ELT
For those critical or curious about ELT performance.... Any radio transmitter needs an external aerial to propagate the signal. Here lies the problem in ELT design.... How to have an aerial attached that won't break off or disconnect in a serious collision or explosion. And none are designed to work under water, as far as I know.
Most modern airliners have one or two installed, that are triggered by a collision, or switched on manually. Each slide/raft also has one in the case of ditching.