NIST was 5 μs off UTC after last week's power cut

It sounds like GPS, and thus a GPS-based stratum 1 server, uses these time servers, but they were successfully failed over:

> Jeff finished off the email mentioning the US GPS system failed over successfully to the WWV-Ft. Collins campus. So again, for almost everyone, there was zero issue, and the redundancy designed into the system worked like it's supposed to.

So failures in these systems are potentially correlated.

The author mentions another solution. Apparently he runs his own atomic clock. I didn’t know this was a thing an individual could do.

> But even with multiple time sources, some places need more. I have two Rubidium atomic clocks in my studio, including the one inside a fancy GPS Disciplined Oscillator (GPSDO). That's good for holdover. Even if someone were jamming my signal, or my GPS antenna broke, I could keep my time accurate to nanoseconds for a while, and milliseconds for months. That'd be good enough for me.

Out of curiosity, can anyone say the most impactful things they've needed incredibly accurate time for?

Yes, either Rb, Cs, or H standards depending on which GNSS system you're using.

For the most critical applications, you can license a system like Fugro AtomiChron that provides enhanced GNSS timing down to the level of a few nanoseconds. There are a couple of products that do similar things, all based on providing better ephemerides than your receiver can obtain from the satellites themselves.

You can get AtomiChron as an optional subscription with the SparkPNT GPSDO, for instance (https://www.sparkfun.com/sparkpnt-gnss-disciplined-oscillato...).

I believe LTE and 5G networks require it to coordinate timeslots between overlapping cells. Of course, they can use whatever reference they want, as long as all the cells are using the same one - it doesn't have to be UTC. Some (parts of) networks transmit it across the network, while others have independent GPS receivers at each cell site.

Synchronization is also required for SDH networks. Don't know if those are still used.

Someone else referenced low power ham radio modes like WSPR, which I also don't know much about, but I can imagine they have timeslots linked to UTC and require accuracy. Those modes have extremely low data rates and narrow bandwidths, requiring accurate synchronization. I don't know if they're designed to self-synchronize, or need an external reference.

When multiple transmitters are transmitting the same radio signal (e.g. TV) they might need to be synchronized to a certain phase relationship. Again, don't know much about it

Nitpick: UTC stands for Coordinated Universal Time. The ordering of the letters was chosen to not match the English or the French names so neither language got preference.