The early phase of the Battle of the Atlantic during which German Navy U-boats enjoyed significant success against the British Royal Navy and its Allies was referred to by U-boat crews as "the Happy Time" ("Die Glückliche Zeit"), and later the First Happy Time, after a second successful period was encountered. The reason for this successful Axis period was the British lack of radar and huff-duff equipped ships which meant that the U-boats were very hard to detect when they made nighttime surface attacks – ASDIC (sonar) could only detect submerged U-boats.