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

Tue Nov 7 15:32:19 EST 2017

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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