This is not the only time signal emitted from a terrestrial tower. For example France uses a similar signal for their legal time https://en.wikipedia.org/wiki/TDF_time_signal , and as mentioned in https://en.wikipedia.org/wiki/Radio_clock there are many others. Have also a look at LORAN-C / eLoran, which recent releases enable a receiver accuracy close to GNSS as far as I understand while there is no way to properly compensate for various propagation effects in DCF77 and alikes (i.e. you'll get millisecond-ish accuracy, but not microsecond even though the original source clock is very stable. Microsecond accuracy may seem overkill but this is actually required in several industrial applications e.g. 4G/5G TDD base stations are often connected to GNSS in order to be phase/time-synchronised).

Last but not least : I should mention that this researcher J.M. Friedt has an amazing amount of wonderful material about clocks, oscillators, SDR and so on... http://jmfriedt.free.fr And more specifically on DCF77, I could find http://jmfriedt.free.fr/dcf77.pdf and http://jmfriedt.free.fr/dcf77.mp4 (among other things)