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

[Geoffrey Baehr]

1 - not to mention that NTP transmissions on a Pi4 are way outside the PLL of ntpd, due to
the enet being hung off a USB bus (read up on resetting/probing every 250m), vs on a Pi 5, where
it’s all internal to the Bcom CPU/everything including enet in one chip and only  L1 cache distance away.

2 - Ditch Pi4’s for timing. Largely bad. Unless uS vs nS matter to you. Not being rude  :-)

3 - I don’t know if I can believe chronyc  saying
it’s 10nS off (from what ?) a timing related UBlox LEA-M8T board off a Pi5. Which I have running
in a temp compensated enclosure (aka my lunchbox with a thermocouple/fan and highly regulated
linear power supply).

4 - One interesting measurement
that I have not done yet is to graph received time for GPS vs Glonass vs BeiDou vs Galileo vs
Cthulu alien (Allan ?  har har) time measurements ( just kidding). The U Blox board is pretty amazing in
that it can handle all these constellations at once. Those guys know what they are doing. Not bad
for $19 off Ebay. M8T, make sure it is the ’T’ version.

5 - RTK makes no difference, time wise. It is after the fact, so to speak.

6 - The California public RTK network is nice (another UBlox board) but 10 cm doesn’t matter
to us. But it is free.  1200 stations. The basic issue is the ‘time' is the time, the RTK comes after the
‘time' to null out the position error. It is of no help whatsoever for precise time, but great for position.
Go read up on Leica Total Station RTK vs handheld RTK sticks, if you are in to surveying.  The
farmer’s shared RTK networks are pretty incredible, loads of them around here. cm accurate farming.
cm accurate watering, planting, fertilizing, tilling, yield mgmt, seed use, seed variety/DNA/plots etc
me correlated through using GPS time for location, the RTK measures the slip distances.

7 - BTW, if you have
not used it, this is the most incredible video cam network across Calif , many with co - located sesimic sensors.
An amazing piece of work and well done. All time syncd. Cal FIRE uses this to spot fires and time stamp
them.

<https://ops.alertcalifornia.org/>
Operations<https://ops.alertcalifornia.org/>
ops.alertcalifornia.org<https://ops.alertcalifornia.org/>
[AW-favicon-180x180.png]<https://ops.alertcalifornia.org/>


rgds to all !

g


On Apr 28, 2025, at 12:58 PM, Poul-Henning Kamp via time-nuts <time-nuts at lists.febo.com> wrote:

--------
Pavel Kořenský via time-nuts writes:


I'm currently using this module in my prototype GPSDO based on a cesium
clock, [...]

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.  […]

But that is not really what you want to know, is it ?

What you want to know is that your PLL has the right time-constant,
so that you derive maximum benefit from both your two sources.

This is what Dave Mills called "The Allan Intercept"

When your PLL tracks near the Allan Intercept, the error output of
the phase detector will have a flat-ish frequency spectrum.

If your PLL is too loose, the spectrum will slope up towards higher
frequencies, if it is too stiff, lower frequencies will dominate.

But be aware that with a Cs-VCO, the signal is in constant peril
from rounding and truncation in floating point math.

--
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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