Abstract: 1. Apparatus for protecting a vessel from a waterborne missile launched urwater by air pressure, comprising means for continuously listening to port and to starboard at three spaced locations at predetermined progressive distances astern of the vessel for any waterborne wavefront radiated as a consequence of a waterborne missile launching, and for detecting the passage of such wavefront across each of said spaced locations, means for signalling aboard the vessel the passage of such wavefront across any of said locations, and means for recording on a time base the instants that such wavefront crosses each of said locations and the approximate arrival direction of the wavefront in terms of port or starboard, whereby proper evasive action for the vessel may be determined and taken.