Thetis (decoy)
Thetis was the name of a floating radar decoy used by German U-boats during the Second World War. The device was stored dismantled in the bow compartment as a pole about two metres long. Assembly usually took place in the U-boat's conning tower, and could be assembled in about four minutes ("FuMT 2 - Thetis IIC"). Later versions could be launched from a standard torpedo tube ("FuMT 4 - Thetis US"). When deployed, it was extended to a total length of eight metres, half of which was submerged. The upper half had a series of reflectors that  were tuned to Allied anti-submarine radar wavelengths to give the same return signal as a U-boat. The number of Thetis decoys carried varied between different U-boats: The first time the Allies knew about the "Thetis" was in a coded radio message to all U-boats transmitted on 11 January 1944. "Thetis" was introduced in January, when large numbers were released into the Bay of Biscay in July to simulate U-boat patrols during the Battle of Normandy.