Submarine echo-sounding apparatus for fishing vessels

789,225. Echo-sounding. PYE MARINE, Ltd., and SOC. GENERALE D'ELECTRICITE ET DE RADIO. Sept. 26, 1955 [Sept. 27, 1954], No. 27827/54. Class 118 (2). [Also in Group XL (c)] Echo-sounding apparatus for accurate positioning of a trawl net 5 with respect to a shoal of fish 4 comprises a transducer 9 on the trawl net instead of or in addition to the transducer 8 on the fishing vessel, and means for producing a magnified indication of echoes over a short sounding range, the echoes being indicated on one or more cathode-ray tubes, with a variable frequency oscillator for triggering the pulse transmitter and providing a time-base for the cathode-ray tube (C.R.T.) display. The oscillator frequency is adjusted so that the echo 30 (Fig. 2a) of the sea bed 3 (Fig. 1) occurs at the end of the scanning trace (Fig. 2b), whereby a maximum length of trace is used and the sounding pulse repetition rate is the maximum possible for the depth concerned, the adjustment being effected by a control knob associated with a scale which gives an indication of the depth of the sea bed or other object producing the echo 30 which has been aligned with a fixed mark 32 on the C.R.T. screen. Fig. 3 shows a block diagram of the apparatus. A variable frequency oscillator 15, which provides a time base for C.R.T. 17, triggers pulse transmitter 18, and through a variable delay network 21 synchronizes a high-speed time-base circuit 22. The output from this may be selectively applied to the C.R.T. 17 by the switch 16, or alternatively to a second C.R.T. 23, to provide a magnified partial range of, for example, seven fathoms, selectable within the main range by variation of the delay in network 21. The control knob of the variable delay network 21 is associated with a scale calibrated to indicate the partial range selected. This scale and the sea-bed depth (oscillator frequency) scale may be arranged vertically one on either side of the C.R.T. 17, as in Fig. 6 (not shown). The output of the high-speed time-base circuit 22 is also fed through a pulse-shaping circuit 24 to provide a marker signal for receiving amplifier 20 either to brighten up the trace or displace a portion of it (Fig. 2c), preferably for a duration corresponding to the range of the high-speed time base. The transmitter 18 and receiving amplifier 20 may be connected to the transducer 8 on the ship or to transducer 9 on the net by means of change-over switch 19. When a shoal of fish has been located (e.g. 34 in Fig. 2b) the marker