[time-nuts] Re: Does cm accurate GPS via RTK give more accurate time?

[Bob Camp]

Hi

If you want a GNSS device for good timing, you need to get a version that is designed for that. Is it a design difference or just a firmware change? You never really know. Either way, you get features you need in the timing version. Usually some or all of them are missing in a version that is not timing focused. 

In the uBlox world you go to a F10T or an F9T to get the timing stuff. In the Septentrio universe you head over to the Mosiac-T. Those are all multi-band devices. You can get any of them as “demo kits” from the manufacturer. You can also get them (sometimes for less money) built into boards from various outfits. The older single band stuff is still very much out there as well. The uBlox single band versions follow the same “T on the end” designators. They do have a few that don’t quite fit that though., 

Only having L1 / L5 gets you going. That’s how uBlox seems to be headed. Most of the RINEX processing folks are still on L1 / L2. I prefer the Mosiac-T for several reasons. One of them is that it will cover L2. (along with everything else). It also will lock up to an external 10 MHz source which makes a lot of things easier. RINEX post processing is useful, you may need to wait for a couple of weeks to get a “full up correct” solution though. Very much not real time. Still a great way to check things. If you really decide go crazy, the Mosaic-T has a real time timing correction service you can subscribe to. 

Accuracy wise, a single band device is stuck with those atmospheric issues running real time. Is that a 10 ns swing today and a 50 ns swing tomorrow? It easily could be. Any module without sawtooth correction will have an uncorrectable jitter on the PPS output. Numbers in the  10 to 20 ns range are not at all uncommon. All of that is on top of whatever the errors are in the “timing solution” on the device. How does all that line up with those top of page bold print claims? …. there’s always footnotes :) :) 

We’re not quite done yet in terms of accuracy. The number any module gives you will (without some sort of fancy correction setup) be relative to the GNSS system time. Depending on the GNSS system there will be somewhat better or not quite as good correlation between GNSS system time and this or that  ground based timescale. Is that X ns or Y ns? It very much depends which one and how things are going. 

Still not quite done though. Time at the ns level is a bit exciting. BIH in Paris works out what the time was after the fact. Yes the various standard organizations around the world do try to stay on time. Welcome to another set of is it X or is it Y ns off numbers. You’ll know in a month or two … 

No, that’s not the whole story. It’s just a quick run past a few of the highlights. It ignores a number of rabbit holes you could (and might) run down here or there. There’s a lot of math / numbers you could dive into, even on what is discussed here. 

Fun !!

Bob

> On Apr 27, 2025, at 6:22 PM, David Cureton via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 
> Hi Drew, 
>  As you probably aware the posittion accurace is largely dependent on time accuracy.  The largest error for domestic GPS recievers has been ionspheric variation in path from satellites to receiver as I understand.   The new crop of cm accurate GPS's achive this step up in accuracy by using a second L5 Band and with the two frequencies in use, the ionsespheric variation can largely be modeled and compensated for by the reciever.  Therefore you would also expect a similar step up in the performance of the timekeeping of these GPS's.   I did have a quick loook at the ZED-F9P from ublox and the datasheets were devoid of ANY characterisation of the TIMEPULSE signal accuracy.  However the receiver will also  be limited by the internal reference clocks  their timebase and the stability/phasen noise of the PLLs.  I think there is a version of the Ublox devices that you can actually provide the recever with a timebase that is much better that the tiny little TCXO that is on the devices.    
> 
> 
> Look forward to hearing of your results. 
> 
> 
> Regards,
> 
> David VK3DCU
> 
> ----- Original Message -----
> From: "drew wollin via time-nuts" <time-nuts at lists.febo.com>
> To: "time-nuts" <time-nuts at lists.febo.com>
> Cc: "drew wollin" <drew_wollin at hotmail.com>
> Sent: Sunday, 27 April, 2025 6:52:12 PM
> Subject: [time-nuts] Does cm accurate GPS via RTK give more accurate time?
> 
> Hi All
> 
> I have a long-term interest in GPS/GNSS for accurate position, frequency and time.
> 
> Conventional GNSS receivers are accurate to a few metres and about 10 nanoseconds.
> 
> The accuracy can be improved by using correction information via RTK over the internet. The receiver needs to be within about 20 km of an RTK base station. The RTK base stations are linked and available as government or private correction networks, some of which are free. See the link for the Australian RTK network.
> 
> The RTK corrections are based on local space weather and the ionosphere's effect on the received satellite signal.
> 
> It is relatively inexpensive and easy to use these networks to get cm accuracy positioning. U-blox and Quectel make RTK-capable receivers, which are available from SparkFun and AliExpress. The receiver boards from AliExpress cost around US$50.
> 
> The receivers can be used with software from the respective manufacturers
> 
> With centimetre position accuracy, presumably, time pulse accuracy also improves. Does anyone know if that is the case? I looked on the web without success
> 
> The Quectel LG290P has a 1 PPS output that could be connected to other timing equipment.
> 
> I have an SRS PRS10 GPSDO cesium clock with an early single-satellite GPS receiver. I was thinking of replacing it with a 1 PPS signal from an RTK-corrected LG290P.
> 
> The other question, of course, is how do I measure if the time is better?
> 
> Regards Drew VK4ZXI
> 
> https://portal.ga.gov.au/persona/pa
> 
> https://docs.sparkfun.com/SparkFun_LG290P_Quadband_GNSS_RTK_Breakout/print_view/
> [https://docs.sparkfun.com/SparkFun_LG290P_Quadband_GNSS_RTK_Breakout/print_view/%22https://i.ytimg.com/vi/a-aU4-Yodzg/default.jpg%22]<https://docs.sparkfun.com/SparkFun_LG290P_Quadband_GNSS_RTK_Breakout/print_view/>
> Introduction - SparkFun LG290P Quadband GNSS RTK Breakout Hookup Guide<https://docs.sparkfun.com/SparkFun_LG290P_Quadband_GNSS_RTK_Breakout/print_view/>
> The SparkFun Quadband GNSS RTK Breakout - LG290P (Qwiic) features the Quectel LG290P GNSS module. The board's dimensions, pin layout, and connectors are exactly the same as our vary popular SparkFun GPS-RTK-SMA Breakout - ZED-F9P (Qwiic); and can be used as a drop-in replacement.The board also accommodates users with a diverse choice of interfaces including UART, SPI 1, and I 2 C 1.
> docs.sparkfun.com
> 
> [https://nonprod.portal.ga.gov.au/cache/images/portal_ga.png]<https://portal.ga.gov.au/persona/pa>
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