 To fool Allied sonar, the Germans deployed "Bold" canisters (which the British called Submarine Bubble Target) to generate false echoes, as well as "Sieglinde" self-propelled decoys.The development of torpedoes also improved with the pattern-running "Flächen-Absuch-Torpedo" (FAT), which ran a pre-programmed course criss-crossing the convoy path and the G7es acoustic torpedo (known to the Allies as German Naval Acoustic Torpedo, GNAT), which homed on the propeller noise of a target.This was initially very effective, but the Allies quickly developed counter-measures, both tactical ("Step-Aside") and technical ("Foxer").