Echo ranging with reference to boundar conditions

Claim 1

ECHO RANGING APPARATUS FOR THE LOCATION OF FISH IN THE SEA, COMPRISING A TRANSMITTER FOR TRANSMITTING PULSED EXPLORING SIGNALS INTO THE SEA, A RECEIVING TRANSDUCER ADAPTED TO PRODUCE ELECTRIC SIGNALS IN RESPONSE TO ECHO SIGNALS RESULTING FROM THE REFLECTION OF THE EXPLORING SIGNALS BY FISH IN THE SEA AND BY THE SEA BED, ELECTRIC AMPLIFYING MEANS CONNECTED TO SAID RECEIVING TRANSDUCER TO AMPLIFY SAID ELECTRIC SIGNALS AND COMPRISING A GATED STAGE CAPABLE OF BEING RENDERED NON-CONDUCTING UPON THE APPLICATION OF A SUITABLE ELECTRIC CONTROL PULSE THERETO, A DISPLAY DEVICE ADAPTED TO DISPLAY THE OUTPUT SIGNALS FROM THE ELECTRIC AMPLIFYING MEANS IN RELATION TO TIME, AN ELECTRIC CONTROL PULSE GENERATOR, CIRCUIT MEANS FOR APPLYING SAID ELECTRIC SIGNALS AS INPUT SIGNALS TO SAID GENERATOR, SAID GENERATOR PRODUCING AN ELECTRIC CONTROL PULSE WHEN THE AMPLITUDE OF SAID ELECTRIC SIGNALS EXCEEDS A PREDETERMINED VALUE, THE SAID PREDETERMINED VALUE BEING SO CHOSEN THAT IT IS EXCEEDED ONLY BY A SIGNAL VALUE CORRESPONDING TO THE RECEIPT OF A SEA BED ECHO, SAID GENERATOR HAVING DELAY MEANS SUCH THAT THE INITIATION OF SAID CONTROL PULSE HAS A PREDETERMINED TIME DELAY IN RELATION TO THE COMMENCEMENT OF THE SEA BED ECHO, CIRCUIT MEANS FOR APPLYING SAID ELECTRIC CONTROL PULSE TO SAID GATED STAGE, SO THAT THE DISPLAY OF RECEIVED SIGNALS IS INHIBITED FOR A PREDETERMINED TIME PERIOD COMMENCING SUBSEQUENTLY TO THE COMMENCEMENT OF THE SEA BED ECHO TO ENSURE THE DISPLAY OF AT LEAST A PORTION OF THE LEADING EDGE OF THE BOTTOM ECHO OF GREATER AMPLITUDE THAN THE DISPLAY OF FISH ECHOES BEFORE SAID INTERRUPTION OCCURS, SO AS TO PROVIDE AN INDEX SUCH THAT FISH ECHOES MERGED WITH SAID BOTTOM ECHO STAND OUT AND CAN BE READILY DETECTED BY THE IRREGULARITIES THEY PRODUCE IN THE LEADING EDGE OF THE BOTTOM ECHO AS DISPLAYED ON SAID DISPLAY DEVICE.