[time-nuts] ✘NEO-M8N vs. NEO-M8T

Bob kb8tq kb8tq at n1k.org
Sun May 20 22:53:37 EDT 2018


Ok, let’s back up a bit. The market for “timing” GPS modules is in GPSDO’s and
similar devices. Your local network hub or cell tower is very much in a fixed location. 
They don’t put them in backpacks. Survey in and position lock is how it’s done,

The sawtooth is the error between the arbitrary ( locked to the TCXO) PPS edge and
the “real time” of the PPS edge. The reason it is commonly called sawtooth is the shape
of the data that comes out went plotted vs time. The sawtooth can get into a problem known
as a hanging bridge when the “tooth” reverses in mid transition.

If you look at the section under “timing (page 79)” in the uBlox manual you will find all the fun stuff
that makes the T different. One of the timing messages includes the time offset between
the pps output and the real GPS time solution. Page 351 and after are the time related commands.
The stuff back around page 358 looks like it’s got the sawtooth data in it.

Using the sawtooth information involves running it into either a control loop ( the normal
case in a GPSDO ) or into some sort of controlled delay line ( far less common ). You need
the information every second to feed into the loop along with your measured phase information. 

There are tons of information about all of this in the archives. There are also a lot of posts
that probably will do a more in depth job of bringing you up to speed on all the various
terms and issues.

Bottom line is that with the sawtooth correction applied, you can get down to below
1x10^-9 at one second on your plot. The T version will automatically output the magic
message with the data in it. 


> On May 20, 2018, at 10:06 PM, Gary E. Miller <gem at rellim.com> wrote:
> Yo Bob!
> On Sun, 20 May 2018 19:26:33 -0400
> Bob kb8tq <kb8tq at n1k.org> wrote:
>> The “big deal” features on the T series are the ability to do single
>> satellite timing
> Which I always thought was pointless, that only works for a fixed
> antenna.  Any GPS in a fixed position lab will have a good rooftop
> antenna with clear skyview.
>> and the auto output of the sawtooth correction
>> information. Cranking sawtooth correction into your data will move
>> the line down most of the way to the “JL” line.
> Except that requires a post process step, so not useful for real time.
> I just looked at the 'U-blox 8 Receiver Description' and it makes no
> mention of sawtooh anything.  Is that in a different doc?
> I'll also test Surevey-In mode to see how much that helps.
> ---------------------------------------------------------------------------
> Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
> 	gem at rellim.com  Tel:+1 541 382 8588
> 	    Veritas liberabit vos. -- Quid est veritas?
>    "If you can’t measure it, you can’t improve it." - Lord Kelvin
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