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close this bookMeans of Identification for Protected Medical Transports (International Committee of the Red Cross , 1994, 18 p.)
close this folder5. Identification by submarines
View the document(introduction...)
View the documenta. Acoustic signature12
View the documentb. Active underwater acoustic identification


The development of submarine technology now allows submarines to stay underwater almost indefinitely and to attack targets well beyond the horizon, without prior visual contact. Ships protected under the Geneva Conventions and their Additional Protocols should therefore be easily identifiable by submarines. There are different methods of doing this.

a. Acoustic signature12

12 Philippe Eberlin. "Underwater acoustic identification of hospital ships''. IRRC, No. 267, November-December 1988, pp. 505-518.

This method is widely used by naval forces to identify ships belonging to their own forces or to friendly forces. Its basic principle relies on the monitoring and analysis of the sound produced when a ship is under way, especially by its main and auxiliary engines, the propeller revolving, and so forth. The combination of these noises constitutes the ship's "acoustic signature". Each ship theoretically has its own unique acoustic signature, a sort of sonic fingerprint which can be used for identification purposes.

However, ships of identical design, built by the same shipyard but sailing under different flags, may have almost identical characteristics and thus acoustic signatures which are so similar as to be very easily confused.

Moreover, a ship's acoustic signature is not immutable. When a ship's load changes, so does its draught; this alters the acoustic signature, as does the ship's age and any damage or modifications made to it. Some experts believe that the acoustic signature should be measured and recorded every six months to make reliable identification possible. The identification is made by comparing the signal recorded by means of hydrophones with a pre-recorded specimen signature.

Acoustic signature is established by recording the noises produced by a ship when it performs manoeuvres in a basin especially fitted for the purpose. The operation requires complex installations and sophisticated measuring and recording instruments. This kind of installation is usually available only in countries which have a well-developed navy and are familiar with the technology involved. In time of war it might therefore be extremely difficult if not impossible for a ship from a small, neutral country, being used as a medical transport, to have its acoustic signature recorded and then to communicate it to the belligerents.

Given the complexity of establishing an acoustic signature and the uncertainties involved in its propagation at sea, only well-trained specialists with sophisticated equipment can make a reliable identification.

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.