Sonar decoy
# Sonar decoy

A sonar decoy is a device for decoying sonar. Most are released from submarines to act as a false target.

## Submarine decoys

The first submarine decoys were the German Bold fitted to U-boats of World War II. These were a pellet of calcium hydride 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 and reflected ASDIC 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, 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 torpedoes. These were usually towed behind the host.

## Example decoys

### Bubble decoys

Reflective bubble targets

* Bold
* Sieglinde

### Hammer and explosive decoys

These were intended to swamp the listening device with noise

* Siegmund

### Signature decoys

* AN/SLQ-25 Nixie
* Mobile submarine simulator

### Towed decoys

* Foxer, a British towed decoy to decoy acoustic torpedoes away from surface ships. Also used by the US as FXR and the Canadians as CAT.
* T-Mk 6 Fanfare, US development of Foxer
