The invention relates to an anti-torpedo defence method which is due to be implemented from a ship (10) which is attacked by torpedoes (16). For that purpose, said ship comprises a sonar system composed of an active on-board sonar (11) having a panoramic view with an astern gap and a towed sonar (12) at a distance from the ship (10). Said sonar system is due to locate a torpedo and a defence means located at a distance from the ship is used to protect said ship against the torpedo. The aim of the invention is to improve the anti-torpedo defence method in order to obtain a reliable protection, even against loud torpedoes from astern, by means of reduced technical means in the on-board sonar (12). For that purpose, said on-board sonar (12) passively locates a torpedo (16) from an astern region of the ship (10) and measures over time the frequency of at least one significant spectral line propagated by said torpedo (16). The distance (aT) to said torpedo (16) is determined continuously on the basis of the bearing excursion speed, the frequency of the spectral line and the rate of frequency change, and the defence means is activated from a predetermined distance.
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