[time-nuts] Re: What GNSS module to buy for a good time reference?

[Ed Marciniak]

Navsparq has the NS-T but that's single band, so no further value discussing in this context.

A look at the Trimble NetRS reveals it's PPS output has 40 ns resolution and says several external factors limit accuracy to +-1 microsecond. While 40ns would come from a 25 MHz clock, it seems to be common reference frequencies are 19.2,26,38.4 MHz. Perhaps it's not exactly 40ns. Elsewhere the manual claims position 5mm+1ppm horizontal and 10mm+1ppm vertical accuracy, which probably represents a fairly decent internal receiver frequency reference. That doesn't seem directly useful for time and frequency but it's interesting for other purposes.

Ah, well...thanks for the interesting reading material the conversation generated.


________________________________
From: John Ackermann N8UR <jra at febo.com>
Sent: Sunday, July 2, 2023 3:00:37 PM
To: Ed Marciniak <ed at nb0m.org>; Discussion of precise time and frequency measurement <time-nuts at lists.febo.com>
Subject: Re: [time-nuts] Re: What GNSS module to buy for a good time reference?

I'm not aware of anything in the ZED-F9T price range that accepts an
external reference.

The closest I know of is the Septentrio Mosaic-T, but the last I heard
the evaluation board was >$1K and I don't think you can buy the bare
modules in one-off quantities.  You can buy a bunch of ZED-F9Ts for
that, but it is thoroughly modern and works really well.  Any completely
packaged receiver with similar capabilities will cost several time that.

As Bob mentioned, there are some used dual frequency receivers that can
do this.  The ones likely to be <$1K are the Trimble NetRS and the
Ashtech Z12 or variants.  The NetRS works pretty well and are easy to
configure and (relatively) easy to get raw observations out of.

The Ashtechs are really, really ancient and very proprietary, and a lot
of them don't work anymore.  I got a couple going, and wrote some python
to convert their serial data stream into RINEX, but I can't really
recommend anyone go down that road these days.  The NetRS is a much less
painful choice.

Now, for those who have are really nuts (time or otherwise), if you take
the lid off a ZED-F9T module, you will see among the three or four ICs
there a separate TCXO.  Don't ask me how I know this; it's not pretty.

Anyway, in theory you could remove that oscillator (I think it's ~60
MHz) and feed in an external reference there.  I don't know anyone who's
tried that, and I don't know if the software has the right hooks to do
anything useful with it.

The Navspark dual-freq is designed for portable RTK applications and
probably does pretty well at that, but from the not-that-great
documentation, its timing performance looks to be pretty basic.  I could
be wrong, but I don't think it supports a 0-D timing mode.

John
----

On 7/2/23 12:31, Ed Marciniak wrote:
> So, what would you recommend for a lower cost module that accepts an
> external reference and has reasonable jitter when using an external
> reference? (I’m specifically calling that out assuming that either a
> rubidium or excellent quartz oscillator is available because I’m not
> necessarily concerned with jitter while using an internal reference).
>
> If considering dual band receivers, is the answer different?
>
> Do you have any opinions or experience to share with respect to Navsparq
> modules? On paper, the specifications are awesome for the price, most
> (maybe all) support binary output, are supported by open-source
> libraries, and have rtk options amenable to using dual receivers for
> spatial orientation. They seem almost too good to be true.
> ------------------------------------------------------------------------
> *From:* John Ackermann N8UR via time-nuts <time-nuts at lists.febo.com>
> *Sent:* Sunday, July 2, 2023 10:41:37 AM
> *To:* time-nuts at lists.febo.com <time-nuts at lists.febo.com>
> *Cc:* John Ackermann N8UR <jra at febo.com>
> *Subject:* [time-nuts] Re: What GNSS module to buy for a good time
> reference?
> Hi Erik --
>
> The ZED-F9T will give you about the best (lowest jitter) PPS accuracy of
> any of the modestly priced modules, but I don't think post-processing
> the results will help you with timing.
>
> You can use the u-blox RAWX message to output raw data which can then be
> converted to RINEX format by one of several means, and then send the
> RINEX file for processing (or process it yourself with something like
> RTKLIB).  The results will give you highly accurate position
> information, usually better with longer observation times.
>
> But the ZED-F9T uses a free-running TCXO for its clock, and can't accept
> an external reference source.  As a result, the clock data returned from
> the post processing service is pretty much meaningless because of the
> TCXO inaccuracy and instability.
>
> By contrast, the clock results from receivers that are locked to a
> quality external reference can be used to determine both time and
> frequency offset and stability down to parts in 10e-15 over long intervals.
>
> I am not sure which post-processing site is best for Europe.  I don't
> think that NRCan has any boundary restrictions (I use them in the U.S.)
> but I don't know if their algorithms cross continents.
>
> In a roundabout way, you can improve your PPS performance by getting a
> good post-processed receiver position, and using that as the ZED-F9T's
> fixed location mode position.  The closer your stated position is to
> actual, the better the PPS results (though a few centimeters won't make
> much difference).  But that's the only timing benefit post-processing
> provides for the ZED-F9T.
>
> John
> ----
>
> On 7/2/23 11:09, Erik Kaashoek via time-nuts wrote:
>> As I needed the accurate PPS in the coming weeks I decided to go for the
>> ZED-F9T L1/L2 and if all is well it will arrive tomorrow.
>> Now I need to understand how all this "postprocessing" works.
>> Some questions:
>>
>> 1: The ublox tools show ability to output basic or full raw data and there
>> are various scripts  to convert ublox data to Rinex and there are some
>> websites listing the commands required to get the required output but is
>> there a dummy's guide somewhere on how to get the RINEX data from the
>> ZED-F9T in the correct version/format for the postprocessing?
>>
>> 2: NRCan seems to process only for Canada (according to their website,
>> correct???). Auspos is listed as processing for the whole world. There are
>> some others. What would be a recommended service? I'm located in Europe.
>>
>> Erik.
>>
>>
>> Op wo 21 jun 2023 om 19:34 schreef Bob Camp via time-nuts <
>> time-nuts at lists.febo.com>:
>>
>>> Hi
>>>
>>> The “big deal” with no L2 is fairly simple:
>>>
>>> One of the great things about dual freq modules is the ability to send off
>>> data
>>> and very quickly get back a corrected version. If you use NRCan, this
>>> includes
>>> clock corrections. They are good to the ~ 0.1 ns level. You can get to <
>>> 1x10^-14
>>> in less than a day.
>>>
>>> The problem is, the free correction services (at the moment) are L1 / L2
>>> based.
>>> For whatever reason, they don’t (yet) understand L5. That may change, or it
>>> may not change. Right now, it’s the way it is ….
>>>
>>> If you want to go crazy, the Mosaic-T is the best of the best in terms of
>>> GNSS
>>> modules at the moment. They apparently are well aware of this and charge
>>> accordingly. You can buy a *lot* of F9T’s for the price of one Mosaic-T.
>>>
>>> Bob
>>>
>>>
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