but not a fixed (automatic) ELT.
jas24zzk,
Making a box that survives is not the problem, transmitting a useable signal is!!
Only VLF/ULF (at huge power outputs) propagates through water, that's why flight recorders and voice recorders have acoustic pingers.
In a land accident, it is the aerial cable or the aerial itself that gets damaged or shielded.
The record is quite clear and unambiguous.
So the failure rate of an ELT in water is 100%, 95%+ on land.
There is no easy (or even hard) answer to the fundamental shortcoming of a fixed ELT.
By contrast, where there are survivors to operate a portable, the technical failure rate of portables to broadcast a signal successfully is near to zero as makes no difference.
Don't forget that, unlike auto accidents, GA accidents generally produce relatively light injuries, of fatalities.
Tootle pip!!