AN/SLQ-25 Nixie

The AN/SLQ-25 Nixie is one of the most widely used naval countermeasure systems designed to protect surface ships from the persistent threat of torpedoes. Developed to provide a reliable and flexible defense, the system operates by creating acoustic signatures that confuse or divert incoming torpedoes away from their intended targets. Its design reflects decades of research into underwater acoustics, electronic warfare, and naval survivability, making it a critical component of modern naval defense strategies.

Photo by Petty Officer 2nd Class Clayton Wren

At its core, the Nixie system functions as a towed acoustic decoy, meaning it is deployed behind the ship at a safe distance to act as a false target. The decoy body, known as the TB-14A, is streamlined to reduce drag and ensure stable towing even at higher speeds. Once deployed, it emits carefully generated signals that mimic the acoustic profile of the ship, effectively tricking torpedoes into pursuing the decoy instead of the vessel.

The system is composed of several key components that work in harmony to achieve its purpose. The towed body is the most visible part, but it relies heavily on the shipboard signal generator that produces the acoustic patterns to be transmitted. These signals are sent through a fiber optic tow cable, which not only provides towing strength but also ensures high-fidelity communication between the ship and the decoy.

Photo by Petty Officer 2nd Class Clayton Wren

The fiber optic tow cable is a vital innovation that allows the system to operate with precision and reliability. Unlike older copper-based cables, fiber optics provide greater bandwidth and resistance to interference, ensuring that the decoy can respond quickly to changing threats. This cable also allows the decoy to be deployed at varying depths, giving operators flexibility in different oceanic environments.

The Nixie system operates in two primary modes: passive and active countermeasures. In passive mode, the decoy emits simulated ship noises such as engine vibrations, propeller cavitation, and other mechanical sounds that torpedoes are programmed to detect. This mode is particularly effective against passive-homing torpedoes, which rely solely on listening for ship-generated noise to guide their attack.

Photo by Petty Officer 2nd Class Clayton Wren

In active countermeasure mode, the system demonstrates its more advanced capabilities. When an active-homing torpedo sends out sonar pings to locate its target, the decoy intercepts these signals, amplifies them, and re-transmits them back to the torpedo. This creates the illusion of a much stronger target signature, drawing the torpedo toward the decoy and away from the ship. This feature is especially important in modern naval warfare, where active-homing torpedoes have become increasingly sophisticated.

The earliest version, the AN/SLQ-25A, laid the foundation for the system’s widespread adoption. It incorporated commercial off-the-shelf components, which made it cost-effective and relatively easy to maintain. However, its capabilities were limited primarily to passive countermeasures, leaving ships vulnerable to more advanced torpedo designs.

The AN/SLQ-25B represented a significant leap forward in capability. By adding the ability to counter active-homing torpedoes, it addressed one of the most pressing vulnerabilities of earlier systems. This upgrade made the Nixie a more versatile and dependable defense mechanism, ensuring that ships could operate with greater confidence in contested waters.

The AN/SLQ-25C introduced further refinements, particularly in its electronics and tow cable design. With a longer and more functional fiber optic cable, it became more effective in shallow water operations, where maneuverability and flexibility are crucial. The improved electronics also allowed for new countermeasure modes, enhancing the system’s adaptability against evolving torpedo threats.

The importance of the Nixie system lies not only in its technical sophistication but also in its role within the broader framework of naval defense. Modern warships face a wide range of threats, from missiles to mines, but torpedoes remain one of the most dangerous due to their stealth and lethality. By providing a reliable means of diverting torpedoes, the Nixie helps ensure the survivability of valuable naval assets and the safety of their crews.

Another advantage of the system is its relatively simple operation compared to other complex electronic warfare tools. Once deployed, the decoy can function autonomously with minimal input, allowing ship crews to focus on other defensive measures. This ease of use has contributed to its longevity and continued relevance in naval fleets around the world.