Sonar alarm system

In combination with a reflective object detecting sonar system including transmitter means for projecting pulses of sonic energy at predetermined sweep intervals and receiver means for providing first electrical signals representative of reflected sonic energy, a target alarm system comprising:

• highlight detector means, responsive to said first electrical signals, for providing a second electrical signal corresponding to each highlight reflection of at least a predetermined strength;
• shadow detector means, responsive to said first electrical signals, for providing a third electrical signal corresponding to each substantial absence of reflected energy for a predetermined time period;
• single shot means, responsive to said second electrical signals, for providing a fourth electrical signal for a predetermined time period after initiation;
• coincidence gate means, responsive to coincidence of said second and said fourth electrical signals, for providing fifth electrical signals each representative of a detected highlight reflection followed by a detected shadow, said fifth electrical signals occurring at times in a given sweep interval corresponding to the ranges of reflective objects giving rise to detected highlights and associated shadows;
• range memory means, responsive to occurrences of said fifth electrical signals in a given sweep interval, for initiating sixth electrical signals at times in the following sweep interval corresponding to said ranges;
• and
• target alarm means, responsive to coincidence of at least one of said fifth signals and a corresponding one of said sixth signals, for providing a perceptible output signal corresponding to detection of a highlight and associated shadow at substantially the same range on at least two successive sweeps.