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

[Bob Camp]

Hi

I think you will find that NRCan is legally a traceable source of data for Canada and
will supply information that links to their national geodetic system. Like John, I use their
service here in the US with no issues. I believe they will happily process data from 
anywhere. 

Many folks have gone looking for a similar service in Europe. We have two here in the
US for positioning, as you have found, there is one in Australia. The only one that feeds
back a nice easy to use timing result is NRCan. 

In terms of a low jitter PPS for testing, your typical GPSDO produces a better 1 PPS than
one of these modules. In terms of “on time” the module wins. However there are a *lot* of
fun and games to get form that time pulse to something like UTC. 

You can indeed use a “sawtooth corrected” pps out of a F9T to compare it’s internal clock
to a standard you already own. Then you can compare that record to an after the fact
processed file on the F9T RINEX data to work out what’s what. 

Off to the cookbook:

1) First you need an ok L1/L2 antenna. $100 or so will get you one from China. Note
that a proper “antenna descriptor” may not be available for a low cost antenna. This
can tangle things a bit. ( = they may want a descriptor )

2) Next you need an antenna location with a reasonable view of the sky. Being able
to see from due east , around past due south to due west and down to about 20 
degrees is in the “great” category if you are in Europe.

3) Get things running and make sure all is well. 

4) As John mentioned, the uBlox tools can grab a RINEX file for you. It will save it
to disk. Everything but the header should be fine. Do a one hour file.

5) Head over to the NRCan website. Sign up for a user / password combo. Without 
that, you can’ t put in data. 

6) Upload your file and look at the error message (if any) that comes back. They may
want info in the header that isn’t there. A text editor will let you put in this or that. 
Possible things they might not like:

	Antenna description, if yours does not have an official designator just put in one that looks close
	Sat system names, at one time uBlox and NRCan didn’t quite agree on this. That may not be true today
	User name, site description, antenna offset … 

Once the one hour file works, you know what to do with the header from then on.
If you are lucky, there may be fields in the F9T or uBlox tools to correct things. If not,
you edit each file.

7) If you are looking at a local standard, you take your log of PPS and sawtooth information
and process it against the clock file that comes back in the zip file from NRCan. 

Alternatives that let you put in a 10 MHz reference and get back data referenced to it:

1) Trimble NetRS. Prices vary from $150 to $900 depending on when you look. At anything
below $250 they are a good deal. They may need a new flash memory card to get running.
The latest firmware is free on the web so there is no reason to get this or that version. 

2) Septentrio Mosaic-T dev kit. You just went up over $1K.

3) Trimble NetR9. Prices range from $1200 to $3000. There are various models. The T1 is
the top of the line. The T3 is the basic one. They all will accept a 10 MHz in. 

All three of these devices have a web interface to make configuration somewhat easier. All
can take a pretty normal 10 MHz sine wave source as a reference input. All can put out /
take in a pps signal. The Mosaic-T does a better job if you plan on doing this.

The NetRS is GPS only. The Mosaic-T will work with any GNSS system that is currently
deployed. The NetR9 will do various systems depending on which one you get and what
options are installed. They all will do GPS. 

At the moment NRCan only does corrections on GPS and Glonass. Will they add more systems
in the future? Who knows ….. Last time I checked the US correction services only ran GPS. 
It’s been a few years since I used them, that may have changed.  

Bob

> On Jul 2, 2023, at 11:09 AM, Erik Kaashoek via time-nuts <time-nuts at lists.febo.com> 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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