[traceroute66]

> you could get time signals from GPS (or GLONASS)

You forgot to mention Galileo ... GLONASS may not be politically attractive :)

But beyond the GNSS ecosystem, there are of course other interesting options:

Safran STL[1] which uses LEO sats. Apparently it works well in places where you can't get a good GNSS signal (i.e. indoors without an external antenna). (This was previously Orolia STL, but they were acquired by Safran).

Most national time labs also offer a leased-line service, e.g. NPL (UK)[2]

There is also a very niche (read: VERY expensive) commercial timing-as-a-service product from a company called Hoptroff[3]. You license both the service/connection plus their software clients, so the $$$$$$ add up. Definitely one of those "if you have to ask you can't afford it" vendors.

[1] https://safran-navigation-timing.com/product/stl-option [2] https://www.npl.co.uk/npltime [3] https://www.hoptroff.com/

[jjtheblunt]

Galileo too! (Pre coffee oversight)

And, recursively, from celltower signals for any CDMA-related technology, i think.