Electronic Sector-scanning Asdic

Extract

A new system of electronic scanning is described in which the narrow receiving beam of an acoustic echo-ranging equipment (i.e. asdic) is swept over a wide sector at a very high speed. Since the transmitted pulse covers the whole sector, and the scanning is done at least once per pulse duration, an effectively instantaneous search of the sector is made without any loss of information. The results of trials at sea are shown and possible applications to surveying and navigation are briefly discussed.

The use of underwater acoustic echo-ranging systems in the form of echo-sounders (as fitted on most ships nowadays) and asdics (as used, for example, for submarine detection during the war and now applied to whale-finding and fish-finding) is well-known.1 These existing systems use a single beam of acoustic energy, the pulse being transmitted by a transducer (or loudspeaker), and received, after reflection from the sea-bottom or whale or other object, by the same or a different transducer (this time used as a microphone). The time difference between transmitted and received pulses measures the range of the bottom or object, and is recorded on a chart by a stylus moving at a speed equal to the velocity of sound in water multiplied by the ratio of the width of the chart to twice the maximum range of detection. The transmission of the pulse marks the left-hand edge of the paper, and any echo received causes a mark at a point in the stylus sweep corresponding to the range of the object causing the echo. The paper is meanwhile moving at right angles to the stylus sweep, so that the next pulse transmission marks on a line adjacent to the previous one.