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

Ole Petter Ronningen opronningen at gmail.com
Tue Nov 7 21:30:41 UTC 2017


Not knowing better, I would expect there to be diurnal effects due to the
ionospere being in the shade or not. I suspect there are people on this
list that know better.

Anyway, the effect I am seeing is also very slowly drifing, see screenshot
of about 20 days of data below. The daily variation varies quite a bit. I
am not sure this can be explained by ionospheric activity, but then again I
dont know much about what goes on up there.

As mentioned, I also have L1/L2 data from the same period, I believe it is
possible to extract or at least estimate the Total Electron Content from
that data somehow, but I do not know how - it gets pretty arcane pretty
quickly for a layman.

Ole

[image: Inline image 1]

On Tue, Nov 7, 2017 at 9:32 PM, Lars Walenius <lars.walenius at hotmail.com>
wrote:

> Hi all,
>
>
>
> If I look on the NIST database for the last days it seems to be a daily
> variation of about 8-10ns. Could this be for the same reason as Ole’s
> variation? Is the daily variations due to not perfect ionosphere
> correction? Can you get much better than the data in the NIST database for
> an ordinary timing receiver like the LEA-6T?
>
>
>
> https://www.nist.gov/pml/time-and-frequency-division/
> services/gps-data-archive
>
>
>
> Another question: If You have an error in the surveyed position of say
> 3meters and you receive all of the available satellites in all directions
> how much will this really affect your timing?
>
>
>
> Sorry for all the silly questions.
>
>
>
> Lars
>
>
>
>
>
> ________________________________
> Från: time-nuts <time-nuts-bounces at febo.com> för Bob Camp <kb8tq at n1k.org>
> Skickat: Tuesday, November 7, 2017 8:21:44 PM
> Till: Discussion of precise time and frequency measurement
> Ämne: Re: [time-nuts] ublox NEO-M8T improved by insulated chamber?
>
> Hi
>
> If you go back into the NIST evaluations of various receiver modules …..
> they don’t always
> work best with the correct coordinates. Some have guessed there are
> residual math errors
> in the devices. Others suggest the “radio side” may be at fault.  Indeed
> varying susceptibility
> to multipath *might* be the answer.
>
> Bob
>
> > On Nov 7, 2017, at 2:12 PM, Ole Petter Ronningen <opronningen at gmail.com>
> wrote:
> >
> > 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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> >>
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