Back in 2013 the U.S. Navy installed a new torpedo warning and countermeasures system on an aircraft carrier (USS Bush, CVN 77) for testing, data collection, and fine tuning. TWS/CAT (Torpedo Warning System/Countermeasures Anti-Torpedo) was to enter service on destroyers, cruisers, carriers, and large amphibious ships by 2015. That has not happened yet because of the technology not performing as effectively as required. .
TWS/CAT consists of a towed sonar array (TWS) that can detect and identify torpedoes so that anti-torpedo torpedoes that intercept the incoming torpedo (CAT) can be launched to destroy the approaching torpedo.
While operating on the USS Bush TWS/CAT was tested against various types of torpedo attack. The tests also provided an opportunity to train sonar operators and collect acoustic data so that the system software can be improved.
The 2015 deadline for deploying TWS/CAT was apparently missed mainly because the CAT anti-torpedo torpedo was not working reliably. The concept behind TWS/CAT has been kicking around in the West for decades. Until recently the main problem was sensors the ship could use to detect an approaching wake homing torpedo and then guide a small anti-torpedo torpedo to intercept. That detection problems appear to have been solved but now the Countermeasures Anti-Torpedo Torpedo has to be perfected. Few details of CAT have been released other than that it is a small 165mm (6.5 inch) torpedo launched from a storage container. This launch container has been show installed in rear of a warship and launching a 165mm torpedo. The main reason for TWS/CAT is to provide a defense against torpedoes that home on the wake of a surface ship. Most wake homers look like standard 533mm (21 inch) torpedoes but they are launched in the general direction of the target and then stalk it. For that reason it is believed that CAT may have worked in theory but is proving less effective in practice. Details of how CAT operates and what the problems are will not be revealed because that would give users (like Russia, China, Iran and North Korea) of wake-homing torpedoes help in modifying their wake homing torpedoes to defeat CAT.
This all began towards the end of World War II when "smart torpedoes" first appeared. These weapons had sensors that homed in on the sound of surface ships. The Germans pioneered this approach. The first such acoustic homing torpedoes followed the sound of the target until the magnetic fuze detected that the torpedo was underneath the ship and detonated the warhead. The acoustic homing torpedoes saw use before the war ended, and even deadlier wake homing torpedoes were perfected and put into service (by Russia) in the 1960s and upgraded ever since. The “wake homing” torpedoes detected the wake of a ship and followed the wake to where the ship currently was and detonated. Other nations also developed wake homing designs so the United States has them available to test TWS/CAT.