Hi Jim

In this case +/- 30m in H and +/- 15m in V would be quite acceptable. A
bit of calculator work and I would be able to determine the minimum
spatial diversity required for those fixes.

yeah the overlap at 0dB ratio  would be  very low or zero. except  for
regions of the lobe that are way down in sensitivity (far down enough
that they may not be included in the solution- IIRC L1 = 30dB is
available fo an isotropic antenna  which provides for substantial off
axis resolution PROVIDING the spreading gain substantially exceeds that,
which it does not  - L1 chip rate 1 Mcps, data =50bps, so gain ~13dB 
ALTHOUGH I see L5 has 10x the chipping rate with consumate increases in
spreading gain, so L5 should be in theory able to provide more fixes as
a higher wanted/unwanted ratio is possible. substantial. wow L5 is good.).

So, the multi antenna combination likely works OK for this application
where antennas have true diversity of sky view.

 Now, some research and I find that the multi receiver solution is
supported using the UBLOX RTK toolkit and the raw output receivers , and
less work to do using the full raw which includes the pseudoranges for
the 9 series.  That would seem a simple solution for high performance fixes.

On 2/07/2023 2:42 am, Lux, Jim via time-nuts wrote:

On 7/1/23 9:42 AM, Lux, Jim via time-nuts wrote:

Here's some samples with the two antennas (cos^2() pattern) angled at 0
(both pointing at zenith), 45 and 60 degrees off zenith. The two
antennas are 5 wavelengths apart (1 meter for 1500 MHz L1)

I clamped the antenna patterns at -20dB (in reality it would be some
sort of weird pattern with lots of lobes and nulls, but all small)

Python code to generate it also attached.

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@lists.febo.com>:

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:

On 7/2/23 8:09 AM, Erik Kaashoek via time-nuts wrote:

JPL has automated precise positioning service. They say it's free.  I've
not used it.

https://pppx.gdgps.net//

/APPS/accepts GPS measurement files, and applies the most advanced GPS
positioning technology from NASA’s Jet Propulsion Laboratory to estimate
the position of your GPS receivers, whether they are static, in motion,
on the ground, or in the air. APPS employs:

  *

    Real-time GPS orbit and clock products from JPL’sGDGPS
    <http://www.gdgps.net/>System

  *

    JPL’s daily and weekly precise GPS orbit and clock products

  *

    JPL’s GipsyX/RTGx software for processing the GPS measurements

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@lists.febo.com
Sent: Sunday, July 2, 2023 10:41:37 AM
To: time-nuts@lists.febo.com time-nuts@lists.febo.com
Cc: John Ackermann N8UR jra@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:

time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com

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

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

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:

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