ATT = Anti-torpedo torpedo.
Also from MARCO POLO through Harry:
"earlier this year, the U.S. Navy was experimenting with ATT (anti-torpedo torpedo) on board USS George H.W. Bush, Nimitz-class nuclear-powered aircraft carrier. The torpedo being used was much smaller than the Russian M-15 -- they said, 6.75 inch (17.145 centimeters). I have no idea how successful the U.S. Navy was in their experiment."
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Surface launched torpedo almost all warships now able to have an ASW [anti-submarine-warfare] capability. NOT ONLY WARSHIPS BUT ANY SAILING VESSEL PERIOD POSSESSING THE APPROPRIATE ELECTRONIC HARDWARE ABLE TO FUNCTION IN THE ASW CAPACITY!