Wake-homing

Wake homing is a technique used to guide torpedoes to their target.

The torpedo is fired to cross behind the stern of the target ship through the wake, as it does so it uses sonar to look for changes in the water caused by the passage of the ship, such as the small air bubbles. When these are detected the torpedo turns toward the ship then follows a zig-zag course, turning when it detects the outer edge of the wake, to keep itself in the wake. This will eventually bring it to the rear of the ship, where its warhead can do the most damage, to propulsion and steering.

The system is difficult to jam, though can be distracted by other ships crossing the wake. In 2013 the US Navy tested prototypes of the Countermeasure Anti-Torpedo (CAT) designed to intercept and destroy the incoming torpedo. Deployment of TWS/CAT has not proceeded as planned due to performance issues.

The main disadvantage of wake homing is that the course taken to the target is non-optimal and the target is always sailing away from the weapon, requiring a fast weapon with a longer range than for direct homing.

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Wikipedia