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

[Bob Camp]

Hi

If position is the only goal (obviously it’s not ….) then things are a bit more simple:

These days, NRCan will accept a single frequency Rinex file. You no longer *must*
have dual freq. device. Plug in any of the uBlox devices to your antenna. Use their
tools to record a couple of days worth of data. Send that off to them.

You *do* have a F9 coming in, use it and record a day’s worth of data. With
a reasonable antenna, the answer should be good to a few mm. 

One of last night’s runs here claims a 7mm x 5mm error in X and Y. Height comes 
in at 22 mm. This is from their “ultra rapid” solution set. If I resubmit in a couple weeks, 
they will have a better solution to work off of and that should reduce all those errors.

Anything else you plug into the same cable will use the antenna location, you only 
need to do this “survey” process once. 

Is the antenna stable to mm? Is the answer *really* good to that level? That’s all 
open to debate. One could also wonder about the antenna mount …. Fortunately,
at 3 ns/m, you don’t really need a location that is good to < 10 cm. 

If we’re talking about real time PPS, things like the ionosphere and troposphere
get into the act. Correction services can help with this. Dual frequency measurements
also can help. Wider frequency spread is better in that case. ( so L1 to L5 is a 
better bet than L1 to L2). Some dual freq devices have the correction calculations
built in, others don’t. 

Last time I played with a F9T (which *was* a while back) it did not have the ability 
to do the calculations internally. That could easily have changed with newer firmware.
It also might be dependent on the unit being L1/L5 vs L1/L2. I have not dug into 
them for a while.

Bob



> On Jul 2, 2023, at 12:41 PM, Erik Kaashoek via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> John,
> Thanks for the reply. The accurate position info to improve the PPS is indeed the goal. This should allow using the ZED-F9T PPS as reference for checking the absolute phase accuracy of the PPS from a cheap GPS.
> I did not try if google earth or open street map can provide any accuracy in location.  The pin in google earth seems to have a resolution of about 50 cm and I can position it on top of my GPS antenna but I'm not sure how accurate google positions their photos.
> Going back in time shows most of the newer high resolution photo's are positioned the same within 50cm, others are shifted up to 10 meters.
> Erik.
> 
> On 2-7-2023 17:41, John Ackermann N8UR via time-nuts wrote:
>> 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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