Wasn't there some problem with PLB's with the EC 225 ditching in the North Sea contributing to issues with locating people?
A PLB only needs to have it's antenna a few inches underwater and it doesn't work.
I see the Mobilarm uses a different technology but again if several are transmitting at once? It seems to use Marine VHF DSC. I seem to remember DCS being very slow to be adopted in the US.
Four portable locator beacons were carried in the helicopter, but these could not be used to their maximum effectiveness because other locator beacons being worn by the passengers in the form of special wrist watches suppressed the signals from more powerful beacons. The beacons were also "unlikely to be orientated and positioned optimally to maximise their broadcast signal," said the AAIB.
The AAIB has concluded that the wrist-worn beacons 'inhibited' the operations of the more powerful emergency beacons, ultimately slowing the recovery operation.
Article here