Thought they used
A whispering gallery to get the exact location for slide windows?
I suppose any sufficiently accurate independent time source would work though.
Incidentally I finally managed to source an FE-5680 but need to find out if using an SDR directly would avoid the need for all that bulky and heavy circuitry from the early Netscape era. This is possible as the actual core which gets to about 100 celsius when in use comparable to a laptop CPU will fit into a far smaller case assuming its not needed all the time ie just for calibration purposes.
For calibration it does not need to output 10 MHz as long as I know what the correct frequency is and add external filters accordingly.