Following the second contact, surface units of the Navy and Coast Guard were sent, in order to deter the intruder from staying in the area in front of Helsinki, where the waters are relatively shallow.
The rules of engagement in peacetime, prescribe some actions to be taken on such occasions: in order to deny their territorial waters to the navigation of the raider boat, this is warned to have trespassed with depth charges generating noise. Exactly what was done by the Finnish navy.
The signal used is the Marine Sound Signal, MSS, and was developed during the Cold War, jointly by NATO and the Warsaw Pact, to alert the commanders of submerged units that they were identified.
The explosive charge used in the device is equivalent to that of a hand grenade, therefore it does not cause any damage to the steel hull of a submarine.
The MSS is designed to explode at around 3 meters below the surface, and creates an audible sound two miles away from the submarine crews.
The custom is in the launch of 5 or 6 of these acoustic devices that should produce the effect of making the intruder ward off.
Giovanni Caprara
Source: FT.com
