[time-nuts] ublox NEO-M8T improved by insulated chamber?

Tue Nov 7 19:12:11 UTC 2017

Yes, I thought so too - but on the same antenna I have a couple of L1/L2
continously logging survey receivers; the position accuracy should be
within 5-10 mm. Unless I've messed something up with coordinate systems,
the position the UBlox thinks it has should be pretty good.

Ole

On Tue, Nov 7, 2017 at 5:43 PM, Jean-Louis Oneto <jl.oneto at free.fr> wrote:

> Hi Ole,
> I think that the long term undulation are caused by a (small) error in
> geodetic position of the antenna. The period should be a sidereal day
> (23:56...)
> Have a good day,
> Jean-Louis
>
>
>
> Envoyé depuis mon appareil mobile Samsung.
>
> -------- Message d'origine --------
> De : Ole Petter Ronningen <opronningen at gmail.com>
> Date :07/11/2017  15:15  (GMT+01:00)
> A : Discussion of precise time and frequency measurement <
> time-nuts at febo.com>
> Objet : Re: [time-nuts] ublox NEO-M8T improved by insulated chamber?
>
> Hi all
>
> Attached is a 24 hour plot of PPS out from a UBlox 6T against a hydrogen
> maser. From 00:00 the bare receiver board was inside a polystyrene box
> where it has soaked for many months, at 16:00 I removed the box exposing
> the board to the airflow in the room, including AC. The box was left off
> for the rest of the day.
>
> The green trace is temperature in the lab. The "long term undulation" in
> phase is normal, although I do not know the precise cause (multipath or
> something else. I am reasonably sure it is not related to temperature in
> the lab.
>
> [image: Inline image 1])
>
> Ole
>
> On Sat, Nov 4, 2017 at 6:06 PM, Denny Page <denny at cococafe.com> wrote:
>
> > [I hate finding unsent email in my folder :-]
> >
> > Others may disagree, but I doubt that the type of small temperature
> > variation you are referring to has any meaningful effect on tracking.
> While
> > the datasheet for the M8T says that there can be "significant impact" to
> > the specifications at “extreme operating temperatures,” it gives the
> > operating temperature as -40 to +85 C. Simply said, if you can stand to
> be
> > in the same room/space with it, I think you are fine.
> >
> > Of much greater interest would be the antenna and it’s placement. I’m
> > afraid I can’t specifically recommend a “good” antenna, but perhaps
> others
> > on the list can. For my EVK-M8T, I’m using the antenna that came with the
> > kit and it works very well. I haven’t tested other antennas with the M8T
> at
> > this time, but I do have a number of other devices with antennas that
> work
> > well. I also have a few antennas that work poorly with all the devices,
> > including the ones with which they came. Unfortunately pretty much all of
> > them lack sufficient identification markings to identify
> manufacture/model
> > info.
> >
> > Regarding placement, I’ve found that in a restricted area even a few
> > inches can have a significant impact on the average number of satellites
> > and signal level. In my case, it’s associated with the single building
> > structure, but it sounds your case is even more restrictive. Although it
> > can be a very lengthy process, performing antenna surveys may help
> improve
> > your situation. For each location, you need to monitor the number of
> > satellites and signal level for 24 hours or more before determining the
> > relative merit of that location. Repeat… and repeat.. and repeat.
> > Determining the very best location for the antenna will likely require as
> > many antenna surveys as you have patience for. :)
> >
> > Hope this helps.
> >
> > Denny
> >
> >
> > > On Nov 02, 2017, at 18:54, MLewis <mlewis000 at rogers.com> wrote:
> > >
> > > 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
> > 33 ns.
> > >
> > > 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
> > to five:
> > > - 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.
> > >
> > > Michael
> > >
> > > p.s.
> > > As I finish this, LH is showing five sats, 23, 30, 31, 32 & 34 dBc,
> Accu
> > 6 ns
> > >
> > > 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:
> > >>> Hi
> > >>>
> > >>> 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.
> > >>>
> > >>> Bob
> > >>>> 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:
> > >>>>> Hi
> > >>>>>
> > >>>>> 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.
> > >>>>>
> > >>>>> Bob
> > >>>>>
> > >>
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> > >
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