GB816119A - Improvements in or relating to sub-aqueous echo sounders for use in fishing - Google Patents

Abstract

816,119. Echo-sounding. MARCONI'S WIRELESS TELEGRAPH CO. Ltd. June 27, 1957 [Aug. 13, 1956], No. 24757/56. Class 118 (2). A sub-aqueous echo sounder for use in fish finding comprises indicating means responsive to the combined strength and number of echoes received in a given time from a predetermined depth interval. By arranging the indicating means to be responsive to both the number and strength of echoes the arrangement according to the invention provides an indication which for any given sort of fish, represents the number and weight of fish per unit volume of sea and thus the weight of fish being caught in a net sweeping through the predetermined depth interval. The depth interval is preferably selected with relation to the sea bottom, although it is possible to select the interval with relation to the bottom of the ship. A first embodiment is shown in Fig. 1. Echo signals from the receiver transducer 44 are applied to two channels, the first 45, 14, 16 incorporating an 8 milliseconds delay circuit 19, and the second 46, 39, 40 incorporating an 8 milliseconds pulse generator 100 which is triggered through a gate stage 6 arranged to pass only powerful echoes of the power of seabottom echoes. Gate 6 is also rendered insensitive to transmission pulses by a blocking pulse applied from a pulse generator 3, 4 triggered by the transmitter trigger pulse applied at 2. The output from delay circuit 19 is applied to the control grid of normally non- conducting gate stage 20 whilst the pulse from generator 100 is applied to the suppressor grid to render the gate stage conducting. By this means the gate is arranged to pass signals during only the 8 milliseconds preceding a bottom pulse and corresponding to a depth interval of just over 3 fathoms in relation to the sea bottom. The signals from the gate are amplified in stage 101, D.C. restored by diode 102 and then applied via a cathode-follower 103 to an integrating circuit 113, 114, 115, the resulting potential being applied to control D.C. motor via a power amplifier 104. The motor drives a counter 106 via a gear-box 107. The total movement of the motor depends on the amplitude and number of fish echoes as integrated by circuit 113, 115, and thus the counter may provide an indication of the catch in a net sweeping through the selected depth interval. The rate of catch is indicated by a voltmeter circuit 108-112 indicating the instantaneous peak potential on capacitor 113. Suitable values for the charge and discharge time constants of the integrating circuit are quoted. The arrangement shown in Fig. 1 suffers from the defect that counter 106 will read lower than it ought for a correct indication of the catch if from time to time no bottom or other echoes are received due for example to bad rolling. The counter reading may be corrected by multiplying it by a factor equal to the ratio of transmitted pulses to bottom pulses. To permit this correction factor to be obtained counters 116, 117 may be associated with the transmitter trigger input 2 and the trigger stage 100. In a second embodiment, Fig. 2 (not shown), the correction is applied automatically. In this embodiment the integrator comprises a Miller integrator, and the integrated voltage is measured by applying it to charge a capacitor which is subsequently discharged through a constant-current valve stage, an oscillator being triggered on for the period of the discharge and the generated cycles of oscillations counted. The measurement is normally made after every six transmitted pulses, but if the number of bottom pulses does not correspond the mesurement is deferred until six bottom pulses have been received and the capacitor is increased in value by one-sixth for each transmitted pulse after the sixth. By this means the charge fora a given voltage is increased and the final count corrected in accordance with the ratio of transmitted to bottom pulses. The arrangement involves the use of relavs and electromagnetic stepping switches. A third embodiment, Fig. 3 (not shown), uses wholly electronic circuits to apply the correction. The circuit is identical with that in Fig. 1 except that the discharge resistor 114 for the integrator capacitor 113 is made very large, so that the charge is retained between signals from the cathode follower, and means responsive to each bottom pulse is provided to completely discharge the capacitor just before the signals appear. With this arrangement, if a bottom pulse is missed, the capacitor retains its charge and the counter operation continues based on the last good sounding. A refinement based on a more complex integrating circuit permits the operation to be based on the average of ten good soundings.