D. Peter Maus wrote:


HF aero comms are not all USB. Some are. Most are still AM. This is due
to the ability of AM signals of very low level to be noticed while other
comms are taking place, facilitating discovery in an emergency when
conditions are less than ideal.




VHF/UHF aero is AM; HF aero is SSB. You can have half a dozen SSB
transmitters all talking at once on the same frequency with much less
harm than there'd be with several AM carriers. I don't think
intelligibility has anything to do with it.