Re: [chrony-users] Re: Any nice example of chrony, WWVB, Raspberry Pi ?

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

Remember that wwv hasno way of telling you how long the travel time was
between the transmitter  your computer.The distance around the world is
140ms, across the US about 10ms, and will depend on how may skips across the
ionosphere it makes ( lengthens it). So getting better than 1 ms means you
have to be less than about 200km from the transmitter.

( note that is milliseconds, not microseconds). A GPS board will be of the
order 5-10 time as expensive and will give you a thousand times higher
accuracy. Now, if all you need is 10s of milliseconds accuracy, WWV should be
fine. Better than that, get a timing  gps card-- unless you cannot get the gps
antenna out above ground (or need hundreds of feet of antenna cable to get it
from the antenna to your gps receiver, or from the receiver to the computer.

If you do it via the network  you can probably get 10microseconds of accuracy.
Useless if you cannot get a network cable to your computer.




William G. Unruh __| Canadian Institute for|____ Tel: +1(604)822-3273
Physics&Astronomy _|___ Advanced Research _|____ Fax: +1(604)822-5324
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On Wed, 23 Oct 2024, Mike Collins wrote:

> [CAUTION: Non-UBC Email]
> [piclock]:</home/pi/> cat /etc/default/gpsd
> # Devices gpsd should collect to at boot time.
>
> # Start the gpsd daemon automatically at boot time
> START_DAEMON="true"
>
> # They need to be read/writeable, either by user gpsd or the group dialout.
> DEVICES="/dev/pps0"
>
> # Other options you want to pass to gpsd
> #GPSD_OPTIONS=""
> GPSD_OPTIONS="-n"
>
> # Automatically hot add/remove USB GPS devices via gpsdctl
> USBAUTO="true"
>
>
> And the pertinent part from chrony.conf:
>
> refclock PPS /dev/pps0 refid PPS
> refclock SHM 0 refid GPS precision 1e-1 offset 0.200 delay 0.6
>
>
> On Tuesday, October 22, 2024 at 10:44:57 AM CDT, infection.many550@xxxxxxxxxxx <infection.many550@xxxxxxxxxxx> wrote:
>
>
> On Tue, Oct 22, 2024 at 09:28:54AM GMT, Miroslav Lichvar wrote:
>
> > These values are bad. A well-working PPS source should show much
> > better skew and std dev. A rpi should be capable of getting below 1
> > ppm and 100 microseconds. I suspect there is still something wrong
> > with your setup. Maybe some noise is picked on the GPIO.
>
> Here are mine:
>
> 3+0 ~$ chronyc -N sourcestats
> Name/IP Address            NP  NR  Span  Frequency  Freq Skew  Offset  Std Dev
> ==============================================================================
> PPS0                      10  7  145    -0.000      0.010    -1ns  292ns
>
> RPi 3B + bookworm, Uputronics GPS hat
>
> --
> Ian
>
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