So, I originally posted this a while back when I was hip-deep in nano-second level time measurement and learning every day how little I knew about this field. We all take for granted that there are a billion nano seconds in a second and that in theory you can count those one-by-one if you have a precise enough oscillator but to actually have stability good enough to track the effects of sub-cycle changes to radio signals (phase changes) is not all that simple. Especially when the measurements themselves run over a day or more.

I eventually got it all to work 'in the lab' but nowhere near stable or accurate enough for a real life application. This was all done at very low radio frequencies with super low radiated power (under the license limits) and that of course complicated matters greatly.

The big game changer was to get hold of an atomic clock that I could run locally. That takes the sting out of synchronizing with external sources (such synchronization usually involves a PLL and while the long term stability is amazing over the short term it usually is quite terrible [1]) and using a relatively affordable SDR I was finally able to do this with enough precision to be able to differentiate between the ground conducted signal and the 'airwave' for one of the EU atomic clocks (DCF-77). It was really neat to see the effect of the day/night cycle on the propagation of the signal (exactly as predicted) as validation.

[1] https://en.wikipedia.org/wiki/Allan_variance

The list archive link seems to 404, but this one works: https://febo.com/pipermail/time-nuts_lists.febo.com/

(Card carrying time-nuts member here.)

Yes, I used to have two Cesium standards in my basement, and that allowed me to make FreeBSD the first kernel which could be wrangled to nano-seconds.

These days timekeeping is much more routine and standard hardware, but even if you miss out on LORAN-C and time-by-telephone and leap-seconds, it can still be lots of fun.

Once you get into it, you can look at the log files, call the remote site, and tell them that somebody left the door to the server room open again.

How ?

A temperature difference of 1°C more or less translates to a 1PPM shift (one microsecond per second) in the kind of crystals used for modern motherboard clock chains.

Most air-con systems will oscillate a couple of degrees with approx 20minute period (gentlemen: Tune your PLLs), so a sudden 5 PPM excursion is either a broken air-con or a door left open.

One of the most amazing feats of applied time-nuttery was Steven J. Murdochs work on fingerprinting the timezone of remote servers, based on their TCP sequences numbers.

(Amongst other things, he found out that a lot of Tor nodes were most probably located in north-eastern USA.)

https://dl.acm.org/doi/10.1145/1180405.1180410

There's a lot of interesting stuff in the time-nut world. There's a fun mailing list at https://time-nuts.febo.narkive.com/ and Jeff Geerling has been doing a lot of stuff with clocks lately on YouTube. An interesting intro project is to set up a GPSDO and track the long-term aging of an OXCO or Rubidium standard (many of which are surplused out of telecom applications for somewhat reasonable prices on eBay).

Also vaguely related, the precision clock kit by Mitxela - https://mitxela.com/shop/clock4

Then there's the EFOS3 Hydrogen Maser by https://www.efos3.com/

There is a very deep rabbithole in here.

Thank you! I've had the idea of aquiring an atomic clock in the back of my head for a while now and I'm glad to see there's a like-minded community out there. I'm not even sure what I'd do with it except measure time dilation effects.

SSL error when loading this site?

this already is a 20yo site or so. why the hype today?