b. Active underwater acoustic identification

Experience during the Second World War and subsequent armed conflicts has prompted some governments, especially those of neutral countries, to develop the idea of an active underwater acoustic identification system. This idea was supported by the ICRC, which is concerned with the safety of hospital ships and other vessels protected by the Geneva Conventions.

The search for a solution led to a system based on underwater transmission of an acoustic signal emitting the ship's call sign in Morse code, preceded by the prefix NNN (for neutral) and YYY for a hospital ship, in accordance with the IMO International Code of Signals.¹³ The transmission is automatically repeated either continuously or at set intervals. The ship's call sign, which is used for all communications, is a group of letters assigned to it under the ITU Radio Regulations. It gives the ship's nationality, while its individual identity can be derived by matching those letters against lists published by the ITU.

¹³ International Code of Signals, Chap. XIV, para. 5, IMO, London, 1985.

Different prototypes have been tested, as has, more recently, a type of equipment which is industrially produced in limited series. The results obtained have confirmed the soundness of the principle and the reliability of the system. Not only does the range of the signal extend to 25 nautical miles, but it has also been possible to take an accurate bearing of the signal at the same distance.

To our knowledge, several States are interested in this method of identification and at least one has decided to equip its merchant vessels with such a device.