A sonar decoy is a device for

decoy A decoy (derived from the Dutch ''de'' ''kooi'', literally "the cage" or possibly ''ende kooi'', " duck cage") is usually a person, device, or event which resembles what an individual or a group might be looking for, but it is only meant to lu ...

ing

sonar Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances (ranging), communicate with or detect objects on o ...

. Most are released from

submarine A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...

s to act as a false target.

Submarine decoys

The first submarine decoys were the German Bold fitted to

U-boat U-boats were naval submarines operated by Germany, particularly in the First and Second World Wars. Although at times they were efficient fleet weapons against enemy naval warships, they were most effectively used in an economic warfare ro ...

s of World War II. These were a pellet of

calcium hydride Calcium hydride is the chemical compound with the formula , and is therefore an alkaline earth hydride. This grey powder (white if pure, which is rare) reacts vigorously with water liberating hydrogen gas. is thus used as a drying agent, i.e. a d ...

in a simple metal container. On contact with sea water, the calcium hydride decomposed to produce a trail of hydrogen gas bubbles that acted as a

bubble curtain A bubble curtain is a system that produces bubbles in a deliberate arrangement in water. It is also called pneumatic barrier. The technique is based on bubbles of air (gas) being let out under the water surface, commonly on the bottom. When the b ...

and reflected

ASDIC Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances ( ranging), communicate with or detect objects on ...

impulses to produce a false target. The container trapped hydrogen and floated, with a crude spring valve to maintain buoyancy to keep it at a constant depth. Later decoys, such as

Sieglinde Sieglinde is a Germanic feminine given name. It is derived from two German words or elements. Those being: "sigu" for victory and "lind" for soft, tender, flexible. The diminutive version is "Sigi" or "Siggie". It is also seen in German mytholog ...

, were motorised and could deploy their false target away from the host submarine, increasing safety.

Torpedo decoys

Decoys were also used by surface ships to decoy the developing

acoustic torpedo An acoustic torpedo is a torpedo that aims itself by listening for characteristic sounds of its target or by searching for it using sonar ( acoustic homing). Acoustic torpedoes are usually designed for medium-range use, and often fired from a sub ...

es. These were usually towed behind the host.

Example decoys

Bubble decoys

Reflective bubble targets * Bold *

Hammer and explosive decoys

These were intended to swamp the listening device with noise * Siegmund

Signature decoys

AN/SLQ-25 Nixie The AN/SLQ-25 Nixie and its variants are towed torpedo decoys used on US and allied warships. It consists of a towed decoy device (TB-14A) and a shipboard signal generator. The decoy emits signals to draw a torpedo away from its intended targe ...

* Mobile submarine simulator

Towed decoys

Foxer Foxer was the code name for a British built acoustic decoy used to confuse German acoustic homing torpedoes like the G7 torpedo during the Second World War. A US version codenamed FXR was deployed at the end of September 1943 on all transatlant ...

, a British towed decoy to decoy

es away from surface ships. Also used by the US as FXR and the Canadians as CAT. * T-Mk 6 Fanfare, US development of Foxer

References

External links

* 01 U-boats {{weapon-stub