kilomikedelta wrote:-
It's unlikely that ultra low frequency beacons will appear in my lifetime.
I suspect you mean VLF (Very Low Frequency) 3 - 30kHz. However, the International and US Radio Frequency Allocation table, published by the FCC, provides only that no frequency below 9 kHz can be allocated.
Frequency Allocation Table
So even though the propagation in water of VLF electromagnetic energy is possible for limited distances, the propagation of acoustic Ultra Sound energy at similar frequencies is not covered, both in air and water. There is no real conflict, whether the US Navy thinks so or not.
mm43