitated, then said "'Yes'. That's
> all I can say."
> For first power up I had obtained an active antenna for
> multi-constellation and a pre-filter that "provides protection from
> near frequency or strong harmonic interfering signals."
> On 01/11/2017 8:45 AM, Bob kb8tq wrote:
>> For NTP levels of accuracy Glonas is quite fine. Combining that with
>> GPS should
>> get you a pretty good “time source” even under your extreme conditions.
>>> On Oct 31, 2017, at 11:14 PM, MLewis <mlewis000 at rogers.com> wrote:
>>> I'm stuck with a near ground level antenna site (~16" above grade?),
>>> with half a sky view (thankfully to the SSE), less some low blocking
>>> buildings with regular mutlipath, plus multipath bouncing off a
>>> taller building to the SE that bounces sats from the NW at me from
>>> low over the Bering Strait. The building I'm in is concrete with
>>> flat steel under each floor from the construction method. As I write
>>> this I'm down to two green sats in LH.
>>> A number of times a day, it will drop to one sat, and there's a few
>>> dropouts a day where it goes to none of sufficient signal. How many
>>> times and for how long varies by the day. It's worse when it's wet
>>> out, which it is right now. If I lower the signal strength
>>> threshold, then I end up with tons of multipath signals.
>>> While I wrote this, LH was typically showing two or three green
>>> sats, once up to five and once down to one. And I just hit a
>>> dropout... for a minute and a half; the one remaining green sat went
>>> behind the corner of the building's entrance canopy, then back out.
>>> On 31/10/2017 10:30 PM, Bob kb8tq wrote:
>>>> Under what conditions would you expect to loose GPS? I seem to be
>>>> able to
>>>> do just fine sitting in an armchair here in the family room. That’s
>>>> hardly a
>>>> fancy setup.
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