JPH05288836

PURPOSE:To clearly display a school of fish in any position and to easily distinguish it by a method wherein a control signal which controls an AGC circuit is changed together with a distance. CONSTITUTION:A seabed reflection level is found at each distance by means of data 110 on an angle of depression and data 111 on the depth of water; a data table is formed in advance in a ROM 12. A clock signal 112 and a transmission trigger 113 are input to a counter 13; they are reset by a transmission trigger from a scanning sonar 4; a counting operation is started; the distance L is found by means of the number of counts. Correction values with reference to a change in the distance L are read out sequentially from the ROM 12; they are converted into an analog voltage signal by means of a D/A converter 14; the signal is multiplied, by means of a multiplier 15, by a variable resistance value which has been set by an operator; an AGC circuit 1 is controlled by a voltage V.AGC. The output signal of the circuit 1 is not controlled in a short distance; it is controlled, from a prescribed distance, by a control value which has been set by means of a setting variable resistor 11.