Patent GB-1011509-A (GB)
Title: Fish detection sonar apparatus
Inventor: HASLETT ROY WILLIAM GEORGE
IPC: G01S15/87
Filed: 1962-12-05
Abstract: 1,011,509. Echo-sounding apparatus. S. SMITH & SONS (ENGLAND) Ltd. Dec. 5, 1963 [Dec. 5, 1962], No. 45991/62. Heading H4D. Fish detecting echo sounding apparatus for use in trawl fishing, comprises a transmitting device 15, Fig. 1, attached to the trawl 10 and generating a sounding beam 16 of pulsed pressure wave energy, downwards and forwards in front of the trawl mouth, and a receiving transducer 18 (or 18<SP>1</SP>), positioned on the trawler 13 to receive echoes of the sounding beam from fish in front of the trawl mouth, and connected to a display device 19 on the trawler to display the received echoes. The first echoes of any one transmitted pulse are received by the receiver transducer 18 from fish within the sounding beam 16 and just below the transmitting device 15, Fig. 2. Further echoes are then received from fish within the sounding beam but nearer to the sea bed. 11. After a certain time, however, stray energy from the transmitted pulse is reflected in a specular fashion from a point 17 on the sea bed. Echoes from fish within the beam 16, but deeper than the line 0<SP>1</SP>, which is the locus of points having the same transmitterpoint-receiver, path length as point 17, are therefore masked by further sea bed echoes. The amount of fish echoes so lost may be lessened by inclining the beam 16 more forward. The transmitter may comprise a pulse generator, gating the output of a 30 kc/s. oscillator to a power amplifier and transmitting transducer, Fig. 3 (not shown), or the pulse generator may be used to control the output of the oscillator, Fig. 4 (not shown). The receiver may comprise, Fig. 5, a rotating magnetic recording drum 34, which can be changed from a recording to a reproducing mode by a switch 32 on the reception thereby of a signal from an amplitude selector 34. The received fish echoes are too small to cause the selector to emit such a signal, and are therefore recorded on the drum. When, however, the large sea bed echo is received the selector emits a signal, and the stored fish echoes are read out to a display means 35, which may be a cathode-ray tube, having a time base triggered by the signal from the selector. The fish echoes are thus always displayed relative to the sea bed, independent of the height of the vessel above the sea bed. An additional transmitter and receiver may be provided on the vessel using the same display means, but operating at a frequency of 20 kc/s. instead of 30 kc/s. Alternative to the time base of the display being triggered by the sea bed echo, the trawl borne transmitter may partly transmit directly to the receiver on the vessel, where the received pulse is used to trigger the time base of the display, such that the subsequently received fish echoes are shown relative to the trawl head line and the sea bed. Due to the inclination of the sounding beam to the sea bed a higher than normal pulse repetition rate can be used without risking the possibility of spurious displays from echoes received via a sea surface reflection. Specifications 785,001, 865,111 and 908,905 are referred to.