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

[Ed Marciniak]

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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