# Geneva Conventions Act 1957, Paragraph 3


It should be possible for medical transports to be identified by submarines by the appropriate underwater acoustic signals transmitted by the medical transports.
The underwater acoustic signal shall consist of the call sign (or any other recognised means of identification of medical transport) of the ship preceded by the single group YYY transmitted in morse on an appropriate acoustic frequency, e.g. 5kHz.
Parties to a conflict wishing to use the underwater acoustic identification signal described above shall inform the Parties concerned of the signal as soon as possible, and shall, when notifying the use of their hospital ships, confirm the frequency to be employed.

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Source: https://www.legislation.gov.uk/ukpga/Eliz2/5-6/52/schedule/FIFTH/part/VI/chapter/23/paragraph/3 (the official version)
Page: https://openlegislation.uk/geneva-conventions-act-1957/schedule-FIFTH/part-VI/chapter-23/paragraph-3
Contains public sector information licensed under the Open Government Licence v3.0. Source: legislation.gov.uk, The National Archives.