Method for classification of sonar echoes

1. A method for distinguishing between submarine and non-submarine sonar targets comprising the steps of producing a series of permanent displays of sonar target echoes having fixed electrical characteristics by projecting a beam of energy pulses of more than one and less than ten milliseconds duration toward a known submarine target of predetermined range along a number of converging paths, then transmitting a succession of energy pulses of the same duration toward an unknown target along a number of converging paths, producing successive signals of durations which successively vary in accordance with the extent of said target along consecutive pulse paths, indicating the maximum and minimum signal duration, and gating a series of sonar echo displays responsive to the same electrical characteristics as those producing the permanent displays, of said unknown target between said maximum and minimum signal duration whereby a homologous echo production of the permanent displays is obtained only when said echo is from said submarine type.