SE357447

1,259,546. Acoustic echo-ranging. BENDIX CORP. 1 Dec., 1969 [27 Dec., 1968], No. 58578/69. Heading H4D. In a sonar system a burst of relatively short pulses, Fig. 2d, replaces each long pulse normally emitted, such that wanted echoes fill in the gaps in the received burst and unwanted echoes, e.g. due to small objects such as fish, do not. In an embodiment transducers 10, 12 provide for arrival angle determination based on the phase shift between their outputs; a phase comparator 22 produces positive and negative rectangular waves of relative duration corresponding to sign and magnitude of echo direction, the position of the target being indicated as a bright spot on cathode-ray tube (C.R.T.) 24 off the centre of the sector being scanned. Range is given by distance from the centre of the C.R.T. A detector 26 feeds an automatic gain control circuit 27 and a filter 28 (see below). In the transmitter a feedback on line 41 is utilized to ensure that pulses start and stop at a zero crossing of the output of oscillator 34. Pulse train control circuit 42 includes manually operable switches for setting pulse duration and the interval between pulses, and co-ordinates with a setting in filter 28; and by manipulating the mark/space ratio in train, Fig. 2d, an operator may estimate the equivalent length of the target. Filter 28, Fig. 4, comprises a transistor 74 such that when it conducts high-frequency components of the signal detected by detector 26 are grounded. An output from pulse control 42, Fig. 1, is applied to the base of transistor 84 which controls the time constant of the filter by adding or removing capacitor 94. Diode 90 is included to inhibit voltage build-up on capacitors 92, 94 between pulses. An input 72, from circuit 30, Fig. 1, is a high-frequency signal including components due to e.g. reverberation and small scattered reflectors; to prevent the display of such a signal a transistor 74 is caused thereby to conduct and thus ground the video output.