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

[Pavel Kořenský]

Hello,

I just wanted to share that u-blox also makes the F9T module (the "T" 
stands for "timing," I believe), which is generally even more precise 
than the F9P. SparkFun has a board based on it here:
https://www.sparkfun.com/sparkfun-gnss-timing-breakout-zed-f9t-qwiic.html

The board isn't cheap, but according to the datasheet, the F9T offers an 
absolute timing accuracy of 5 ns (1-sigma, fixed position mode), 
depending on things like temperature, atmospheric conditions, baseline 
length, GNSS antenna quality, multipath, satellite visibility, and 
geometry. In differential timing mode, the accuracy can go down to 2.5 ns.

I'm currently using this module in my prototype GPSDO based on a cesium 
clock, and so far, the results have been very good. I'm not using the 
standard 1PPS output — instead, I use a programmable output from the GPS 
receiver set to 1.25 MHz, and I feed it into a PLL with a Kalman filter 
at the input.

Now, regarding how to measure which clock is "better": that's a tough 
question. If you have only two clocks, you can't really tell which one 
is more accurate — you need a third, better reference. Ideally, that 
would be a cesium standard or, even better, a hydrogen maser. 
Unfortunately, getting access to something like that is very difficult 
for a home lab.
What I did was use my rubidium (Rb) standard as a reference and measured 
my GPS-PLL design disciplining an old, reliable HP10811 crystal 
oscillator. Later, I used the same setup to stabilize the Rb oscillator 
and compared it against a commercial GPSDO. I'm still hoping to get 
access to a cesium clock in the future (as far as I know, there's no 
hydrogen maser available in Prague or anywhere in the Czech Republic).
The main reason I haven’t published my design yet is that I haven't been 
able to make a full comparison against a top-tier reference clock.

Best regards,

PavelK


Dne 27.04.2025 v 10:52 drew wollin via time-nuts napsal(a):
> 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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