And to be even more precise in the pre-CAE days the main source for disciplining...

dfox on Sept 17, 2018 | parent | context | favorite | on: LeapSecond – A web site dedicated to precise Time ...

And to be even more precise in the pre-CAE days the main source for disciplining was SDH/SONET clock of the network interfaces. It is higher stratum clock source than GPS, but as the frequency is higher it ends in more accurate PLL lock.