Which reminds me, in the '70s a transatlantic flight picked up a 121.5 ELT signal, mid ocean. Either Shannon or Gander asked all flights to monitor 121.5 and to report their INS positions when the signal was first heard and when it was lost. After a couple of hours they had several dozen plots and were able to divert a Shell tanker to a spot in the Atlantic. The tanker found a lone yachtsman with a broken mast. If I remember rightly a Concorde was even more involved in the airborne plot production - anyone?