[smsm42]

>>> battery life is measured in days and hours since they have to send out a fairly powerful signal using the weakest antenna configuration.

They don't need to send it continuously, so I wonder why it is only hours. Sending a loud short ping once an hour shouldn't consume too much energy. Of course, I don't know enough about radio physics to know if it'd be enough, say, if it is 1km underwater - that may be a problem. Maybe supplement it with acoustic ping too?

GPS is not needed continuously too - it can record last reading, say, before high-g acceleration event and then let the GPS unit be destroyed, burned, starved of power or whatever happens to it. I.e. continuous GPS is needed only when everything is OK and the device is connected to the plane's powerplant - once it is disconnected, record last known GPS and shut down everything - we're in trouble, so the only task for it now is to scream loudly until it is found.