APPARATUS FOR AIMING A PELAGIC TRAWL AT A SHOAL OF FISH

摘要

1318537 Nets SIMRAD AS 9 Sept 1970 [15 Sept 1969] 43175/70 Heading A1A [Also in Division H4] Apparatus for aiming a pelagic trawl at an echo-producing object such as a fish school comprises two sonar transducers mounted, near the trawl mouth, in the horizontal plate through the central axis of the trawl and directed so that their respective sound radiations overlap and have approximately the same intensity in the vertical plane through the central axis, two further transducers mounted near the trawl mouth in said vertical plane and directed so that their radiations overlap and have approximately equal intensities in a common plane which is inclined to said axis and intersects the axis at a predetermined distance from the trawl mouth, and echo indicating means on the trawling vessel having an indicator screen with four separate time base axes for displaying echoes from each of the transducers. Respective distance markers representing said predetermined distance are provided on each time base axis. The occurrence of pairs of approxiately equal strength echo signal indications at the markers indicates that the trawl is aimed at an echo-producing object at the predetermined distance from the trawl mouth. In the embodiment described the transducers are preferably identical and may be energised sequentially at the same carrier frquency or may be energised simultaneously at different respective carrier frequencies derived from a common trigger pulse. The echo indicator is a cathode ray tube with four deflection-modulated radial time bases arranged as a rectangular cross, the distance markers being provided by a range ring. The tube's luminance peristance is preferably long enough for the displayed echoes of several echo-sounding periods to be visible at the same time.